FM Communication

FM Communication basic concepts of FM Communications

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Contents Articles Fundamentals Telecommunication

1 1

Electromagnetic radiation

15

Electromagnetic induction

23

Frequency

25

Frequency synthesizer

25

Frequency mixer

29

Very high frequency

32

Ultra high frequency

37

Super high frequency

49

Extremely high frequency

50

Modulation Modulation

Transmitter Transmitter

Antenna Antenna (radio)

Reciever

54 54 60 60 67 67 87

Receiver (radio)

87

Tuned radio frequency receiver

93

Radar

96

Radar

96

Applications

114

Transistor radio

114

Walkie-talkie

119

Extra Knowledge

127

Noise (electronics)

127

Induction plasma technology

129

References Article Sources and Contributors

135

Image Sources, Licenses and Contributors

138

Article Licenses License

141

1

Fundamentals Telecommunication Telecommunication is the transmission of information over significant distances to communicate. In earlier times, telecommunications involved the use of visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, and optical heliographs, or audio messages via coded drumbeats, lung-blown horns, or sent by loud whistles, for example. In the modern age of electricity and electronics, telecommunications now also includes the use of electrical devices such as telegraphs, telephones, and teletypes, the use of radio and microwave communications, as well as fiber optics and their associated electronics, plus the use of the orbiting satellites and the Internet. A Gower telephone, at the Musée des Arts et A revolution in wireless telecommunications began in the first decade Métiers in Paris of the 20th century, with Guglielmo Marconi winning the Nobel Prize in Physics in 1909 for his pioneering developments in wireless radio communications. Other highly notable pioneering inventors and developers in the field of electrical and electronic telecommunications include Charles Wheatstone and Samuel Morse (telegraph), Alexander Graham Bell (telephone), Nikola Tesla, Edwin Armstrong, and Lee de Forest (radio), as well as John Logie Baird and Philo Farnsworth (television).

Telecommunications play an important role in the world economy and the worldwide telecommunication industry's revenue was estimated to be $3.85 trillion in 2008.[1] The service revenue of the global telecommunications industry was estimated to be $1.7 trillion in 2008, and is expected to touch $2.7 trillion by 2013.[1]

History Ancient systems Greek hydraulic semaphore systems were used as early as the 4th century BC. The hydraulic semaphores, which worked with water filled vessels and visual signals, functioned as optical telegraphs. However, they could only utilize a very limited range of pre-determined messages, and as with all such optical telegraphs could only be deployed during good visibility conditions.[2] During the Middle Ages, chains of beacons were commonly used on hilltops as a means of relaying a signal. Beacon chains suffered the drawback that they could only pass a single bit of information, so the meaning of the message such as "the enemy has been sighted" had to be agreed upon in advance. One notable instance of their use was during the Spanish Armada, when a beacon chain relayed a signal from Plymouth to London that signaled the arrival of the Spanish warships.[3]

Telecommunication

Systems since the Middle Ages In 1792, Claude Chappe, a French engineer, built the first fixed visual telegraphy system (or semaphore line) between Lille and Paris.[4] However semaphore systems suffered from the need for skilled operators and the expensive towers at intervals of 10–30 kilometers (6–20 mi). As a result of competition from the electrical telegraph, Europe's last commercial semaphore line in Sweden was abandoned in 1880.[5]

The telegraph and telephone The first commercial electrical telegraph was constructed by Sir Charles Wheatstone and Sir William Fothergill Cooke, and its use began on April 9, 1839. Both Wheatstone and Cooke viewed their device as "an improvement to the [already-existing, so-called] electromagnetic telegraph" not as a new device.[6] A replica of one of Chappe's semaphore towers in The businessman Samuel F.B. Morse and the physicist Joseph Henry Nalbach of the United States developed their own, simpler version of the electrical telegraph, independently. Morse successfully demonstrated this system on September 2, 1837. Morse's most important technical contribution to this telegraph was the rather simple and highly efficient Morse Code, which was an important advance over Wheatstone's complicated and significantly more expensive telegraph system. The communications efficiency of the Morse Code anticipated that of the Huffman code in digital communications by over 100 years, but Morse and his associate Alfred Vail developed the code purely empirically, unlike Huffman, who gave a detailed theoretical explanation of how his method worked.

The first permanent transatlantic telegraph cable was successfully completed on 27 July 1866, allowing transatlantic electrical communication for the first time.[7] An earlier transatlantic cable had operated for a few months in 1859, and among other things, it carried messages of greeting back and forth between President James Buchanan of the United States and Queen Victoria of the United Kingdom. However, that transatlantic cable failed soon, and the project to lay a replacement line was delayed for five years by the American Civil War. Also, these transatlantic cables would have been completely incapable of carrying telephone calls even had the telephone already been invented. The first transatlantic telephone cable (which incorporated hundreds of electronic amplifiers) was not operational until 1956.[8] The conventional telephone now in use worldwide was first patented by Alexander Graham Bell in March 1876.[9] That first patent by Bell was the master patent of the telephone, from which all other patents for electric telephone devices and features flowed. Credit for the invention of the electric telephone has been frequently disputed, and new controversies over the issue have arisen from time-to-time. As with other great inventions such as radio, television, the light bulb, and the digital computer, there were several inventors who did pioneering experimental work on voice transmission over a wire, and then they improved on each other's ideas. However, the key innovators were Alexander Graham Bell and Gardiner Greene Hubbard, who created the first telephone company, the Bell Telephone Company in the United States, which later evolved into American Telephone & Telegraph (AT&T). The first commercial telephone services were set up in 1878 and 1879 on both sides of the Atlantic in the cities of New Haven, Connecticut, and London, England.[10] [11]

2

Telecommunication

Radio and television In 1832, James Lindsay gave a classroom demonstration of wireless telegraphy via conductive water to his students. By 1854, he was able to demonstrate a transmission across the Firth of Tay from Dundee, Scotland, to Woodhaven, a distance of about two miles (3 km), again using water as the transmission medium.[12] In December 1901, Guglielmo Marconi established wireless communication between St. John's, Newfoundland and Poldhu, Cornwall (England), earning him the Nobel Prize in Physics for 1909, one which he shared with Karl Braun.[13] However small-scale radio communication had already been demonstrated in 1893 by Nikola Tesla in a presentation before the National Electric Light Association.[14] On March 25, 1925, John Logie Baird of Scotland was able to demonstrate the transmission of moving pictures at the Selfridge's department store in London, England. Baird's system relied upon the fast-rotating Nipkow disk, and thus it became known as the mechanical television. It formed the basis of experimental broadcasts done by the British Broadcasting Corporation beginning September 30, 1929.[15] However, for most of the 20th century, television systems were designed around the cathode ray tube, invented by Karl Braun. The first version of such an electronic television to show promise was produced by Philo Farnsworth of the United States, and it was demonstrated to his family in Idaho on September 7, 1927.[16]

Computer networks and the Internet On 11 September 1940, George Stibitz was able to transmit problems using teletype to his Complex Number Calculator in New York and receive the computed results back at Dartmouth College in New Hampshire.[17] This configuration of a centralized computer or mainframe computer with remote "dumb terminals" remained popular throughout the 1950s and into the 60's. However, it was not until the 1960s that researchers started to investigate packet switching — a technology that allows chunks of data to be sent between different computers without first passing through a centralized mainframe. A four-node network emerged on December 5, 1969. This network soon became the ARPANET, which by 1981 would consist of 213 nodes.[18] ARPANET's development centred around the Request for Comment process and on 7 April 1969, RFC 1 was published. This process is important because ARPANET would eventually merge with other networks to form the Internet, and many of the communication protocols that the Internet relies upon today were specified through the Request for Comment process. In September 1981, RFC 791 introduced the Internet Protocol version 4 (IPv4) and RFC 793 introduced the Transmission Control Protocol (TCP) — thus creating the TCP/IP protocol that much of the Internet relies upon today. However, not all important developments were made through the Request for Comment process. Two popular link protocols for local area networks (LANs) also appeared in the 1970s. A patent for the token ring protocol was filed by Olof Soderblom on October 29, 1974, and a paper on the Ethernet protocol was published by Robert Metcalfe and David Boggs in the July 1976 issue of Communications of the ACM.[19] [20] The Ethernet protocol had been inspired by the ALOHAnet protocol which had been developed by electrical engineering researchers at the University of Hawaii.

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4

Key concepts Etymology The word telecommunication was adapted from the French word télécommunication. It is a compound of the Greek prefix tele- (τηλε-), meaning [21] "far off", and the Latin communicare, meaning "to share". The French word télécommunication was coined in 1904 by the French engineer and [22] novelist Édouard Estaunié.

A number of key concepts reoccur throughout the literature on modern telecommunication systems. Some of these concepts are discussed below.

Basic elements A basic telecommunication system consists of three primary units that are always present in some form: • A transmitter that takes information and converts it to a signal. • A transmission medium, also called the "physical channel" that carries the signal. An example of this is the "free space channel". • A receiver that takes the signal from the channel and converts it back into usable information. For example, in a radio broadcasting station the station's large power amplifier is the transmitter; and the broadcasting antenna is the interface between the power amplifier and the "free space channel". The free space channel is the transmission medium; and the receiver's antenna is the interface between the free space channel and the receiver. Next, the radio receiver is the destination of the radio signal, and this is where it is converted from electricity to sound for people to listen to. Sometimes, telecommunication systems are "duplex" (two-way systems) with a single box of electronics working as both a transmitter and a receiver, or a transceiver. For example, a cellular telephone is a transceiver.[23] The transmission electronics and the receiver electronics in a transceiver are actually quite independent of each other. This can be readily explained by the fact that radio transmitters contain power amplifiers that operate with electrical powers measured in the watts or kilowatts, but radio receivers deal with radio powers that are measured in the microwatts or nanowatts. Hence, transceivers have to be carefully designed and built to isolate their high-power circuitry and their low-power circuitry from each other. Telecommunication over telephone lines is called point-to-point communication because it is between one transmitter and one receiver. Telecommunication through radio broadcasts is called broadcast communication because it is between one powerful transmitter and numerous low-power but sensitive radio receivers.[23] Telecommunications in which multiple transmitters and multiple receivers have been designed to cooperate and to share the same physical channel are called multiplex systems.

Analog versus digital communications Communications signals can be either by analog signals or digital signals. There are analog communication systems and digital communication systems. For an analog signal, the signal is varied continuously with respect to the information. In a digital signal, the information is encoded as a set of discrete values (for example, a set of ones and zeros). During the propagation and reception, the information contained in analog signals will inevitably be degraded by undesirable physical noise. (The output of a transmitter is noise-free for all practical purposes.) Commonly, the noise in a communication system can be expressed as adding or subtracting from the desirable signal in a completely random way. This form of noise is called "additive noise", with the understanding that the noise can be negative or positive at different instants of time. Noise that is not additive noise is a much more difficult situation to describe or analyze, and these other kinds of noise will be omitted here. On the other hand, unless the additive noise disturbance exceeds a certain threshold, the information contained in digital signals will remain intact. Their resistance to noise represents a key advantage of digital signals over analog

Telecommunication signals.[24]

Telecommunication networks A communications network is a collection of transmitters, receivers, and communications channels that send messages to one another. Some digital communications networks contain one or more routers that work together to transmit information to the correct user. An analog communications network consists of one or more switches that establish a connection between two or more users. For both types of network, repeaters may be necessary to amplify or recreate the signal when it is being transmitted over long distances. This is to combat attenuation that can render the signal indistinguishable from the noise.[25]

Communication channels The term "channel" has two different meanings. In one meaning, a channel is the physical medium that carries a signal between the transmitter and the receiver. Examples of this include the atmosphere for sound communications, glass optical fibers for some kinds of optical communications, coaxial cables for communications by way of the voltages and electric currents in them, and free space for communications using visible light, infrared waves, ultraviolet light, and radio waves. This last channel is called the "free space channel". The sending of radio waves from one place to another has nothing to do with the presence or absence of an atmosphere between the two. Radio waves travel through a perfect vacuum just as easily as they travel through air, fog, clouds, or any other kind of gas besides air. The other meaning of the term "channel" in telecommunications is seen in the phrase communications channel, which is a subdivision of a transmission medium so that it can be used to send multiple streams of information simultaneously. For example, one radio station can broadcast radio waves into free space at frequencies in the neighborhood of 94.5 MHz (megahertz) while another radio station can simultaneously broadcast radio waves at frequencies in the neighborhood of 96.1 MHz. Each radio station would transmit radio waves over a frequency bandwidth of about 180 kHz (kilohertz), centered at frequencies such as the above, which are called the "carrier frequencies". Each station in this example is separated from its adjacent stations by 200 kHz, and the difference between 200 kHz and 180 kHz (20 kHz) is an engineering allowance for the imperfections in the communication system. In the example above, the "free space channel" has been divided into communications channels according to frequencies, and each channel is assigned a separate frequency bandwidth in which to broadcast radio waves. This system of dividing the medium into channels according to frequency is called "frequency-division multiplexing" (FDM). Another way of dividing a communications medium into channels is to allocate each sender a recurring segment of time (a "time slot", for example, 20 milliseconds out of each second), and to allow each sender to send messages only within its own time slot. This method of dividing the medium into communication channels is called "time-division multiplexing" (TDM), and is used in optical fiber communication.[25] [26] Some radio communication systems use TDM within an allocated FDM channel. Hence, these systems use a hybrid of TDM and FDM.

Modulation The shaping of a signal to convey information is known as modulation. Modulation can be used to represent a digital message as an analog waveform. This is commonly called "keying" – a term derived from the older use of Morse Code in telecommunications – and several keying techniques exist (these include phase-shift keying, frequency-shift keying, and amplitude-shift keying). The "Bluetooth" system, for example, uses phase-shift keying to exchange information between various devices.[27] [28] In addition, there are combinations of phase-shift keying and amplitude-shift keying which is called (in the jargon of the field) "quadrature amplitude modulation" (QAM) that are used in high-capacity digital radio communication systems.

5

Telecommunication Modulation can also be used to transmit the information of low-frequency analog signals at higher frequencies. This is helpful because low-frequency analog signals cannot be effectively transmitted over free space. Hence the information from a low-frequency analog signal must be impressed into a higher-frequency signal (known as the "carrier wave") before transmission. There are several different modulation schemes available to achieve this [two of the most basic being amplitude modulation (AM) and frequency modulation (FM)]. An example of this process is a disc jockey's voice being impressed into a 96 MHz carrier wave using frequency modulation (the voice would then be received on a radio as the channel "96 FM").[29] In addition, modulation has the advantage of being about to use frequency division multiplexing (FDM).

Society and telecommunication Telecommunication has a significant social, cultural. and economic impact on modern society. In 2008, estimates placed the telecommunication industry's revenue at $3.85 trillion or just under 3 percent of the gross world product (official exchange rate).[1] Several following sections discuss the impact of telecommunication on society.

Economic impact Microeconomics On the microeconomic scale, companies have used telecommunications to help build global business empires. This is self-evident in the case of online retailer Amazon.com but, according to academic Edward Lenert, even the conventional retailer Wal-Mart has benefited from better telecommunication infrastructure compared to its competitors.[30] In cities throughout the world, home owners use their telephones to organize many home services ranging from pizza deliveries to electricians. Even relatively-poor communities have been noted to use telecommunication to their advantage. In Bangladesh's Narshingdi district, isolated villagers use cellular phones to speak directly to wholesalers and arrange a better price for their goods. In Côte d'Ivoire, coffee growers share mobile phones to follow hourly variations in coffee prices and sell at the best price.[31] Macroeconomics On the macroeconomic scale, Lars-Hendrik Röller and Leonard Waverman suggested a causal link between good telecommunication infrastructure and economic growth.[32] Few dispute the existence of a correlation although some argue it is wrong to view the relationship as causal.[33] Because of the economic benefits of good telecommunication infrastructure, there is increasing worry about the inequitable access to telecommunication services amongst various countries of the world—this is known as the digital divide. A 2003 survey by the International Telecommunication Union (ITU) revealed that roughly a third of countries have fewer than one mobile subscription for every 20 people and one-third of countries have fewer than one land-line telephone subscription for every 20 people. In terms of Internet access, roughly half of all countries have fewer than one out of 20 people with Internet access. From this information, as well as educational data, the ITU was able to compile an index that measures the overall ability of citizens to access and use information and communication technologies.[34] Using this measure, Sweden, Denmark and Iceland received the highest ranking while the African countries Nigeria, Burkina Faso and Mali received the lowest.[35]

Social impact Telecommunication has played a significant role in social relationships. Nevertheless devices like the telephone system were originally advertised with an emphasis on the practical dimensions of the device (such as the ability to conduct business or order home services) as opposed to the social dimensions. It was not until the late 1920s and 1930s that the social dimensions of the device became a prominent theme in telephone advertisements. New promotions started appealing to consumers' emotions, stressing the importance of social conversations and staying

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connected to family and friends.[36] Since then the role that telecommunications has played in social relations has become increasingly important. In recent years, the popularity of social networking sites has increased dramatically. These sites allow users to communicate with each other as well as post photographs, events and profiles for others to see. The profiles can list a person's age, interests, sexual preference and relationship status. In this way, these sites can play important role in everything from organising social engagements to courtship.[37] Prior to social networking sites, technologies like short message service(SMS) and the telephone also had a significant impact on social interactions. In 2000, market research group Ipsos MORI reported that 81% of 15 to 24 year-old SMS users in the United Kingdom had used the service to coordinate social arrangements and 42% to flirt.[38]

Other impacts In cultural terms, telecommunication has increased the public's ability to access to music and film. With television, people can watch films they have not seen before in their own home without having to travel to the video store or cinema. With radio and the Internet, people can listen to music they have not heard before without having to travel to the music store. Telecommunication has also transformed the way people receive their news. A survey by the non-profit Pew Internet and American Life Project found that when just over 3,000 people living in the United States were asked where they got their news "yesterday", more people said television or radio than newspapers. The results are summarised in the following table (the percentages add up to more than 100% because people were able to specify more than one source).[39] Local TV

National TV

Radio

Local paper

Internet

National paper

59%

47%

44%

38%

23%

12%

Telecommunication has had an equally significant impact on advertising. TNS Media Intelligence reported that in 2007, 58% of advertising expenditure in the United States was spent on mediums that depend upon telecommunication.[40] The results are summarised in the following table. Internet

Radio

Cable TV

Syndicated TV

Spot TV

Network TV

Newspaper

Magazine

Outdoor

Total

Percent 7.6%

7.2%

12.1%

2.8%

11.3%

17.1%

18.9%

20.4%

2.7%

100%

Dollars $11.31 billion

$10.69 billion

$18.02 billion

$4.17 billion

$16.82 billion

$25.42 billion

$28.22 billion

$30.33 billion

$4.02 billion

$149 billion

Telecommunication and government Many countries have enacted legislation which conform to the International Telecommunication Regulations establish by the International Telecommunication Union (ITU), which is the "leading UN agency for information and communication technology issues."[41] In 1947, at the Atlantic City Conference, the ITU decided to "afford international protection to all frequencies registered in a new international frequency list and used in conformity with the Radio Regulation." According to the ITU's Radio Regulations adopted in Atlantic City, all frequencies referenced in the International Frequency Registration Board, examined by the board and registered on the International Frequency List "shall have the right to international protection from harmful interference."[42] From a global perspective, there have been political debates and legislation regarding the management of telecommunication and broadcasting. The history of broadcasting discusses some of debates in relation to balancing

Telecommunication

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conventional communication such as printing and telecommunication such as radio broadcasting.[43] The onset of World War II brought on the first explosion of international broadcasting propaganda.[43] Countries, their governments, insurgents, terrorists, and militiamen have all used telecommunication and broadcasting techniques to promote propaganda.[43] [44] Patriotic propaganda for political movements and colonization started the mid 1930s. In 1936, the BBC did broadcast propaganda to the Arab World to partly counter similar broadcasts from Italy, which also had colonial interests in North Africa.[43] Modern insurgents, such as those in the latest Iraq war, often use intimidating telephone calls, SMSs and the distribution of sophisticated videos of an attack on coalition troops within hours of the operation. "The Sunni insurgents even have their own television station, Al-Zawraa, which while banned by the Iraqi government, still broadcasts from Erbil, Iraqi Kurdistan, even as coalition pressure has forced it to switch satellite hosts several times." [44]

Modern telecommunication Telephone In an analog telephone network, the caller is connected to the person he wants to talk to by switches at various telephone exchanges. The switches form an electrical connection between the two users and the setting of these switches is determined electronically when the caller dials the number. Once the connection is made, the caller's voice is transformed to an electrical signal using a small microphone in the caller's handset. This electrical signal is then sent through the network to the user at the other end where it is transformed back into sound by a small speaker in that person's handset. There is a separate electrical connection that works in reverse, allowing the users to converse.[45] [46] The fixed-line telephones in most residential homes are analog — that is, the speaker's voice directly determines the signal's voltage. Although short-distance calls may be handled from end-to-end as analog signals, increasingly telephone service providers are transparently converting the signals to digital for transmission before converting them back to analog for reception. The advantage of this is that digitized voice data can travel side-by-side with data from the Internet and can be perfectly reproduced in long distance communication (as opposed to analog signals that are inevitably impacted by noise).

Optical fiber provides cheaper bandwidth for long distance communication

Mobile phones have had a significant impact on telephone networks. Mobile phone subscriptions now outnumber fixed-line subscriptions in many markets. Sales of mobile phones in 2005 totalled 816.6 million with that figure being almost equally shared amongst the markets of Asia/Pacific (204 m), Western Europe (164 m), CEMEA (Central Europe, the Middle East and Africa) (153.5 m), North America (148 m) and Latin America (102 m).[47] In terms of new subscriptions over the five years from 1999, Africa has outpaced other markets with 58.2% growth.[48] Increasingly these phones are being serviced by systems where the voice content is transmitted digitally such as GSM or W-CDMA with many markets choosing to depreciate analog systems such as AMPS.[49] There have also been dramatic changes in telephone communication behind the scenes. Starting with the operation of TAT-8 in 1988, the 1990s saw the widespread adoption of systems based on optic fibres. The benefit of communicating with optic fibers is that they offer a drastic increase in data capacity. TAT-8 itself was able to carry 10 times as many telephone calls as the last copper cable laid at that time and today's optic fibre cables are able to carry 25 times as many telephone calls as TAT-8.[50] This increase in data capacity is due to several factors: First, optic fibres are physically much smaller than competing technologies. Second, they do not suffer from crosstalk

Telecommunication which means several hundred of them can be easily bundled together in a single cable.[51] Lastly, improvements in multiplexing have led to an exponential growth in the data capacity of a single fibre.[52] [53] Assisting communication across many modern optic fibre networks is a protocol known as Asynchronous Transfer Mode (ATM). The ATM protocol allows for the side-by-side data transmission mentioned in the second paragraph. It is suitable for public telephone networks because it establishes a pathway for data through the network and associates a traffic contract with that pathway. The traffic contract is essentially an agreement between the client and the network about how the network is to handle the data; if the network cannot meet the conditions of the traffic contract it does not accept the connection. This is important because telephone calls can negotiate a contract so as to guarantee themselves a constant bit rate, something that will ensure a caller's voice is not delayed in parts or cut-off completely.[54] There are competitors to ATM, such as Multiprotocol Label Switching (MPLS), that perform a similar task and are expected to supplant ATM in the future.[55]

Radio and television In a broadcast system, the central high-powered broadcast tower transmits a high-frequency electromagnetic wave to numerous low-powered receivers. The high-frequency wave sent by the tower is modulated with a signal containing visual or audio information. The receiver is then tuned so as to pick up the high-frequency wave and a demodulator is used to retrieve the signal containing the visual or audio Digital television standards and their adoption worldwide. information. The broadcast signal can be either analog (signal is varied continuously with respect to the information) or digital (information is encoded as a set of discrete values).[23] [56] The broadcast media industry is at a critical turning point in its development, with many countries moving from analog to digital broadcasts. This move is made possible by the production of cheaper, faster and more capable integrated circuits. The chief advantage of digital broadcasts is that they prevent a number of complaints common to traditional analog broadcasts. For television, this includes the elimination of problems such as snowy pictures, ghosting and other distortion. These occur because of the nature of analog transmission, which means that perturbations due to noise will be evident in the final output. Digital transmission overcomes this problem because digital signals are reduced to discrete values upon reception and hence small perturbations do not affect the final output. In a simplified example, if a binary message 1011 was transmitted with signal amplitudes [1.0 0.0 1.0 1.0] and received with signal amplitudes [0.9 0.2 1.1 0.9] it would still decode to the binary message 1011 — a perfect reproduction of what was sent. From this example, a problem with digital transmissions can also be seen in that if the noise is great enough it can significantly alter the decoded message. Using forward error correction a receiver can correct a handful of bit errors in the resulting message but too much noise will lead to incomprehensible output and hence a breakdown of the transmission.[57] [58] In digital television broadcasting, there are three competing standards that are likely to be adopted worldwide. These are the ATSC, DVB and ISDB standards; the adoption of these standards thus far is presented in the captioned map. All three standards use MPEG-2 for video compression. ATSC uses Dolby Digital AC-3 for audio compression, ISDB uses Advanced Audio Coding (MPEG-2 Part 7) and DVB has no standard for audio compression but typically uses MPEG-1 Part 3 Layer 2.[59] [60] The choice of modulation also varies between the schemes. In digital audio broadcasting, standards are much more unified with practically all countries choosing to adopt the Digital Audio Broadcasting standard (also known as the Eureka 147 standard). The exception being the United States which has

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chosen to adopt HD Radio. HD Radio, unlike Eureka 147, is based upon a transmission method known as in-band on-channel transmission that allows digital information to "piggyback" on normal AM or FM analog transmissions.[61] However, despite the pending switch to digital, analog television remains being transmitted in most countries. An exception is the United States that ended analog television transmission (by all but the very low-power TV stations) on 12 June 2009[62] after twice delaying the switchover deadline. For analog television, there are three standards in use for broadcasting color TV (see a map on adoption here). These are known as PAL (British designed), NTSC (North American designed), and SECAM (French designed). (It is important to understand that these are the ways from sending color TV, and they do not have anything to do with the standards for black & white TV, which also vary from country to country.) For analog radio, the switch to digital radio is made more difficult by the fact that analog receivers are sold at a small fraction of the price of digital receivers.[63] [64] The choice of modulation for analog radio is typically between amplitude modulation (AM) or frequency modulation (FM). To achieve stereo playback, an amplitude modulated subcarrier is used for stereo FM.

The Internet The Internet is a worldwide network of computers and computer networks that can communicate with each other using the Internet Protocol.[65] Any computer on the Internet has a unique IP address that can be used by other computers to route information to it. Hence, any computer on the Internet can send a message to any other computer using its IP address. These messages carry with them the originating computer's IP address allowing for two-way communication. The Internet is thus an exchange of messages between computers.[66] As of 2008, an estimated 21.9% of the world population has access to the Internet with the highest access rates (measured as a percentage of the population) in North America (73.6%), Oceania/Australia (59.5%) and Europe (48.1%).[67] In terms of broadband access, Iceland (26.7%), South Korea (25.4%) and the Netherlands (25.3%) led the world.[68]

The OSI reference model

The Internet works in part because of protocols that govern how the computers and routers communicate with each other. The nature of computer network communication lends itself to a layered approach where individual protocols in the protocol stack run more-or-less independently of other protocols. This allows lower-level protocols to be customized for the network situation while not changing the way higher-level protocols operate. A practical example of why this is important is because it allows an Internet browser to run the same code regardless of whether the computer it is running on is connected to the Internet through an Ethernet or Wi-Fi connection. Protocols are often talked about in terms of their place in the OSI reference model (pictured on the right), which emerged in 1983 as the first step in an unsuccessful attempt to build a universally adopted networking protocol suite.[69] For the Internet, the physical medium and data link protocol can vary several times as packets traverse the globe. This is because the Internet places no constraints on what physical medium or data link protocol is used. This leads to the adoption of media and protocols that best suit the local network situation. In practice, most intercontinental

Telecommunication communication will use the Asynchronous Transfer Mode (ATM) protocol (or a modern equivalent) on top of optic fibre. This is because for most intercontinental communication the Internet shares the same infrastructure as the public switched telephone network. At the network layer, things become standardized with the Internet Protocol (IP) being adopted for logical addressing. For the World Wide Web, these "IP addresses" are derived from the human readable form using the Domain Name System (e.g. 72.14.207.99 [70] is derived from www.google.com [71]). At the moment, the most widely used version of the Internet Protocol is version four but a move to version six is imminent.[72] At the transport layer, most communication adopts either the Transmission Control Protocol (TCP) or the User Datagram Protocol (UDP). TCP is used when it is essential every message sent is received by the other computer where as UDP is used when it is merely desirable. With TCP, packets are retransmitted if they are lost and placed in order before they are presented to higher layers. With UDP, packets are not ordered or retransmitted if lost. Both TCP and UDP packets carry port numbers with them to specify what application or process the packet should be handled by.[73] Because certain application-level protocols use certain ports, network administrators can manipulate traffic to suit particular requirements. Examples are to restrict Internet access by blocking the traffic destined for a particular port or to affect the performance of certain applications by assigning priority. Above the transport layer, there are certain protocols that are sometimes used and loosely fit in the session and presentation layers, most notably the Secure Sockets Layer (SSL) and Transport Layer Security (TLS) protocols. These protocols ensure that the data transferred between two parties remains completely confidential and one or the other is in use when a padlock appears in the address bar of your web browser.[74] Finally, at the application layer, are many of the protocols Internet users would be familiar with such as HTTP (web browsing), POP3 (e-mail), FTP (file transfer), IRC (Internet chat), BitTorrent (file sharing) and OSCAR (instant messaging).

Local Area Networks and Wide Area Networks Despite the growth of the Internet, the characteristics of local area networks ("LANs" – computer networks that do not extend beyond a few kilometers in size) remain distinct. This is because networks on this scale do not require all the features associated with larger networks and are often more cost-effective and efficient without them. When they are not connected with the Internet, they also have the advantages of privacy and security. However, purposefully lacking a direct connection to the Internet will not provide 100% protection of the LAN from hackers, military forces, or economic powers. These threats exist if there are any methods for connecting remotely to the LAN. There are also independent wide area networks ("WANs" – private computer networks that can and do extend for thousands of kilometers.) Once again, some of their advantages include their privacy, security, and complete ignoring of any potential hackers – who cannot "touch" them. Of course, prime users of private LANs and WANs include armed forces and intelligence agencies that must keep their information completely secure and secret. In the mid-1980s, several sets of communication protocols emerged to fill the gaps between the data-link layer and the application layer of the OSI reference model. These included Appletalk, IPX, and NetBIOS with the dominant protocol set during the early 1990s being IPX due to its popularity with MS-DOS users. TCP/IP existed at this point, but it was typically only used by large government and research facilities.[75] As the Internet grew in popularity and a larger percentage of traffic became Internet-related, LANs and WANs gradually moved towards the TCP/IP protocols, and today networks mostly dedicated to TCP/IP traffic are common. The move to TCP/IP was helped by technologies such as DHCP that allowed TCP/IP clients to discover their own network address — a function that came standard with the AppleTalk/ IPX/ NetBIOS protocol sets.[76] It is at the data-link layer, though, that most modern LANs diverge from the Internet. Whereas Asynchronous Transfer Mode (ATM) or Multiprotocol Label Switching (MPLS) are typical data-link protocols for larger networks such as WANs; Ethernet and Token Ring are typical data-link protocols for LANs. These protocols differ from the former protocols in that they are simpler (e.g. they omit features such as Quality of Service guarantees) and offer collision prevention. Both of these differences allow for more economical systems.[77] Despite the modest popularity

11

Telecommunication of IBM token ring in the 1980s and 90's, virtually all LANs now use either wired or wireless Ethernets. At the physical layer, most wired Ethernet implementations use copper twisted-pair cables (including the common 10BASE-T networks). However, some early implementations used heavier coaxial cables and some recent implementations (especially high-speed ones) use optical fibers.[78] When optic fibers are used, the distinction must be made between multimode fibers and single-mode fiberes. Multimode fibers can be thought of as thicker optical fibers that are cheaper to manufacture devices for but that suffers from less usable bandwidth and worse attenuation – implying poorer long-distance performance.[79]

References [1] Worldwide Telecommunications Industry Revenues (http:/ / www. plunkettresearch. com/ Telecommunications/ TelecommunicationsStatistics/ tabid/ 96/ Default. aspx), Internet Engineering Task Force, June 2010. [2] Lahanas, Michael, Ancient Greek Communication Methods (http:/ / www. mlahanas. de/ Greeks/ Communication. htm), Mlahanas.de website. Retrieved July 14, 2009. [3] David Ross, The Spanish Armada (http:/ / www. britainexpress. com/ History/ tudor/ armada. htm), Britain Express, October 2008. [4] Les Télégraphes Chappe (http:/ / chappe. ec-lyon. fr/ ), Cédrick Chatenet, l'Ecole Centrale de Lyon, 2003. [5] CCIT/ITU-T 50 Years of Excellence (http:/ / www. google. com/ url?sa=t& ct=res& cd=19& url=http:/ / www. itu. int/ itudoc/ gs/ promo/ tsb/ 88192. pdf& ei=WmQKRc6wEqL4ggP_6bHTDQ& sig=__RpZ0L0hbqjtzZfVWEAMZVhduDBw=& sig2=dzK2J3-3WNRc0o63DXwciQ#search="semaphore 1880 Sweden"), International Telecommunication Union, 2006. [6] The Electromagnetic Telegraph (http:/ / www. du. edu/ ~jcalvert/ tel/ morse/ morse. htm), J. B. Calvert, 19 May 2004. [7] The Atlantic Cable (http:/ / www. sil. si. edu/ digitalcollections/ hst/ atlantic-cable/ ), Bern Dibner, Burndy Library Inc., 1959 [8] Sir Arthur C. Clarke. Voice Across the Sea (http:/ / books. google. ca/ books?id=L2UNAQAAIAAJ), Harper & Brothers, New York City, 1958. [9] Brown, Travis (1994). Historical first patents: the first United States patent for many everyday things (http:/ / books. google. com/ ?id=V-NUAAAAMAAJ& dq) (illustrated ed.). University of Michigan: Scarecrow Press. pp. 179. ISBN 9780810828988. . [10] Connected Earth: The telephone (http:/ / www. connected-earth. com/ Galleries/ Telecommunicationsage/ Thetelephone/ index. htm), BT, 2006. [11] History of AT&T (http:/ / www. att. com/ history/ milestones. html), AT&T, 2006. [12] Dundee City Council. Biography: James Bowman Lindsay 1799 – 1862 (http:/ / www. dundeecity. gov. uk/ jbl/ ), Macdonald Black, Dundee City Council website. Retrieved September 9, 2010. [13] Tesla Biography (http:/ / www. teslasociety. com/ biography. htm), Ljubo Vujovic, Tesla Memorial Society of New York, 1998. [14] Tesla's Radio Controlled Boat (http:/ / www. tfcbooks. com/ teslafaq/ q& a_025. htm), Twenty First Century Books, 2007. [15] The Pioneers (http:/ / www. mztv. com/ newframe. asp?content=http:/ / www. mztv. com/ pioneers. html), MZTV Museum of Television, 2006. [16] Philo Farnsworth (http:/ / www. time. com/ time/ magazine/ article/ 0,9171,990620,00. html), Neil Postman, TIME Magazine, 29 March 1999. [17] George Stlibetz (http:/ / www. kerryr. net/ pioneers/ stibitz. htm), Kerry Redshaw, 1996. [18] Hafner, Katie (1998). Where Wizards Stay Up Late: The Origins Of The Internet. Simon & Schuster. ISBN 0-684-83267-4. [19] Data transmission system (http:/ / patft1. uspto. gov/ netacgi/ nph-Parser?Sect1=PTO2& Sect2=HITOFF& p=1& u=/ netahtml/ PTO/ search-bool. html& r=1& f=G& l=50& co1=AND& d=PTXT& s1=4293948. PN. & OS=PN/ 4293948& RS=PN/ 4293948), Olof Solderblom, PN 4,293,948, October 1974. [20] Ethernet: Distributed Packet Switching for Local Computer Networks (http:/ / www. acm. org/ classics/ apr96/ ), Robert M. Metcalfe and David R. Boggs, Communications of the ACM (pp 395–404, Vol. 19, No. 5), July 1976. [21] Telecommunication, tele- and communication, New Oxford American Dictionary (2nd edition), 2005. [22] Jean-Marie Dilhac, From tele-communicare to Telecommunications (http:/ / www. ieee. org/ portal/ cms_docs_iportals/ iportals/ aboutus/ history_center/ conferences/ che2004/ Dilhac. pdf), 2004. [23] Haykin, Simon (2001). Communication Systems (4th ed.). John Wiley & Sons. pp. 1–3. ISBN 0-471-17869-1. [24] Ambardar, Ashok (1999). Analog and Digital Signal Processing (2nd ed.). Brooks/Cole Publishing Company. pp. 1–2. ISBN 0-534-95409-X. [25] ATIS Telecom Glossary 2000 (http:/ / www. atis. org/ tg2k/ ), ATIS Committee T1A1 Performance and Signal Processing (approved by the American National Standards Institute), 28 February 2001. [26] Yao, Colin (June 14, 2008). "Introduction to SONET (Synchronous Optical Networking)" (http:/ / www. articlesbase. com/ computers-articles/ introduction-to-sonet-synchronous-optical-networking-fiber-optic-technologies-tutorial-series-449247. html). Articlesbase.com. . [27] Haykin, pp 344–403. [28] Bluetooth Specification Version 2.0 + EDR (http:/ / www. bluetooth. org/ foundry/ adopters/ document/ Core_v2. 0_EDR/ en/ 1/ Core_v2. 0_EDR. zip) (p 27), Bluetooth, 2004.

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Telecommunication [29] Haykin, pp 88–126. [30] Lenert, Edward (10.1111/j.1460-2466.1998.tb02767.x). "A Communication Theory Perspective on Telecommunications Policy". Journal of Communication 48 (4): 3–23. doi:10.1111/j.1460-2466.1998.tb02767.x. [31] Mireille Samaan (April 2003) (PDF). The Effect of Income Inequality on Mobile Phone Penetration (http:/ / web. archive. org/ web/ 20070214102055/ http:/ / dissertations. bc. edu/ cgi/ viewcontent. cgi?article=1016& context=ashonors). Boston University Honors thesis. Archived from the original (http:/ / dissertations. bc. edu/ cgi/ viewcontent. cgi?article=1016& context=ashonors) on February 14, 2007. . Retrieved June 8, 2007. [32] Röller, Lars-Hendrik; Leonard Waverman (2001). "Telecommunications Infrastructure and Economic Development: A Simultaneous Approach". American Economic Review 91 (4): 909–923. doi:10.1257/aer.91.4.909. ISSN 0002-8282. [33] Riaz, Ali (10.1177/016344397019004004). "The role of telecommunications in economic growth: proposal for an alternative framework of analysis". Media, Culture & Society 19 (4): 557–583. doi:10.1177/016344397019004004. [34] "Digital Access Index (DAI)" (http:/ / www. itu. int/ ITU-D/ ict/ dai/ ). itu.int. . Retrieved March 6, 2008. [35] World Telecommunication Development Report 2003 (http:/ / www. itu. int/ ITU-D/ ict/ publications/ wtdr_03/ index. html), International Telecommunication Union, 2003. [36] Fischer, Claude S.. "'Touch Someone': The Telephone Industry Discovers Sociability." Technology and Culture 29.1 (January 1988): 32–61. JSTOR. Web. 4 October 2009. [37] "How do you know your love is real? Check Facebook" (http:/ / www. cnn. com/ 2008/ LIVING/ personal/ 04/ 04/ facebook. love/ index. html). CNN. April 4, 2008. . [38] I Just Text To Say I Love You (http:/ / www. ipsos-mori. com/ content/ polls-2000/ i-just-text-to-say-i-love-you. ashx), Ipsos MORI, September 2005. [39] "Online News: For many home broadband users, the internet is a primary news source" (http:/ / www. pewinternet. org/ pdfs/ PIP_News. and. Broadband. pdf). Pew Internet Project. March 22, 2006. . [40] "100 Leading National Advertisers" (http:/ / adage. com/ images/ random/ datacenter/ 2008/ spendtrends08. pdf) (PDF). Advertising Age. June 23, 2008. . Retrieved June 21, 2009. [41] International Telecommunication Union : About ITU (http:/ / www. itu. int/ net/ about/ index. aspx). ITU. Accessed 21 July 2009. ( PDF (https:/ / www. itu. int/ osg/ csd/ wtpf/ wtpf2009/ documents/ ITU_ITRs_88. pdf) of regulation) [42] Codding, George A. Jr.. " Jamming and the Protection of Frequency Assignments (http:/ / www. jstor. org/ pss/ 2194872)". The American Journal of International Law, Vol. 49, No. 3 (Jul., 1955), Published by: American Society of International Law. pp. 384–388. Republished by JSTOR.org The American Journal of International Law". Accessed 21 July 2009. [43] Wood, James & Science Museum (Great Britain) " History of international broadcasting (http:/ / books. google. ca/ books?id=WUO4U8L5N_cC& pg=PA3& lpg=PA3& dq="countries+ use+ telecommunications+ for+ propaganda& source=bl& ots=xZ23AbxMud& sig=elOIe1XUeivJ4fvrB5DPXDA6H54& hl=en& ei=CglmSpWjMIraNu3r9ZsB& sa=X& oi=book_result& ct=result& resnum=2)". IET 1994, Volume 1, p.2 of 258 ISBN 0-86341-302-1, ISBN 978-0-86341-302-5. Republished by Googlebooks. Accessed 21 July 2009. [44] Garfield, Andrew. " The U.S. Counter-propaganda Failure in Iraq (http:/ / www. meforum. org/ 1753/ the-us-counter-propaganda-failure-in-iraq)", FALL 2007, The Middle East Quarterly, Volume XIV: Number 4, Accessed 21 July 2009. [45] How Telephone Works (http:/ / electronics. howstuffworks. com/ telephone1. htm), HowStuffWorks.com, 2006. [46] Telephone technology page (http:/ / www. epanorama. net/ links/ telephone. html), ePanorama, 2006. [47] Gartner Says Top Six Vendors Drive Worldwide Mobile Phone Sales to 21% Growth in 2005 (http:/ / www. gartner. com/ press_releases/ asset_145891_11. html), Gartner Group, 28 February 2006. [48] Africa Calling (http:/ / www. spectrum. ieee. org/ may06/ 3426), Victor and Irene Mbarika, IEEE Spectrum, May 2006. [49] Ten Years of GSM in Australia (http:/ / www. amta. org. au/ default. asp?Page=142), Australia Telecommunications Association, 2003. [50] Milestones in AT&T History (http:/ / www. att. com/ history/ milestones. html), AT&T Knowledge Ventures, 2006. [51] Optical fibre waveguide (http:/ / www. cs. ucl. ac. uk/ staff/ S. Bhatti/ D51-notes/ node21. html), Saleem Bhatti, 1995. [52] Fundamentals of DWDM Technology (http:/ / www. cisco. com/ univercd/ cc/ td/ doc/ product/ mels/ cm1500/ dwdm/ dwdm_ovr. pdf), CISCO Systems, 2006. [53] Report: DWDM No Match for Sonet (http:/ / www. lightreading. com/ document. asp?doc_id=31358), Mary Jander, Light Reading, 2006. [54] Stallings, William (2004). Data and Computer Communications (7th edition (intl) ed.). Pearson Prentice Hall. pp. 337–366. ISBN 0-13-183311-1. [55] MPLS is the future, but ATM hangs on (http:/ / www. networkworld. com/ columnists/ 2002/ 0812edit. html), John Dix, Network World, 2002 [56] How Radio Works (http:/ / www. howstuffworks. com/ radio. htm), HowStuffWorks.com, 2006. [57] Digital Television in Australia (http:/ / www. digitaltv. com. au/ ), Digital Television News Australia, 2001. [58] Stallings, William (2004). Data and Computer Communications (7th edition (intl) ed.). Pearson Prentice Hall. ISBN 0-13-183311-1. [59] HDV Technology Handbook (http:/ / www. dynamix. ca/ doc/ HDVhandbook1. pdf), Sony, 2004. [60] Audio (http:/ / www. dvb. org/ technology/ standards_specifications/ audio/ ), Digital Video Broadcasting Project, 2003. [61] Status of DAB (USA) (http:/ / www. worlddab. org/ cstatus. aspx), World DAB Forum, March 2005. [62] Brian Stelter (June 13, 2009). "Changeover to Digital TV Off to a Smooth Start" (http:/ / www. nytimes. com/ 2009/ 06/ 14/ business/ media/ 14digital. html?_r=2& hp). New York Times. .

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Telecommunication [63] GE 72664 Portable AM/FM Radio (http:/ / www. amazon. com/ dp/ B00000J060), Amazon.com, June 2006. [64] DAB Products (http:/ / www. worlddab. org/ dabprod. aspx), World DAB Forum, 2006. [65] Robert E. Kahn and Vinton G. Cerf, What Is The Internet (And What Makes It Work) (http:/ / www. cnri. reston. va. us/ what_is_internet. html), December 1999. (specifically see footnote xv) [66] How Internet Infrastructure Works (http:/ / computer. howstuffworks. com/ internet-infrastructure. htm), HowStuffWorks.com, 2007. [67] World Internet Users and Population Stats (http:/ / www. internetworldstats. com/ stats. htm), internetworldstats.com, 19 March 2007. [68] OECD Broadband Statistics (http:/ / www. oecd. org/ document/ 39/ 0,2340,en_2649_34225_36459431_1_1_1_1,00. html), Organisation for Economic Co-operation and Development, December 2005. [69] History of the OSI Reference Model (http:/ / www. tcpipguide. com/ free/ t_HistoryoftheOSIReferenceModel. htm), The TCP/IP Guide v3.0, Charles M. Kozierok, 2005. [70] http:/ / 72. 14. 207. 99/ [71] http:/ / www. google. com/ [72] Introduction to IPv6 (http:/ / www. microsoft. com/ technet/ itsolutions/ network/ ipv6/ introipv6. mspx), Microsoft Corporation, February 2006. [73] Stallings, pp 683–702. [74] T. Dierks and C. Allen, The TLS Protocol Version 1.0, RFC 2246, 1999. [75] Martin, Michael (2000). Understanding the Network ( The Networker's Guide to AppleTalk, IPX, and NetBIOS (http:/ / www. informit. com/ content/ images/ 0735709777/ samplechapter/ 0735709777. pdf)), SAMS Publishing, ISBN 0-7357-0977-7. [76] Ralph Droms, Resources for DHCP (http:/ / www. dhcp. org/ ), November 2003. [77] Stallings, pp 500–526. [78] Stallings, pp 514–516. [79] Fiber Optic Cable Tutorial (http:/ / www. arcelect. com/ fibercable. htm), Arc Electronics. Retrieved June, 2007.

Further reading • OECD, Universal Service and Rate Restructuring in Telecommunications (http://books.google.com/ books?id=WpmzcqmgMbAC&dq=universal+service+and+rate+restructuring+in+telecommunications& printsec=frontcover&source=bl&ots=S2USGNAune&sig=Alh7pDRwI3Rk4iYVYuMq9rZlIZc&hl=en& sa=X&oi=book_result&resnum=1&ct=result#PPP1,M1), Organisation for Economic Co-operation and Development (OECD) Publishing, 1991. ISBN 92-64-13497-2

External links • • • • • • •

ATIS Telecom Glossary (http://www.atis.org/tg2k/) Communications Engineering Tutorials (http://www.complextoreal.com/tutorial.htm) Federal Communications Commission (http://www.fcc.gov/) Unified Communications (http://www.radvision.com/Unified-Communications/) IEEE Communications Society (http://www.comsoc.org/) International Telecommunication Union (http://www.itu.int/home/) Ericsson's Understanding Telecommunications (http://web.archive.org/web/20040413074912/www.ericsson. com/support/telecom/index.shtml) at archive.org (Ericsson removed the book from their site in September 2005)

14

Electromagnetic radiation

15

Electromagnetic radiation Electromagnetic radiation (often abbreviated E-M radiation or EMR) is a form of energy exhibiting wave-like behavior as it travels through space. EMR has both electric and magnetic field components, which oscillate in phase perpendicular to each other and perpendicular to the direction of energy propagation. Electromagnetic radiation is classified according to the frequency of its wave. The electromagnetic spectrum, in order of increasing frequency and decreasing wavelength, consists of radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays and gamma rays. The eyes of various organisms sense a small and somewhat variable window of frequencies called the visible spectrum. The photon is the quantum of the electromagnetic interaction and the basic "unit" of light and all other forms of electromagnetic radiation and is also the force carrier for the electromagnetic force. Electromagnetic radiation carries energy and momentum that may be imparted to matter with which it interacts.

Physics Theory James Clerk Maxwell first formally postulated electromagnetic waves. These were subsequently confirmed by Heinrich Hertz. Maxwell derived a wave form of the electric and magnetic equations, thus uncovering the wave-like nature of electric and magnetic fields, and their symmetry. Because the speed of EM waves predicted by the wave equation coincided with the measured speed of light, Maxwell concluded that light itself is an EM wave. According to Maxwell's equations, a spatially varying electric field causes the magnetic field to change over time. Likewise, a spatially varying magnetic field causes changes over time in the electric field. In an electromagnetic wave, the changes induced by the electric field shift the wave in the magnetic field in one direction; the action of the magnetic field shifts the electric field in the same direction. Together, these fields form a propagating electromagnetic wave. A quantum theory of the interaction between electromagnetic radiation and matter such as electrons is described by the theory of quantum electrodynamics.

Properties

Shows the relative wavelengths of the electromagnetic waves of three different colors of light (blue, green and red) with a distance scale in micrometres along the x-axis.

Electromagnetic radiation

The physics of electromagnetic radiation is electrodynamics. Electromagnetism is the physical phenomenon associated with the theory of electrodynamics. Electric and magnetic fields obey the properties of Electromagnetic waves can be imagined as a self-propagating transverse oscillating wave superposition. Thus, a field due to any of electric and magnetic fields. This diagram shows a plane linearly polarized wave propagating from right to left. The electric field is in a vertical plane and the magnetic particular particle or time-varying field in a horizontal plane. electric or magnetic field contributes to the fields present in the same space due to other causes. Further, as they are vector fields, all magnetic and electric field vectors add together according to vector addition. For example, in optics two or more coherent lightwaves may interact and by constructive or destructive interference yield a resultant irradiance deviating from the sum of the component irradiances of the individual lightwaves. Since light is an oscillation it is not affected by travelling through static electric or magnetic fields in a linear medium such as a vacuum. However in nonlinear media, such as some crystals, interactions can occur between light and static electric and magnetic fields — these interactions include the Faraday effect and the Kerr effect. In refraction, a wave crossing from one medium to another of different density alters its speed and direction upon entering the new medium. The ratio of the refractive indices of the media determines the degree of refraction, and is summarized by Snell's law. Light of composite wavelengths (natural sunlight) disperses into a visible spectrum passing through a prism, because of the wavelength dependent refractive index of the prism material (dispersion); that is, each component wave within the composite light is bent a different amount. EM radiation exhibits both wave properties and particle properties at the same time (see wave-particle duality). Both wave and particle characteristics have been confirmed in a large number of experiments. Wave characteristics are more apparent when EM radiation is measured over relatively large timescales and over large distances while particle characteristics are more evident when measuring small timescales and distances. For example, when electromagnetic radiation is absorbed by matter, particle-like properties will be more obvious when the average number of photons in the cube of the relevant wavelength is much smaller than 1. Upon absorption of light, it is not too difficult to experimentally observe non-uniform deposition of energy. Strictly speaking, however, this alone is not evidence of "particulate" behavior of light, rather it reflects the quantum nature of matter.[1] There are experiments in which the wave and particle natures of electromagnetic waves appear in the same experiment, such as the self-interference of a single photon. True single-photon experiments (in a quantum optical sense) can be done today in undergraduate-level labs.[2] When a single photon is sent through an interferometer, it passes through both paths, interfering with itself, as waves do, yet is detected by a photomultiplier or other sensitive detector only once.

Wave model Electromagnetic radiation is a transverse wave meaning that the oscillations of the waves are perpendicular to the direction of energy transfer and travel. An important aspect of the nature of light is frequency. The frequency of a wave is its rate of oscillation and is measured in hertz, the SI unit of frequency, where one hertz is equal to one oscillation per second. Light usually has a spectrum of frequencies which sum together to form the resultant wave. Different frequencies undergo different angles of refraction. A wave consists of successive troughs and crests, and the distance between two adjacent crests or troughs is called the wavelength. Waves of the electromagnetic spectrum vary in size, from very long radio waves the size of buildings to very short gamma rays smaller than atom nuclei. Frequency is inversely proportional to wavelength,

16

Electromagnetic radiation according to the equation:

where v is the speed of the wave (c in a vacuum, or less in other media), f is the frequency and λ is the wavelength. As waves cross boundaries between different media, their speeds change but their frequencies remain constant. Interference is the superposition of two or more waves resulting in a new wave pattern. If the fields have components in the same direction, they constructively interfere, while opposite directions cause destructive interference. The energy in electromagnetic waves is sometimes called radiant energy.

Particle model Because energy of an electromagnetic wave is quantized (see second quantization), electromagnetic energy is emitted and absorbed as discrete packets of energy, or quanta, called photons. The energy of the photons is proportional to the frequency of the wave.[3] Conversely, in a first-quantized treatment, because a photon acts as a transporter of energy, it is associated with a probability wave with frequency proportional to the energy carried. In both treatments, the energy per photon is related to the frequency via the Planck–Einstein equation:[4]

where E is the energy, h is Planck's constant, and f is frequency. The energy is commonly expressed in the unit of electronvolt (eV). This photon-energy expression is a particular case of the energy levels of the more general electromagnetic oscillator, whose average energy, which is used to obtain Planck's radiation law, can be shown to differ sharply from that predicted by the equipartition principle at low temperature, thereby establishes a failure of equipartition due to quantum effects at low temperature.[5] As a photon is absorbed by an atom, it excites the atom, elevating an electron to a higher energy level. If the energy is great enough, so that the electron jumps to a high enough energy level, it may escape the positive pull of the nucleus and be liberated from the atom in a process called photoionisation. Conversely, an electron that descends to a lower energy level in an atom emits a photon of light equal to the energy difference. Since the energy levels of electrons in atoms are discrete, each element emits and absorbs its own characteristic frequencies. Together, these effects explain the emission and absorption spectra of light. The dark bands in the absorption spectrum are due to the atoms in the intervening medium absorbing different frequencies of the light. The composition of the medium through which the light travels determines the nature of the absorption spectrum. For instance, dark bands in the light emitted by a distant star are due to the atoms in the star's atmosphere. These bands correspond to the allowed energy levels in the atoms. A similar phenomenon occurs for emission. As the electrons descend to lower energy levels, a spectrum is emitted that represents the jumps between the energy levels of the electrons. This is manifested in the emission spectrum of nebulae. Today, scientists use this phenomenon to observe what elements a certain star is composed of. It is also used in the determination of the distance of a star, using the red shift.

Speed of propagation Any electric charge which accelerates, or any changing magnetic field, produces electromagnetic radiation. Electromagnetic information about the charge travels at the speed of light. Accurate treatment thus incorporates a concept known as retarded time (as opposed to advanced time, which is not physically possible in light of causality), which adds to the expressions for the electrodynamic electric field and magnetic field. These extra terms are responsible for electromagnetic radiation. When any wire (or other conducting object such as an antenna) conducts alternating current, electromagnetic radiation is propagated at the same frequency as the electric current. At the quantum level, electromagnetic radiation is produced when the wavepacket of a charged particle oscillates or otherwise accelerates. Charged particles in a stationary state do not move, but a superposition of such states may result in oscillation, which is responsible for the phenomenon of radiative transition between quantum states of a

17

Electromagnetic radiation charged particle. Depending on the circumstances, electromagnetic radiation may behave as a wave or as particles. As a wave, it is characterized by a velocity (the speed of light), wavelength, and frequency. When considered as particles, they are known as photons, and each has an energy related to the frequency of the wave given by Planck's relation E = hν, where E is the energy of the photon, h = 6.626 × 10−34 J·s is Planck's constant, and ν is the frequency of the wave. One rule is always obeyed regardless of the circumstances: EM radiation in a vacuum always travels at the speed of light, relative to the observer, regardless of the observer's velocity. (This observation led to Albert Einstein's development of the theory of special relativity.) In a medium (other than vacuum), velocity factor or refractive index are considered, depending on frequency and application. Both of these are ratios of the speed in a medium to speed in a vacuum.

Thermal radiation and electromagnetic radiation as a form of heat The basic structure of matter involves charged particles bound together in many different ways. When electromagnetic radiation is incident on matter, it causes the charged particles to oscillate and gain energy. The ultimate fate of this energy depends on the situation. It could be immediately re-radiated and appear as scattered, reflected, or transmitted radiation. It may also get dissipated into other microscopic motions within the matter, coming to thermal equilibrium and manifesting itself as thermal energy in the material. With a few exceptions such as fluorescence, harmonic generation, photochemical reactions and the photovoltaic effect, absorbed electromagnetic radiation simply deposits its energy by heating the material. This happens both for infrared and non-infrared radiation. Intense radio waves can thermally burn living tissue and can cook food. In addition to infrared lasers, sufficiently intense visible and ultraviolet lasers can also easily set paper afire. Ionizing electromagnetic radiation can create high-speed electrons in a material and break chemical bonds, but after these electrons collide many times with other atoms in the material eventually most of the energy gets downgraded to thermal energy, this whole process happening in a tiny fraction of a second. That infrared radiation is a form of heat and other electromagnetic radiation is not, is a widespread misconception in physics. Any electromagnetic radiation can heat a material when it is absorbed. The inverse or time-reversed process of absorption is responsible for thermal radiation. Much of the thermal energy in matter consists of random motion of charged particles, and this energy can be radiated away from the matter. The resulting radiation may subsequently be absorbed by another piece of matter, with the deposited energy heating the material. Radiation is an important mechanism of heat transfer. The electromagnetic radiation in an opaque cavity at thermal equilibrium is effectively a form of thermal energy, having maximum radiation entropy. The thermodynamic potentials of electromagnetic radiation can be well-defined as for matter. Thermal radiation in a cavity has energy density (see Planck's Law) of

Differentiating the above with respect to temperature, we may say that the electromagnetic radiation field has an effective volumetric heat capacity given by

18

Electromagnetic radiation

19

Electromagnetic spectrum Generally, EM radiation (the designation 'radiation' excludes static electric and magnetic and near fields) is classified by wavelength into radio, microwave, infrared, the visible region we perceive as light, ultraviolet, X-rays and gamma rays. Arbitrary electromagnetic waves can always be expressed by Fourier analysis in terms of sinusoidal monochromatic waves which can be classified into these regions of the spectrum. The behavior of EM radiation depends on its wavelength. Higher frequencies have shorter wavelengths, and lower frequencies have longer wavelengths. When EM radiation interacts with single atoms and molecules, its behavior depends on the amount of energy per quantum it carries. Spectroscopy can detect a much wider region of the EM spectrum than the visible range of 400 nm to 700 nm. A common laboratory spectroscope can detect wavelengths from 2 nm to 2500 nm. Detailed information about the physical properties of objects, gases, or even stars can be obtained from this type of device. It is widely used in astrophysics. For example, hydrogen atoms emit radio waves of wavelength 21.12 cm.

Electromagnetic spectrum with light highlighted

Legend: γ = Gamma rays HX = Hard X-rays SX = Soft X-Rays EUV = Extreme ultraviolet NUV = Near ultravioletVisible light NIR = Near infrared MIR = Moderate infrared FIR = Far infraredRadio waves: EHF = Extremely high frequency (Microwaves) SHF = Super high frequency (Microwaves) UHF = Ultrahigh frequency VHF = Very high frequency HF = High frequency MF = Medium frequency LF = Low frequency VLF = Very low frequency VF = Voice frequency ULF = Ultra low frequency SLF = Super low frequency ELF = Extremely low frequency

Soundwaves are not electromagnetic radiation. At the lower end of the electromagnetic spectrum, about 20 Hz to about 20 kHz, are frequencies that might be considered in the audio range. However, electromagnetic waves cannot be directly perceived by human ears. Sound waves are the oscillating compression of molecules. To be heard, electromagnetic radiation must be converted to air pressure waves, or if the ear is submerged, water pressure waves.

Light

Electromagnetic radiation EM radiation with a wavelength between approximately 400 nm and 700 nm is directly detected by the human eye and perceived as visible light. Other wavelengths, especially nearby infrared (longer than 700 nm) and ultraviolet (shorter than 400 nm) are also sometimes referred to as light, especially when visibility to humans is not relevant. If radiation having a frequency in the visible region of the EM spectrum reflects off of an object, say, a bowl of fruit, and then strikes our eyes, this results in our visual perception of the scene. Our brain's visual system processes the multitude of reflected frequencies into different shades and hues, and through this not-entirely-understood psychophysical phenomenon, most people perceive a bowl of fruit. At most wavelengths, however, the information carried by electromagnetic radiation is not directly detected by human senses. Natural sources produce EM radiation across the spectrum, and our technology can also manipulate a broad range of wavelengths. Optical fiber transmits light which, although not suitable for direct viewing, can carry data that can be translated into sound or an image. To be meaningful both transmitter and receiver must use some agreed-upon encoding system - especially so if the transmission is digital as opposed to the analog nature of the waves.

Radio waves Radio waves can be made to carry information by varying a combination of the amplitude, frequency and phase of the wave within a frequency band. When EM radiation impinges upon a conductor, it couples to the conductor, travels along it, and induces an electric current on the surface of that conductor by exciting the electrons of the conducting material. This effect (the skin effect) is used in antennas. EM radiation may also cause certain molecules to absorb energy and thus to heat up; this is exploited in microwave ovens. Radio waves are not ionizing radiation, as the energy per photon is too small.

Derivation Electromagnetic waves as a general phenomenon were predicted by the classical laws of electricity and magnetism, known as Maxwell's equations. Inspection of Maxwell's equations without sources (charges or currents) results in, along with the possibility of nothing happening, nontrivial solutions of changing electric and magnetic fields. Beginning with Maxwell's equations in free space:

where is a vector differential operator (see Del). One solution,

is trivial. For a more useful solution, we utilize vector identities, which work for any vector, as follows:

To see how we can use this, take the curl of equation (2):

Evaluating the left hand side:

20

Electromagnetic radiation

21

where we simplified the above by using equation (1). Evaluate the right hand side:

Equations (6) and (7) are equal, so this results in a vector-valued differential equation for the electric field, namely

Applying a similar pattern results in similar differential equation for the magnetic field:

These differential equations are equivalent to the wave equation:

where c0 is the speed of the wave in free space and f describes a displacement Or more simply:

where

is d'Alembertian:

Notice that in the case of the electric and magnetic fields, the speed is:

Which, as it turns out, is the speed of light in vacuum. Maxwell's equations have unified the vacuum permittivity , the vacuum permeability , and the speed of light itself, c0. Before this derivation it was not known that there was such a strong relationship between light and electricity and magnetism. But these are only two equations and we started with four, so there is still more information pertaining to these waves hidden within Maxwell's equations. Let's consider a generic vector wave for the electric field.

Here

is the constant amplitude,

propagation, and

is any second differentiable function,

is a position vector. We observe that

is a unit vector in the direction of is a generic solution to the wave

equation. In other words

for a generic wave traveling in the

direction.

This form will satisfy the wave equation, but will it satisfy all of Maxwell's equations, and with what corresponding magnetic field?

Electromagnetic radiation

The first of Maxwell's equations implies that electric field is orthogonal to the direction the wave propagates.

The second of Maxwell's equations yields the magnetic field. The remaining equations will be satisfied by this choice of . Not only are the electric and magnetic field waves traveling at the speed of light, but they have a special restricted orientation and proportional magnitudes, , which can be seen immediately from the Poynting vector. The electric field, magnetic field, and direction of wave propagation are all orthogonal, and the wave propagates in the same direction as . From the viewpoint of an electromagnetic wave traveling forward, the electric field might be oscillating up and down, while the magnetic field oscillates right and left; but this picture can be rotated with the electric field oscillating right and left and the magnetic field oscillating down and up. This is a different solution that is traveling in the same direction. This arbitrariness in the orientation with respect to propagation direction is known as polarization. On a quantum level, it is described as photon polarization. The direction of the polarization is defined as the direction of the electric field. More general forms of the second order wave equations given above are available, allowing for both non-vacuum propagation media and sources. A great many competing derivations exist, all with varying levels of approximation and intended applications. One very general example is a form of the electric field equation,[6] which was factorized into a pair of explicitly directional wave equations, and then efficiently reduced into a single uni-directional wave equation by means of a simple slow-evolution approximation.

References [1] [2] [3] [4]

(http:/ / www. qo. phy. auckland. ac. nz/ talks/ photoelectric. pdf) (http:/ / people. whitman. edu/ ~beckmk/ QM/ grangier/ Thorn_ajp. pdf) Weinberg, S. (1995). The Quantum Theory of Fields. 1. Cambridge University Press. pp. 15–17. ISBN 0-521-55001-7. Paul M. S. Monk (2004). Physical Chemistry (http:/ / books. google. com/ ?id=LupAi35QjhoC& pg=PA435& dq="planck+ einstein+ equation"). John Wiley and Sons. p. 435. ISBN 9780471491804. . [5] Vu-Quoc, L., Configuration integral (statistical mechanics) (http:/ / clesm. mae. ufl. edu/ wiki. pub/ index. php/ Configuration_integral_(statistical_mechanics)), 2008. [6] Kinsler, P. (2010). "Optical pulse propagation with minimal approximations". Phys. Rev. A 81: 013819. arXiv:0810.5689. Bibcode 2010PhRvA..81a3819K. doi:10.1103/PhysRevA.81.013819.

• Hecht, Eugene (2001). Optics (4th ed.). Pearson Education. ISBN 0-8053-8566-5. • Serway, Raymond A.; Jewett, John W. (2004). Physics for Scientists and Engineers (6th ed.). Brooks Cole. ISBN 0-534-40842-7. • Tipler, Paul (2004). Physics for Scientists and Engineers: Electricity, Magnetism, Light, and Elementary Modern Physics (5th ed.). W. H. Freeman. ISBN 0-7167-0810-8. • Reitz, John; Milford, Frederick; Christy, Robert (1992). Foundations of Electromagnetic Theory (4th ed.). Addison Wesley. ISBN 0-201-52624-7. • Jackson, John David (1999). Classical Electrodynamics (3rd ed.). John Wiley & Sons. ISBN 0-471-30932-X. • Allen Taflove and Susan C. Hagness (2005). Computational Electrodynamics: The Finite-Difference Time-Domain Method, 3rd ed. Artech House Publishers. ISBN 1-58053-832-0.

22

Electromagnetic radiation

External links • Electromagnetism (http://www.lightandmatter.com/html_books/0sn/ch11/ch11.html) - a chapter from an online textbook • Electromagnetic Radiation (http://www.cvel.clemson.edu/emc/tutorials/Radiation/EM_Radiation.html) - an introduction for electrical engineers • Electromagnetic Waves from Maxwell's Equations (http://www.physnet.org/modules/pdf_modules/m210. pdf) on Project PHYSNET (http://www.physnet.org). • Radiation of atoms? e-m wave, Polarisation, ... (http://www.hydrogenlab.de/elektronium/HTML/ einleitung_hauptseite_uk.html) • An Introduction to The Wigner Distribution in Geometric Optics (http://scripts.mit.edu/~raskar/lightfields/ index.php?title=An_Introduction_to_The_Wigner_Distribution_in_Geometric_Optics) • The windows of the electromagnetic spectrum, on Astronoo (http://www.astronoo.com/articles/ electromagneticSpectrum-en.html)

Electromagnetic induction Electromagnetic induction is the production of voltage across a conductor moving through a magnetic field. It underlies the operation of generators, transformers, induction motors, all electric motors, synchronous motors, and solenoids. Michael Faraday is generally credited with the discovery of the induction phenomenon in 1831 though it may have been anticipated by the work of Francesco Zantedeschi in 1829. Around 1830[1] to 1832[2] Joseph Henry made a similar discovery, but did not publish his findings until later.

Overview Michael Faraday stated that electromotive force (EMF) produced around a closed path is proportional to the rate of change of the magnetic flux through any surface bounded by that path. In practice, this means that an electric current will be induced in any closed circuit when the magnetic flux through a surface bounded by the conductor changes. This applies whether the field itself changes in strength or the conductor is moved through it. In mathematical form, Faraday's law states that:

where is the electromotive force ΦB is the magnetic flux. For the special case of a coil of wire, composed of N loops with the same area, the equation becomes

A corollary of Faraday's Law, together with Ampère's law and Ohm's law is Lenz's law: The EMF induced in an electric circuit always acts in such a direction that the current it drives around the circuit opposes the change in magnetic flux which produces the EMF.[3]

23

Electromagnetic induction

Applications The principles of electromagnetic induction are applied in many devices and systems, including: • • • • • • • • • • • • • • •

Current clamp Electrical generators Electromagnetic forming Graphics tablet Hall effect meters Induction cookers Induction motors Induction sealing Induction welding Inductors Magnetic flow meters Mechanically powered flashlight Pickups Rowland ring Transcranial magnetic stimulation

• Transformers • Wireless energy transfer

References [1] "Magnets" (http:/ / library. thinkquest. org/ 13526/ c3c. htm). ThinkQuest. . Retrieved 2009-11-06. [2] "Joseph Henry" (http:/ / www. nndb. com/ people/ 671/ 000096383/ ). Notable Names Database. . Retrieved 2009-11-06. [3] Brauer, John R. (2006). Magnetic actuators and sensors (http:/ / books. google. com/ books?id=Wwk1EeZubdUC). John Wiley and Sons. p. 20. ISBN 0-471-73169-2. ., Extract of page 20 (http:/ / books. google. com/ books?id=Wwk1EeZubdUC& pg=PA20)

External links • A free java simulation on motional EMF (http://www.phy.hk/wiki/englishhtm/Induction.htm) • Two videos demonstrating Faraday's and Lenz's laws at EduMation (http://msdaif.googlepages.com/physics)

24

25

Frequency Frequency synthesizer A frequency synthesizer is an electronic system for generating any of a range of frequencies from a single fixed timebase or oscillator. They are found in many modern devices, including radio receivers, mobile telephones, radiotelephones, walkie-talkies, CB radios, satellite receivers, GPS systems, etc. A frequency synthesizer can combine frequency multiplication, frequency division, and frequency mixing (the frequency mixing process generates sum and difference frequencies) operations to produce the desired output signal.

Types Three types of synthesizer can be distinguished. The first and second type are routinely found as stand-alone architecture: Direct Analog Synthesis (also called a mix-filter-divide architecture[1] as found in the 1960s HP 5100A) and by comparison the more modern Direct Digital Synthesizer (DDS) (Table-Look-Up). The third type are routinely used as communication system IC building-blocks: indirect digital (PLL) synthesizers including integer-N and fractional-N.[2]

Digiphase Synthesizer It is in some ways similar to a DDS, but it has architectural differences. One of its big advantages is to allow a much finer resolution than other types of synthesizers with a given reference frequency.[3]

History Although frequency as the inverse of a wave period is a relatively recent idea[4] , the origins of frequency synthesis can be found in the much older concept of angular velocity.[4] The wheel trains of timekeeping devices have gear ratio relationships that were well-studied at least as far back as the time of Christian Huygens, who died in 1695.[4] Prior to widespread use of synthesizers, radio and television receivers relied on manual tuning of a local oscillator, such as with the turret tuner commonly used in television receivers prior to the 1980s. Variations in temperature and aging of components caused frequency drift. Automatic frequency control (AFC) solves some of the drift problem, but manual retuning was often necessary. Since transmitter frequencies are well known and very stable, an accurate means of generating fixed, stable frequencies would solve the problem. A simple and effective solution employs the use of many stable resonators or oscillators, one for each tuning frequency. Quartz crystals offer good stability and are often used for this purpose. This "brute force" technique is practical when only a handful of frequencies are required, but quickly becomes costly and impractical in many applications. For example, the FM radio band in many countries supports 100 individual frequencies from about 88 MHz to 108 MHz. Cable television can support even more frequencies or channels over a much wider band. A large number of crystals increases cost and requires greater space. Many coherent and incoherent techniques have been devised over the years. Some approaches include phase locked loops, double mix, triple mix, harmonic, double mix divide, and direct digital synthesis (DDS). The choice of approach depends on several factors, such as cost, complexity, frequency step size, switching rate, phase noise, and spurious output. Coherent techniques generate frequencies derived from a single, stable master oscillator. In most applications, crystal oscillator are common, but other resonators and frequency sources can be used. Incoherent techniques derive

Frequency synthesizer frequencies from a set of several stable oscillators.[5] The vast majority of synthesizers in commercial applications use coherent techniques due to simplicity and low cost. Synthesizers used in commercial radio receivers are largely based on phase-locked loops or PLLs. Many types of frequency synthesiser are available as integrated circuits, reducing cost and size. High end receivers and electronic test equipment use more sophisticated techniques, often in combination.

System analysis and design A well-thought-out design procedure is considered to be the first significant step to a successful synthesizer project.[6] In the system design of a frequency synthesizer, states Manassewitsch, there are as many "best" design procedures as there are experienced synthesizer designers.[6] System analysis of a frequency synthesizer involves output frequency range (or frequency bandwidth or tuning range), frequency increments (or resolution or frequency tuning), frequency stability (or phase stability, compare spurious outputs), phase noise performance (e.g., spectral purity), switching time (compare settling time and rise time), and size, power consumption, and cost.[7] [8] James A. Crawford says that these are mutually contradictive requirements[8]

Trial-and-error superseded by calculation and control theory The trial and error method was once the work-horse for designers of frequency synthesizers[4] . This began to change with the works of Floyd M. Gardner (his 1966 Phaselock techniques)[9] and Venceslav F. Kroupa (his 1973 Frequency Synthesis)[4] . Manassewitsch calls this the Brute-force approach.[5] Techniques and formulae have been provided by Dean Banerjee[10] .

Gearbox approach Surprisingly sophisticated mathematical techniques analogous to mechanical gear ratio relationships can be employed in frequency synthesis when the frequency synthesis factor is composed of multiplicative integers in the numerator and denominator.[4] This method allows for effective planning of distribution and suppression of spectral spurs.

Modulo-N approach Variable frequency synthesizers including DDS are routinely designed using this method.

Principle of PLL synthesizers See main article: Phase-locked loop A phase locked loop is a feedback control system. It compares the phases of two input signals and produces an error signal that is proportional to the difference between their phases.[11] The error signal is then low pass filtered and used to drive a voltage-controlled oscillator (VCO) which creates an output frequency. The output frequency is fed through a frequency divider back to the input of the system, producing a negative feedback loop. If the output frequency drifts, the phase error signal will increase, driving the frequency in the opposite direction so as to reduce the error. Thus the output is locked to the frequency at the other input. This other input is called the reference and is usually derived from a crystal oscillator, which is very stable in frequency. The block diagram below shows the basic elements and arrangement of a PLL based frequency synthesizer.

26

Frequency synthesizer

The key to the ability of a frequency synthesizer to generate multiple frequencies is the divider placed between the output and the feedback input. This is usually in the form of a digital counter, with the output signal acting as a clock signal. The counter is preset to some initial count value, and counts down at each cycle of the clock signal. When it reaches zero, the counter output changes state and the count value is reloaded. This circuit is straightforward to implement using flip-flops, and because it is digital in nature, is very easy to interface to other digital components or a microprocessor. This allows the frequency output by the synthesizer to be easily controlled by a digital system.

Example Suppose the reference signal is 100 kHz, and the divider can be preset to any value between 1 and 100. The error signal produced by the comparator will only be zero when the output of the divider is also 100 kHz. For this to be the case, the VCO must run at a frequency which is 100 kHz x the divider count value. Thus it will produce an output of 100 kHz for a count of 1, 200 kHz for a count of 2, 1 MHz for a count of 10 and so on. Note that only whole multiples of the reference frequency can be obtained with the simplest integer N dividers. Fractional N dividers are readily available [10] .

Practical considerations In practice this type of frequency synthesiser cannot operate over a very wide range of frequencies, because the comparator will have a limited bandwidth and may suffer from aliasing problems. This would lead to false locking situations, or an inability to lock at all. In addition, it is hard to make a high frequency VCO that operates over a very wide range. This is due to several factors, but the primary restriction is the limited capacitance range of varactor diodes. However, in most systems where a synthesiser is used, we are not after a huge range, but rather a finite number over some defined range, such as a number of radio channels in a specific band. Many radio applications require frequencies that are higher than can be directly input to the digital counter. To overcome this, the entire counter could be constructed using high-speed logic such as ECL, or more commonly, using a fast initial division stage called a prescaler which reduces the frequency to a manageable level. Since the prescaler is part of the overall division ratio, a fixed prescaler can cause problems designing a system with narrow channel spacings - typically encountered in radio applications. This can be overcome using a dual-modulus prescaler.[10] Further practical aspects concern the amount of time the system can switch from channel to channel, time to lock when first switched on, and how much noise there is in the output. All of these are a function of the loop filter of the system, which is a low-pass filter placed between the output of the frequency comparator and the input of the VCO.

27

Frequency synthesizer Usually the output of a frequency comparator is in the form of short error pulses, but the input of the VCO must be a smooth noise-free DC voltage. (Any noise on this signal naturally causes frequency modulation of the VCO.). Heavy filtering will make the VCO slow to respond to changes, causing drift and slow response time, but light filtering will produce noise and other problems with harmonics. Thus the design of the filter is critical to the performance of the system and in fact the main area that a designer will concentrate on when building a synthesiser system.[10]

References [1] Popiel-Gorski (1975, p. 25) [2] Egan (2000, pp. 14–27) [3] Egan (2000, pp. 372–376) [4] Kroupa (1999, p. 3) [5] Manassewitsch (1987, p. 7) [6] Manassewitsch (1987, p. 151) [7] Manassewitsch (1987, p. 51) [8] Crawford (1994, p. 4) [9] Gardner (1966) [10] Banerjee (2006) [11] Phase is the integral of frequency. Controling the phase will also control the frequency.

• Banerjee, Dean (2006), PLL Performance, Simulation and Design Handbook (http://www.national.com/ analog/timing/pll_designbook) (4th ed.), National Semiconductor. Also PDF version (http://www.national. com/appinfo/wireless/files/deansbook4.pdf). • Crawford, James A. (1994), Frequency Synthesizer Design Handbook, Artech House, ISBN 0-89006-440-7 • Egan, William F. (2000), Frequency Synthesis by Phase-lock (2nd ed.), John Wiley & Sons, ISBN 0-471-32104-4 • Gardner, Floyd M. (1966), Phaselock Techniques, John Wiley and Sons • Kroupa, Venceslav F. (1999), Direct Digital Frequency Synthesizers, IEEE Press, ISBN 0-7803-3438-8 • Kroupa, Venceslav F. (1973), Frequency Synthesis: Theory, Design & Applications (http://books.google.com/ books?id=hipTAAAAMAAJ&pgis=1), Griffin, ISBN 0-4705-0855-8 • Manassewitsch, Vadim (1987), Frequency Synthesizers: Theory and Design (3rd ed.), John Wiley & Sons, ISBN 0-471-01116-9 • Popiel-Gorski, Jerzy (1975), Frequency Synthesis: Techniques and Applications, IEEE Press, ISBN 0-87942-039-1

Further reading • Ulrich L. Rohde "Digital PLL Frequency Synthesizers - Theory and Design ", Prentice-Hall, Inc., Englewood Cliffs, NJ, January 1983 • Ulrich L. Rohde " Microwave and Wireless Synthesizers: Theory and Design ", John Wiley & Sons, August 1997, ISBN 0-471-52019-5

External links • Hewlett-Packard 5100A (http://www.hpmemory.org/news/5100/hp5100_page_00.htm) (tunable, 0.01 Hz-resolution Direct Frequency Synthesizer introduced in 1964; to HP, direct synthesis meant PLL not used, while indirect meant a PLL was used) • FREQUENCY SYNTHESIZER (http://www.google.com/patents?id=2oFzAAAAEBAJ) U.S. Patent 3,555,446, Braymer, N. B., (1971, January 12) [1] [1] Egan, 2000, p. 372, 570

28

Frequency mixer

Frequency mixer In electronics a mixer or frequency mixer is a nonlinear electrical circuit that creates new frequencies from two signals applied to it. In its most common application, two signals at frequencies f1 and f2 are applied to a mixer, and it produces new signals at the sum f1 + f2 and difference f1 - f2 of the original frequencies. Other frequency components may also be produced in a practical frequency mixer. Mixers are widely used to shift signals from one frequency range to another, a Frequency Mixer Symbol. process known as heterodyning, for convenience in transmission or further signal processing. For example, a key component of a superheterodyne receiver is a mixer used to move received signals to a common intermediate frequency. Frequency mixers are also used to modulate a carrier frequency in radio transmitters.

Types Passive mixers use one or more diodes and rely on the non-linear relation between voltage and current to provide the multiplying element. In a passive mixer, the desired output signal is always of lower power than the input signals. Active mixers can increase the strength of the product signal. Active mixers improve isolation between the ports, but may have higher noise and more power consumption; an active mixer can be less tolerant of overload. Mixers may be built of discrete components, may be part of integrated circuits, or can be delivered as hybrid modules. Mixers may also be classified by their topology. Unbalanced mixers allow some of the input signal power to pass through to the output. A single-balanced mixer is arranged so that the local oscillator, (or RF) signal port, cancels and cannot pass through to the output. A doubly balanced mixer has symmetrical paths for both inputs, and will have no output if either input signal is not present. Selection of a mixer type is a trade off for a particular application. Mixer circuits are characterized by conversion gain, and noise figure.[1] Balanced and double-balanced designs allow less of the input signals to feed through to the output. Nonlinear electronic components that are used as mixers include Schematic diagram of a double-balanced passive diodes, transistors biased near cutoff, and at lower frequencies, analog diode mixer. There is no output unless both f1 and f2 inputs are present. multipliers. Ferromagnetic-core inductors driven into saturation have also been used. In nonlinear optics, crystals with nonlinear characteristics are used to mix two frequencies of laser light to create optical heterodynes.

29

Frequency mixer

30

Diode A diode can be used to create a simple unbalanced mixer. This type of mixer produces the original frequencies as well as their sum and their difference. The importance of the diode is that it is non-linear (or non-Ohmic), which means its response (current) is not proportional to its input (voltage). The diode therefore does not reproduce the frequencies of its driving voltage in the current through it, which allows the desired frequency manipulation. Certain other non-linear devices such as tunnel diodes or Gunn diodes can be utilized similarly. The current I through an ideal diode as a function of the voltage V across it is given by

where what is important is that V appears in e's exponent. The exponential can be expanded as

and can be approximated for small x (that is, small voltages) by the first few terms of that series:

Suppose that the sum of the two input signals

is applied to a diode, and that an output voltage is generated

that is proportional to the current through the diode (perhaps by providing the voltage that is present across a resistor in series with the diode). Then, disregarding the constants in the diode equation, the output voltage will have the form

The first term on the right is the original two signals, as expected, followed by the square of the sum, which can be rewritten as , where the multiplied signal is obvious. The ellipsis represents all the higher powers of the sum which we assume to be negligible for small signals.

Switching Another form of mixer operates by switching, with the smaller input signal being passed inverted or uninverted according to the phase of the local oscillator (LO). This would be typical of the normal operating mode of a packaged double balanced mixer module such as an SBL-1, with the local oscillator drive considerably higher than the signal amplitude. The aim of a switching mixer is to achieve linear operation over the signal level, and hard switching driven by the local oscillator. Mathematically the switching mixer is not much different from a multiplying mixer, just because instead of the LO sine wave term we would use the signum function. In the frequency domain the switching mixer operation leads to the usual sum and difference frequencies, but also to further terms e.g. +-3*fLO, +-5*fLO, etc. The advantage of a switching mixer is that it can achieve - with the same effort - a lower noise figure (NF) and larger conversion gain. This come because the switching diodes or transistors act either like a low resistor (switch closed) or large resistor (switch open) and in both cases only minimum noise is added. From the circuit perspective many multiplying mixers can be used as switching mixers, just by increasing the LO amplitude. So RF engineers simply talk about mixers, and mean switching mixers.

Frequency mixer

Applications The mixer circuit can be used not only to shift the frequency of an input signal as in a receiver, but also as a product detector, modulator, phase detector or frequency multiplier.[2] For example a communications receiver might contain two mixer stages for conversion of the input signal to an intermediate frequency, and another mixer employed as a detector for demodulation of the signal.

References [1] D.S. Evans, G. R. Jessop, VHF-UHF Manual Third Edition, [[Radio Society of Great Britain, 1976, page 4-12 [2] Paul Horowitz, Winfred Hill The Art of Electronics Second Edition, Cambridge University Press 1989, pp. 885-887

External links  This article incorporates public domain material from websites or documents of the General Services Administration.

31

Very high frequency

32

Very high frequency Very high frequency Frequency range

30 to 300 MHz

Wavelength range 1 to 10 m

ITU Radio Band Numbers

1 2 3 4 5 6 7 8 9 10 11 ITU Radio Band Symbols

ELF SLF ULF VLF LF MF HF VHF UHF SHF EHF NATO Radio bands

ABCDEFGHIJKLM IEEE Radar bands

HF VHF UHF L S C X Ku K Ka Q V W

Very high frequency (VHF) is the radio frequency range from 30 MHz to 300 MHz. Frequencies immediately below VHF are denoted High frequency (HF), and the next higher frequencies are known as Ultra high frequency (UHF). The frequency allocation is done by ITU. These names referring to high-end frequency usage originate from mid-20th century, when regular radio service used MF, Medium Frequencies, better known as "AM" in USA, below the HF. Currently VHF is at the low-end of practical frequency usage, new systems tending to use frequencies in SHF and EHF above the UHF range. See Radio spectrum for full picture. Common uses for VHF are FM radio broadcast, television broadcast, land mobile stations (emergency, business, and military), long range data communication with radio modems, Amateur Radio, marine communications, air traffic control communications and air navigation systems (e.g. VOR, DME & ILS).

Propagation characteristics VHF propagation characteristics are ideal for short-distance terrestrial communication, with a range generally somewhat farther than line-of-sight from the transmitter (see formula below). Unlike high frequencies (HF), the ionosphere does not usually reflect VHF radio and thus transmissions are restricted to the local area (and don't interfere with transmissions thousands of kilometres away). VHF is also less affected by atmospheric noise and interference from electrical equipment than lower frequencies. Whilst it is more easily blocked by land features than HF and lower frequencies, it is less affected by buildings and other less substantial objects than UHF frequencies. Two unusual propagation conditions can allow much farther range than normal. The first, tropospheric ducting, can occur in front of and parallel to an advancing cold weather front, especially if there is a marked difference in humidities between the cold and warm air masses. A duct can form approximately 250 km (155 mi) in advance of the cold front, much like a ventilation duct in a building, and VHF radio frequencies can travel along inside the duct, bending or refracting, for hundreds of kilometers. For example, a 50 watt Amateur FM transmitter at 146 MHz can talk from Chicago, to Joplin, Missouri, directly, and to Austin, Texas, through a repeater. In a July 2006 incident, a NOAA Weather Radio transmitter in north central Wisconsin was blocking out local transmitters in west central Michigan, quite far out of its normal range. In midsummer 2006, central Iowa stations were heard in Columbus, Nebraska and blocked out Omaha radio and TV stations for several days, while WBNX-TV in Akron, Ohio, a

Very high frequency

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television station on Channel 55 in the analog age, was noted for bleeding over other Channel 55 stations in Wausau and Kenosha, Wisconsin as far west as the Wisconsin River valley for hours at a time. Similar propagation effects can affect land-mobile stations in this band, rarely causing interference well beyond the usual coverage area. The second type, much more rare, is called Sporadic E, referring to the E-layer of the ionosphere. Phenomena still not completely understood (as of 2010) may allow the formation of ionized "patches" in the ionosphere, dense enough to reflect back VHF frequencies the same way HF frequencies are usually reflected (skywave). For example, KMID (TV Channel 2; 54–60 MHz) from Midland, Texas was seen around Chicago, pushing out Chicago's WBBM-TV. These patches may last for seconds, or extend into hours. FM stations from Miami, Florida; New Orleans, Louisiana; Houston, Texas and even Mexico were heard for hours in central Illinois during one such event.

Line-of-sight calculation For analog TV, VHF transmission range is a function of transmitter power, receiver sensitivity, and distance to the horizon, since VHF signals propagate under normal conditions as a near line-of-sight phenomenon. The distance to the radio horizon is slightly extended over the geometric line of sight to the horizon, as radio waves are weakly bent back toward the Earth by the atmosphere. An approximation to calculate the line-of-sight horizon distance (on Earth) is: • distance in miles =

where

• distance in kilometres =

is the height of the antenna in feet where

is the height of the antenna in metres.

These approximations are only valid for antennas at heights that are small compared to the radius of the Earth. They may not necessarily be accurate in mountainous areas, since the landscape may not be transparent enough for radio waves. In engineered communications systems, more complex calculations are required to assess the probable coverage area of a proposed transmitter station. The accuracy of these calculations for digital TV signals is being debated.[1]

Universal use Certain subparts of the VHF band have the same use around the world. Some national uses are detailed below. • 108–118 MHz: Air navigation beacons VOR and Instrument Landing System localiser. • 118–137 MHz: Airband for air traffic control, AM, 121.5 MHz is emergency frequency

By country

A plan showing VHF use in television, FM radio, amateur radio, marine radio and aviation.

Very high frequency

Australia The VHF TV band in Australia was originally allocated channels 1 to 10 - with channels 2, 7 and 9 assigned for the initial services in Sydney and Melbourne, and later the same channels were assigned in Brisbane, Adelaide and Perth. Other capital cities and regional areas used a combination of these and other frequencies as available. For some strange reason, the initial commercial services in Hobart and Darwin were respectively allocated channels 6 and 8 rather than 7 or 9. By the early 1960s it became apparent that the 10 VHF channels were insufficient to support the growth of television services. This was rectified by the addition of three additional frequencies - channels 0, 5A and 11. Older television sets using rotary dial tuners required adjustment to receive the new channels. Several TV stations were allocated to VHF channels 3, 4 and 5A, which were within the FM radio bands although not yet used for that purpose. A couple of notable examples were NBN Newcastle, WIN-4 Wollongong and ABC Illawarra on channel 5A. Most TVs of that era were not equipped to receive these broadcasts, and so were modified at the owners' expense to be able to tune into these bands; otherwise the owner had to buy a new TV. Beginning in the 1990s, the Australian Broadcasting Authority began a process to move these stations to UHF bands to free up valuable VHF spectrum for its original purpose of FM radio. In addition, by 1985 the federal government decided new TV stations are to be broadcast on the UHF band. Two new VHF frequencies, 9A and 12, have since been made available and are being used primarily for digital services (e.g. ABC in capital cities) but also for some new analogue services in regional areas. Because channel 9A is not used for television services in or near Sydney, Melbourne, Brisbane, Adelaide or Perth, digital radio in those cities are broadcast on DAB frequencies blocks 9A, 9B and 9C.

New Zealand • 44–51, 54–68 MHz: Band I Television (channels 1–3) • 87.5–108 MHz: Band II Radio • 174–230 MHz: Band III Television (channels 4–11) In New Zealand, the four main Free-to-Air TV stations still use the VHF Television bands (Band I and Band III) to transmit their programmes to New Zealand households. Other stations, including a variety of pay and regional free-to-air stations, are forced to broadcast their programmes in the UHF band, since the VHF band is very overloaded with four stations sharing a very small frequency band, which can be so overcrowded that one or more channels, more often than not one of the MediaWorks-owned channels (TV3 and FOUR), is unavailable in some smaller towns.

United Kingdom British television originally used VHF band I and band III. Television on VHF was in black and white with 405-line format (although there were experiments with all three colour systems—NTSC, PAL, and SECAM—adapted for the 405-line system in the late 1950s and early 60s). British colour television was broadcast on UHF (channels 21–69), beginning in the late 1960s. From then on, TV was broadcast on both VHF and UHF (VHF being a monochromatic downconversion from the 625-line colour signal), with the exception of BBC2 (which had always broadcast solely on UHF). The last British VHF TV transmitters closed down on January 3, 1985. VHF band III is now used in the UK for digital audio broadcasting, and VHF band II is used for FM radio, as it is in most of the world. Unusually, the UK has an amateur radio allocation at 4 metres, 70-70.5 MHz.

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Very high frequency

United States and Canada Frequency assignments between US and Canadian users are closely coordinated since much of the Canadian population is within VHF radio range of the US border. Certain discrete frequencies are reserved for radio astronomy. The general services in the VHF band are: • 30–46 MHz: Licensed 2-way land mobile communication.[2] • 30–88 MHz: Military VHF-FM, including SINCGARS • 43–50 MHz: Cordless telephones, 49 MHz FM walkie-talkies and radio controlled toys, and mixed 2-way mobile communication. The FM broadcast band originally operated here (42-50 MHz) before moving to 88-108 MHz. • 50–54 MHz: Amateur radio 6 meter band • 54-72 and 76-88 MHz TV channels 2 through 6 (VHF-Lo), known as "Band I" internationally; some DTV stations will appear here. See North American broadcast television frequencies • 72–76 MHz: Radio controlled models, industrial remote control, and other devices. Model aircraft operate on 72 MHz while surface models operate on 75 MHz in the USA and Canada, air navigation beacons 74.8-75.2 MHz. • 88–108 MHz: FM radio broadcasting (88–92 non-commercial, 92–108 commercial in the United States) (known as "Band II" internationally) • 108–118 MHz: Air navigation beacons VOR • • • • • • • • • •

• • • •

118–137 MHz: Airband for air traffic control, AM, 121.5 MHz is emergency frequency 137-138 Space research, space operations, meteorological satellite [3] 138–144 MHz: Land mobile, auxiliary civil services, satellite, space research, and other miscellaneous services 144–148 MHz: Amateur radio 2 Meters band 148-150 Land mobile, fixed, satellite 150–156 MHz: "VHF business band," the unlicensed Multi-Use Radio Service (MURS), and other 2-way land mobile, FM 156–158 MHz VHF Marine Radio; narrow band FM, 156.8 MHz (Channel 16) is the maritime emergency and contact frequency. 160-161 MHz Railways [4] 162.40–162.55: NOAA Weather Stations, narrowband FM 174-216 MHz television channels 7 - 13 (VHF-Hi), known as "Band III" internationally. A number of DTV channels have begun broadcasting here, especially many of the stations which were assigned to these channels for previous analog operation. 174–216 MHz: professional wireless microphones (low power, certain exact frequencies only) 216–222 MHz: land mobile, fixed, maritime mobile,[5] 222–225 MHz: 1.25 meters (US) (Canada 219-220, 222-225 MHz) amateur radio 225 MHz and above: Military aircraft radio (225–400 MHz) AM, including HAVE QUICK, dGPS RTCM-104

VHF television It is considered that one of the most significant events in the history of broadcast television regulation was the creation of an artificial scarcity of VHF licenses. The FCC's decision to locate television service on the limited VHF band changed the ways of television service and network competition in the industry. The rationale of this policy was to create a situation of increased competition and viewer choice. Television was added to the VHF band in 1941 on channels one through six. During the war freeze, channel one was removed and used only for war purposes. Later, in 1945, channels seven through thirteen were added.[6] The large technically and commercially valuable slice of the VHF spectrum taken up by television broadcasting has attracted the attention of many companies and governments recently, with the development of more efficient digital television broadcasting standards. In some countries much of this spectrum will likely become available (probably for sale) within the next decade or so (June 12, 2009, in the United States).

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Very high frequency 87.5-87.9 MHz 87.5-87.9 MHz is a radio frequency which, in most of the world, is used for FM broadcasting. In North America, however, this bandwidth is allocated to VHF television channel 6 (82-88 MHz). The audio for TV channel 6 is broadcast at 87.75 MHz (adjustable down to 87.74). Several stations, most notably those joining the Pulse 87 franchise, operate on this frequency as radio stations, though they use television licenses. As a result, FM radio receivers such as those found in automobiles which are designed to tune into this frequency range can receive the audio for programming on the local TV channel 6 while in North America. 87.9 MHz is normally off-limits for FM audio broadcasting except for displaced class D stations which have no other frequencies in the normal 88.1-107.9 MHz subband on which to move. So far, only 2 stations have qualified to operate on 87.9 MHz: 10-watt KSFH in Mountain View, California and 34-watt translator K200AA in Sun Valley, Nevada.

Unlicensed operation In some countries, particularly the United States and Canada, limited low-power license-free operation is available in the FM broadcast band for purposes such as micro-broadcasting and sending output from CD or digital media players to radios without auxiliary-in jacks, though this is illegal in some other countries. This practice was legalised in the United Kingdom on 8 December 2006.[7]

References [1] Grotticelli, Michael (2009-06-22). "DTV Transition Not So Smooth in Some Markets" (http:/ / broadcastengineering. com/ news/ dtv-transition-not-smooth-markets-0622/ ). Broadcast Engineering. . Retrieved 2009-06-24. [2] The 42 MHz Segment is still currently used by the California Highway Patrol, New Jersey State Police, Tennessee Highway Patrol and other state law enforcement agencies. [3] Industry Canada, Canadian Table of Frequency Allocations 9 kHz - 275 GHz, 2005 Edition (revised February 2007) pg. 29 [4] The 160 and 161 areas are AAR 99 channel railroad radios issued to the railroad (Sample, AAR 21 is 160.425 and that is issued to TVRM and other railroads that want AAR 21) [5] Canadian table pg. 30 [6] William Boddy, Fifties Television: The Industry and Its Critics, University of Illinois Press, 1992, ISBN 9780252062995 [7] http:/ / media. ofcom. org. uk/ 2006/ 11/ 23/ change-to-the-law-to-allow-the-use-of-low-power-fm-transmitters-for-mp3-players/

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Ultra high frequency

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Ultra high frequency Ultra high frequency Frequency range 0.3 to 3 GHz

ITU Radio Band Numbers

1 2 3 4 5 6 7 8 9 10 11 ITU Radio Band Symbols

ELF SLF ULF VLF LF MF HF VHF UHF SHF EHF NATO Radio bands

ABCDEFGHIJKLM IEEE Radar bands

HF VHF UHF L S C X Ku K Ka Q V W

Ultra High Frequency (UHF) designates the ITU Radio frequency range of electromagnetic waves between 300 MHz and 3 GHz (3,000 MHz), also known as the decimetre band or decimetre wave as the wavelengths range from one to ten decimetres (10 cm to 1 metre). Radio waves with frequencies above the UHF band fall into the SHF (super high frequency) and EHF (extremely high frequency) bands, all of which fall into the microwave frequency range. Lower frequency signals fall into the VHF (very high frequency) or lower bands. See Electromagnetic spectrum and Radio spectrum for a full listing of frequency bands.

Characteristics, advantages, and disadvantages The point to point transmission and reception of TV and radio signals is affected by many variables. Atmospheric moisture; solar wind; physical obstructions, such as mountains and buildings; and time of day all affect the signal transmission and the degradation of signal reception. All radio waves are partly absorbed by atmospheric moisture. Atmospheric absorption reduces, or attenuates, the strength of radio signals over long distances. The effects of attenuation degradation increases with frequency. UHF TV signals are generally more degraded by moisture than lower bands, such as VHF TV signals. The ionosphere, a layer of the Earth's atmosphere, is filled with charged particles that can reflect some radio waves. Amateur radio enthusiasts primarily use this quality of the ionosphere to help propagate lower frequency HF signals around the world: the waves are trapped, bouncing around in the upper layers of the ionosphere until they are refracted down at another point on the Earth. This is called skywave transmission. UHF TV signals are not carried along the ionosphere but can be reflected off of the charged particles down at another point on Earth in order to reach farther than the typical line-of-sight transmission distances; this is the skip distance. UHF transmission and reception are enhanced or degraded by tropospheric ducting as the atmosphere warms and cools throughout the day. The main advantage of UHF transmission is the physically short wave that is produced by the high frequency. The size of transmission and reception antennas is related to the size of the radio wave. The UHF antenna is stubby and short. Smaller and less conspicuous antennas can be used with higher frequency bands. The major disadvantage of UHF is its limited broadcast range and reception, often called line-of-sight between the TV station's transmission antenna and customer's reception antenna, as opposed to VHF's very long broadcast range and reception, which is less restricted by line of sight. UHF is widely used in two-way radio systems and cordless telephones, whose transmission and reception antennas are closely spaced. UHF signals travel over line-of-sight distances. Transmissions generated by two-way radios and

Ultra high frequency cordless telephones do not travel far enough to interfere with local transmissions. Several public-safety and business communications are handled on UHF. Civilian applications, such as GMRS, PMR446, UHF CB, 802.11b ("WiFi") and the widely adapted GSM and UMTS cellular networks, also use UHF frequencies. A repeater propagates UHF signals when a distance greater than the line of sight is required. • See "Radio horizon".

History Australia In Australia, UHF was first anticipated in the mid 1970s with TV channels 27-69. The first UHF TV broadcasts in Australia were operated by Special Broadcasting Service (SBS) on channel 28 in Sydney and Melbourne starting in 1980, and translator stations for the Australian Broadcasting Corporation (ABC). The UHF band is now used extensively as ABC, SBS, commercial and public-access television services have expanded, particularly through regional areas. Australia also provides the UHF CB service for general-purpose two-way communications.

Canada The first Canadian television network was publicly owned Radio-Canada, the Canadian Broadcasting Corporation. Its stations, as well as that of the first private networks (CTV and TVA, created in 1961), are primarily VHF. More recent third-network operators initially signing-on in the 1970s or 1980s were often relegated to UHF, or (if they were to attempt to deploy on VHF) to reduced power or stations in outlying areas. Canada's VHF spectrum was already crowded with both domestic broadcasts and numerous foreign border stations. The use of UHF to provide programming which otherwise would not be available, such as province-wide educational services (Knowledge Channel, TVOntario, Télé-Québec), French language programming (outside Québec) and ethnic/multilingual television, has therefore become common. Third networks such as Quatre-Saisons or Global often will rely heavily on UHF stations as repeaters or as a local presence in large cities where VHF spectrum is largely already full. The handful of digital terrestrial television stations currently on-air in Canada as of 2008 are also all UHF broadcasts, although some digital broadcasts will return to VHF channels vacated after the digital transition is completed in August 2011.[1] Digital Audio Broadcasting, deployed on a very limited scale in Canada in 2005, uses UHF frequencies in the L band from 1452 to 1492 MHz. There are currently no VHF Band III digital radio stations in Canada as, unlike in much of Europe, these frequencies are among the most popular for use by television stations.[2]

Ireland In the Republic of Ireland, UHF was introduced in 1978 to augment the existing RTÉ One VHF 625-line transmissions and to provide extra frequencies for the new RTÉ Two channel. The first UHF transmitter site was Cairn Hill in Co. Longford, followed by Three Rock Mountain in South Co. Dublin. These sites were followed by Clermont Carn in Co. Louth and Holywell Hill in Co. Donegal in 1981. Elsewhere in Ireland, both the RTÉ channels are available on VHF. Since then RTÉ have migrated nearly all their low-power relay sites to UHF. TV3 and TG4 are transmitted entirely in UHF only. When Digital Terrestrial TV is introduced, it is intended to broadcast this on UHF only initially, although VHF allocations exist. VHF TV is likely to cease whenever the existing analogue broadcasts are switched off. The UHF band is also used in parts of Ireland for Television deflector systems bringing British television signals to towns and rural areas which cannot receive these signals directly

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Ultra high frequency

Japan In Japan, an Independent UHF Station (ja:全国独立UHF放送協議会 Zenkoku Dokuritsu Yū-eichi-efu Hōsō Kyōgi-kai, literally National Independent UHF Broadcasting Forum) is one of a loosely knit group of free commercial terrestrial television stations which is not a member of the major national networks keyed in Tokyo and Osaka. Japan's original broadcasters were VHF. Although some experimental broadcasts were made as early as 1939, NHK (founded in 1926 as a radio network modeled on the BBC) began regular VHF television broadcasting in 1953. Its two terrestrial television services (NHK General TV and NHK Educational TV) appear on VHF 1 and 3 respectively in the Tokyo region. Privately owned Japanese VHF TV stations were most often built by large national newspapers with Tokyo stations exerting a large degree of control over national programming. The independent stations broadcast in analogue UHF, unlike major networks which were historically primarily broadcast in analogue VHF. The loose coalition of UHF independents is operated mostly by local governments or metropolitan newspapers with less outside control. Compared with major network stations, Japan's UHF independents have more restrictive programming acquisition budgets and lower average ratings; they are also more likely to broadcast single episode or short-series UHF anime (many of which serve to promote DVD's or other product tie-ins) and brokered programming such as religion and infomercials. Japanese terrestrial television is in the process of switching entirely to digital UHF, with all analogue television (both VHF and UHF) planned to shut down in 2011.

Malaysia UHF broadcasting was used outside Kuala Lumpur and the Klang Valley by private TV station TV3 in the late 80s, with the government stations only transmitting in VHF (Bands 1 and 3) and the 450 MHz range being occupied by the ATUR cellular phone service operated by Telekom Malaysia. The ATUR service ceased operation in the late 90s, freeing up the frequency for other uses. UHF was not commonly used in the Klang Valley until 1994 (despite TV3's signal also being available over UHF Channel 29, as TV3 transmitted over VHF Channel 12 in the Klang Valley). 1994 saw the introduction of the channel MetroVision (which ceased transmission in 1999, got bought over by TV3's parent company - System Televisyen Malaysia Berhad - and relaunched as 8TV in 2004). This was followed by Ntv7 in 1998 (also acquired by TV3's parent company in 2005) and recently Channel 9 (which started in 2003, ceased transmission in 2005, was also acquired by TV3's parent company shortly after, and came back as TV9 in early 2006). At current count, there are 4 distinct UHF signals receivable by an analog TV set in the Klang Valley: Channel 25 (8TV), Channel 29 (TV3 UHF transmission), Channel 37 (NTV7) and Channel 39 (TV9). Channel 39 is usually allocated for VCRs, decoder units (i.e. the ASTRO and MiTV set top boxes) and other devices that have an RF signal generator (i.e. game consoles).

United Kingdom In the UK, UHF television began in 1964 following a plan by the General Post Office to allocate sets of frequencies for 625-lined television to regions across the country, so as to accommodate four national networks with regional variations (the VHF allocations allowed for only two such networks using 405 lines). The UK UHF channels would range from 21 to 68 (later extended to 69) and regional allocations were generally grouped close together to allow for the use of aerials designed to receive a specific sub-band with greater efficiency than wider-band aerials could. Aerial manufacturers would therefore divide the band into over-lapping groups; A (channels 21-34), B (39-53), C/D (48-68) and E (39-68). The first service to use UHF was BBC2 in 1964 followed by BBC1 and ITV (already broadcast on VHF) in 1969 and Channel 4/S4C in 1982. PAL colour was introduced on UHF only in 1967 (for BBC2) and 1969 (for BBC1 & ITV). As a consequence of achieving maximum national coverage, signals from one region would typically over-lap with that of another, which was accommodated for by allocating a different set of channels in each adjacent area, often

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Ultra high frequency resulting in greater choice for viewers when a network in one region aired different programmes to the neighbouring region. Initial uptake of UHF television was very slow: Differing propagation characteristics between VHF and UHF meant new additional transmitters needed to be built, often at different locations to the then-established VHF sites, and generally with a larger number of relay stations to fill the greater number of gaps in coverage that came with the new band. This led to poor picture quality in bad coverage areas, and many years before the service achieved full national coverage. In addition to this, the only exclusively UHF service, BBC2, would run for only a few hours a day and run alternative programming for minority audiences in contrast to the more populist schedules of BBC1 and ITV. However the 1970s saw a large increase in UHF TV viewing while VHF took a significant decline: The appeal of colour, which was never introduced to VHF (despite preliminary plans to do so in the late 1950s and early 1960s) and the fall in television prices saw most households use a UHF set by the end of that decade. With the second and last VHF television service having launched in 1955, VHF TV was finally decommissioned for good in 1985 with no plans for it to return to use. The launch of Channel 5 in 1997 added a fifth national television network to UHF, requiring deviation from the original frequency allocation plan of the early 1960s and the allocation of UHF frequencies previously not used for television (such as UK Channels 35 and 37, previously reserved for RF modulators in devices such as domestic videocassette recorders, requiring an expensive VCR re-tuning programme funded by the new network). A lack of capacity within the band to accommodate a fifth service with the complex over-lapping led to the fifth and final network having a significantly reduced national coverage compared to the other networks, with reduced picture quality in many areas and the use of wide-band aerials often required. The launch of digital terrestrial television in 1998 saw the continued use of UHF for television, with six multiplexes allocated for the service, all within the UHF band. However analogue transmissions have been planned to cease completely by 2012 after which time it is uncertain as to whether the vacated capacity will be used for additional digital television services or put into alternative use, such as mobile telecommunications or internet services.

United States Television On December 29, 1949, KC2XAK of Bridgeport, Connecticut, became the first UHF television station to operate on a regular daily schedule. The first commercially licensed UHF television station was WWLP in Springfield, Massachusetts; [3] however, the first commercially licensed TV station on the air was KPTV, Channel 27, in Portland, Oregon, on September 18, 1952. This TV station used much of the equipment, including the transmitter, from KC2XAK. American television broadcasting, which began experimentally in the 1930s with some regular commercial broadcasting in just a few cities (such as New York and Chicago) in 1941, was originally allocated (by the Federal Communication Commission - the FCC) broadcasting channels solely in the VHF (Very High Frequency) band. All VHF TV channels except channel 1 through 13 had been removed from the FCC allocation list during World War II and those frequencies re-allocated for military use, leaving thirteen channels as of May 1945.[4] While efforts at TV broadcasting on any channel were drastically curtailed for the duration of WW II, largely due to lack of available receivers, the post-war era would bring rapid expansion in the nascent broadcast television industry. After VHF Channel 1 was re-allocated to land-mobile radio systems in 1948 due to radio-interference problems, a mere one dozen TV channels remained. These were found to be not enough to serve the needs of television broadcasting as it grew nationwide during the latter 1940s and the 1950s. For example, these cities were never able to be allocated any VHF-TV stations at all, due to technical reasons found by the FCC: Fort Wayne, Indiana, Lexington, Kentucky, Huntsville, Alabama, and Fresno, California. In addition, scores more cities were able to receive only one VHF broadcast station, for example High Point, North Carolina, Montgomery, Alabama,

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Ultra high frequency Wilmington, Delaware, Bakersfield, California, and Santa Barbara, California. Also, the entire state of New Jersey would receive only one VHF broadcast station of its own (which was to ultimately become WNET 13 Newark), leaving much of the state to be served from New York City or Philadelphia, and Delaware has had only one VHF station. Clearly, there was a problem with an insufficient number of TV channels being available to cover all of the United States. With a grand total of 106 VHF stations broadcasting by the end of the 1940s in the U.S., problems with interference between stations due to some overcrowding of stations were already becoming apparent in the densely populated areas, such as the eastern mid-Atlantic states (New York, Pennsylvania, New Jersey, Delaware, Maryland, and Connecticut) and Southern California. In 1949, the Federal Communications Commission stopped accepting applications for licensing new stations (a freeze that lasted until 1952) in order to address questions such as the allocation of additional channel frequencies, and also the selection of an electronic system for color television. Allocating more of the VHF band (30 to 300 MHz) by moving existing radio communication users off this band seemed to be impossible. For example, FM radio broadcasting had already suffered a huge setback after a forced move from its original 42-50 MHz allocated band to the current 88-108 MHz band in 1946[5] rendered all existing FM transmitters and receivers obsolete. Furthermore, several other important radio communications services use the VHF band. For example, in aeronautical radio use, a so-called "UHF radio" system for voice communications actually falls in VHF spectrum as all of its frequencies are below 300 MHz. The aeronautical radio VHF radio system, located above 108 MHz, is among the frequencies fall into the wide band that is in between Channel 6 and Channel 7 of VHF broadcast TV. Police and fire department radios, land-mobile users and two-meter amateur radio operators also occupy VHF Band II, along with the entire FM broadcast band. It was impractical and uneconomic to require these well-established users to move to other frequencies, such as to the genuine UHF band (300 MHz-3 GHz). The U.S. Army and Navy did not need to keep their huge wartime UHF spectrum allocation simply because they had never used most of it. That allocation had been done hastily in 1942 in the face of the emergency of a huge war of unknown duration - and with the presence of very new and poorly understood electronic technologies like radar. In 1942, nobody knew how much bandwidth that the Army and the Navy might need for radar and for radio communications, so the Federal Government took a wise expedient: it allocated a huge amount of radio spectrum to the uniformed services for the time being, in case the service might need it. Then, it could make adjustments later. After the War ended, and after the growth of civilian TV broadcasting in the years after the war, by 1950 expansion of TV channels into the UHF band of frequencies became inevitable. However, lots of UHF TV technology remained unproven at that time, though plenty had been learned about UHF electronics during the war, especially in the development and improvement of radar. (There are significant advantages to using shorter wavelengths, hence higher frequencies, for radars.) Also, the question of which owners should retain the more-valuable (at that time) VHF TV channels remained hotly contested between several different competing interests. To incumbent corporations, such as the Radio Corporation of America and its National Broadcasting Company subsidiary, UHF-TV and FM radio represented disruptive technologies - competition to their existing and long-established manufacturing and broadcast interests in VHF-TV and AM radio. In the fall of 1944, Columbia Broadcasting System pressed a high-definition black and white system on the UHF band employing 750-1,000 scanning lines which offered the possibility of higher-definition monochrome and color broadcasting, both then were precluded from the VHF band because of their bandwidth demands; more significantly, it offered the possibility for sufficient numbers of conventional 6MHz channels to support the FCC's goals of a "truly nationwide and competitive service."[6] CBS was not trying maximize broadcast (or network) competition through freer market entry in the UHF system, but instead CBS's 16MHz channels would have allowed only 27 UHF channels versus the 82 channels possible under the standard 6MHz bandwidth. [7] Vice President of CBS, Adrian Murphy, told the FCC: "I would say that it would be better to have two networks in color" instead of the four or more networks possible with narrower bandwidths in UHF.[8] To newer entrants into TV broadcasting such as the DuMont Laboratories company

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Ultra high frequency and its fourth-ranked DuMont Television Network, however, the need for additional TV channels in major markets was urgent.[9] For proponents of educational TV broadcasting, the difficulties in competing with commercial broadcasters for the increasingly scarce VHF channels becoming a key problem.[10] Any attempt to pursue the objective of broadcast localism on the VHF-TV channels threatened in many regions to push the third-network TV companies such as the American Broadcasting Company onto stations in outlying communities, if they could be accommodated on VHF channels at all. A key question in the FCC's allocation of TV channels was hence that of intermixture. To allocate four to as many as seven VHF channels to each of the largest cities would mean forcing the smaller, intervening cities completely onto the UHF channels, while an allocation scheme that sought to assign one or two VHF channels in each smaller city would force VHF and UHF stations to compete in most markets. (Some may find it hard to believe, but the large metropolitan areas of New York City, Washington-Baltimore, Los Angeles, and San Francisco received seven VHF stations apiece, and Chicago was allocated five, with the other two possible ones going to Milwaukee and Rockford, Illinois.) Hopes that UHF-TV would allow dozens of television stations in every media market were thwarted not only by poor image frequency rejection in superheterodyne receivers with the standard intermediate frequency of 45.75 MHz, but also by very poor adjacent-channel rejection and channel selectivity by early tuner designs and manufactures. UHF-TV stations in the same immediate area were usually assigned by the FCC a minimum of six channels apart due to inadequate TV receiver manufacture. Technical problems with the design of vacuum tubes for operation at high UHF frequencies were beginning to be addressed as late as 1954.[11] These shortcomings led to "UHF taboos", which in effect limited each metropolitan area to only moderately more UHF stations than VHF ones, despite the theoretically much higher number of channels.[12] When the Freeze ended in 1952, the television industry exploded. It grew from the 108 pre-Freeze stations to more than 530 in 1960. These stations were established on the UHF band despite the fact it did not have near the coverage of their VHF competitors. The FCC tried solving this problem by allowing the lower powered UHF stations more power, but it did not work, VHF still had more coverage. At the same time, advertisers had caught on to this and did most of their business with VHF stations. In all, the FCC’s effort to try to intermix VHF and UHF stations in the same market had failed[13] . While the more-established broadcasters were operating profitably on VHF channels as affiliates of the largest TV networks (at the time, NBC and CBS), most of the original UHF local stations of the 1950s soon went bankrupt, limited by the range their signals could supposedly travel, the lack of UHF tuners in most TV sets, and difficulties in finding advertisers willing to spend money on them. UHF stations fell quickly behind the VHF stations. UHF station revenues in 1953 were recorded as having a loss of $10,500,000. More stations left the air than began broadcasting and 60 percent of the industry losses were by UHF stations from 1953 to 1956.[14] TV network affiliations were also difficult to get in many locations; the UHF stations with major-network affiliation would often lose these affiliations in favour of any viable new VHF TV station which entered the same market. Of the 82 new UHF-TV stations in the United States broadcasting as of June 1954, only 24 of them remained on the air a year later.[15] The fraction of new TV receivers that were factory-equipped with all-channel tuners dropped from 35% in early 1953 to 9% by 1958, a drop that was only partially compensated for by field upgrades or the availability of UHF converters for separate purchase. The majority of the 165 UHF stations to begin telecasting between 1952 and 1959 did not survive. Under the All-Channel Receiver Act, FCC regulations by 1965 would ensure that all new TV sets sold in the U.S. had built-in UHF tuners that could receive channels 14-83. In spite of this, by 1971, there were just more than 170 full-service UHF broadcast stations nationwide.[16]

42

Ultra high frequency Independent and educational stations In the United States, the UHF stations gained a reputation for being locally owned, less polished and professional, not as popular, and having weaker signal propagation than their VHF channel counterparts. While UHF-TV has been available to American TV broadcasters since 1952, affiliates of the four major American TV networks (NBC, CBS, ABC, and DuMont) continued to transmit their programs primarily on VHF channels wherever they were available. With the availability of the twelve VHF television channels limited by FCC spacing rules to avoid co-channel and adjacent channel interference between TV stations in the same or nearby cities, all available VHF-TV allocations were already in use in most large TV markets by the mid-1950s. To be more specific, two TV stations on the same channel needed to be about 160 or more miles apart, and two TV stations on adjacent channels needed to be about 60 or more miles apart. Exceptions to this rule occurred with VHF channels 4 and 5, and VHF channels 6 and 7, because there are additional "guard bands" between these two pairs that are allocated to other uses. Thus, the pair channel 4 and 5 was found in New York City, Washington, D.C., St. Louis, Los Angeles, San Francisco, and many other places. Likewise, the pair channel 6 and 7 was found in Denver and several other places. With the most financially affluent and network-connected TV broadcasters all on VHF channels, UHF stations in major population centers of the United States were usually unable to get big TV-network affiliations (ABC, CBS, & NBC), and thus were usually either educational network or independent TV stations.[17] Other UHF stations did for a time affiliate with less-affluent broadcast networks that didn't last very long; for example, the fourth-ranked DuMont Network, which operated from 1946 to 1956, and then went out of business. The movie UHF, starring "Weird Al" Yankovic and Michael Richards, parodied the independent UHF station phenomenon.[18] However, there were significant cities that had few or no VHF channels allocated to them. Hence, these cities did get UHF stations that did get major network affiliations and did become financially sound businesses. Some of these stations have been located in or near state capital cities or served nearby major rural regions, such as Montgomery, Alabama, Frankfort, Kentucky, Dover, Delaware, Lincoln, Nebraska, Topeka, Kansas, Jefferson City, Missouri, Lansing, Michigan, Harrisburg, Pennsylvania, Madison, Wisconsin, and Springfield, Missouri. In the United States, television stations of or near state capital cities are important because they closely cover the operations of the state governments and spread the information to the residents of a wide region of the states. TV antenna manufacturers of years ago often rated their top-of-the-line "deep-fringe" antenna models with phrases like "100 miles VHF/60 miles UHF" if the antenna included UHF reception at all. (In the practice of electrical engineering, the frequency range in which an antenna is to be used is an important factor in its design.) TV set manufacturers of years ago often treated UHF tuners as extra-charge optional-items until the All-Channel Receiver Act of 1964 forced their inclusion in all new TV sets as a standard. By 1964, many pioneering UHF broadcasters had already gone bankrupt. Various attempts were made by the FCC regulators to stem the tide of UHF station failures met with mixed results: • Limits on the number of owned-and-operated stations controlled by one corporation were raised from five stations to seven, provided that two of them were UHF stations. Both NBC-TV (WBUF 17 Buffalo, WNBC 30 Hartford) and CBS-TV (WHCT 18 Hartford, WXIX 19 Milwaukee) acquired pairs of UHF stations as an experiment in the mid-1950s, only to abandon the stations in 1958-59. Their commercial network programming soon returned to VHF channel affiliates. WBUF's allocation on channel 17 was donated to the public-TV broadcaster WNED-TV, which now broadcasts as a Public Broadcasting Service station.[19] • The UHF television impact policy (1960–1988) allowed applications for new VHF TV stations to be opposed in cases where licensure could lead to the economic failure of an existing UHF TV broadcaster.[20] • The secondary affiliation rule (1971–1995) prohibited a network entering a market with two existing VHF TV network affiliates and one UHF independent TV station from placing its programs on a secondary basis on one or both VHF stations without offering them to the UHF station.[21]

43

Ultra high frequency • Limits on UHF effective radiated power, originally very restrictive, were relaxed. A UHF TV station could be licensed for up to five megawatts of carrier power, unlike VHF TV stations, which were limited to 100 (Channels 2-6) or 316 kilowatts of carrier power (Channels 7-13) depending on their channel. • More recent limits on station ownership are based on the combined percentage of the American population (originally 35% maximum, now increased to 45%) reached by one group of stations under common ownership. A UHF discount, by which only half of the audience of a UHF station would be counted against these limits, would ultimately allow groups such as PAX to reach the majority of the American audience using owned-and-operated UHF stations.[22] The situation was to begin to improve in the 1960s and 1970s, but progress was to be slow and difficult. While ABC-TV and the short-lived DuMont Network, being smaller and less prosperous networks, had had a number of UHF affiliates,[23] National Educational Television and the later PBS had even more. The original SIN (Spanish International Network), which was established in 1962 as the predecessor of the modern Univision network, was built primarily by UHF-TV broadcasters, such as charter stations KWEX-TV, Channel 41 in San Antonio and KMEX-TV, Channel 34, in Los Angeles. Ultimately, in addition to providing TV service where VHF channels simply were not possible because of the limitations on the technology, UHF-TV also became a means to obtain programming which was not being provided by the "Big Three" commercial networks. For example, there were educational services like the Public Broadcasting Service, religious broadcasting, and Spanish language or multilingual broadcasting that all relied primarily on UHF channels to offer programming alternatives. Fourth networks, satellite and cable television In 1970, Ted Turner had acquired a struggling independent station on Channel 17 in Atlanta, Georgia, purchasing reruns of popular television shows, the Atlanta Braves baseball team and the Atlanta Hawks basketball team in order to provide access to entertainment for broadcast. This station, renamed as WTBS, was uplinked in 1975 to satellite alongside new premium channels such as HBO, gaining access to distant cable television markets and becoming the first of various superstations to obtain national coverage. In 1986 Turner purchased the entire MGM film library, and Turner Broadcasting System's access to movie rights was to prove commercially valuable as home videocassette rental became ubiquitous in the 1980s. In 1986, the DuMont owned-and-operated station group Metromedia was acquired by News Corporation and used as the foundation to relaunch a fourth commercial network which obtained affiliation with many former big-city independent stations as Fox TV. While largely built from former independents and UHF stations in its early years, Fox had the large programming budgets that the original DuMont lacked. Ultimately it was able in some markets to draw existing long-standing VHF affiliates away from established big-three networks, outbidding CBS for National Football Conference programming in 1994 and attracting many of that network's affiliates. Various smaller networks were created with the intent to follow in its footsteps, often assembling a fledgling network by affiliating with a disparate collection of formerly independent UHF stations which otherwise would have no network programming. Fox launched in 1986. The film UHF portrayed a fictional station on channel 62 in 1989. By 1994, New World Communications was moving its established stations from CBS to Fox affiliations in multiple markets, including WJBK-TV 2 Detroit. In many cases, this pushed CBS onto UHF; "U-62" as the new home of CBS in Detroit became CBS owned-and-operated station WWJ-TV in 1995, obtaining access to audiences thousands of miles distant through satellite and cable television. The concentration of media ownership, the proliferation of cable and satellite television and the digital television transition have contributed to the quality equalization of VHF and UHF broadcasts. The distinction between UHF and VHF characteristics has declined in importance with the emergence of additional broadcast television networks

44

Ultra high frequency (Fox, The CW, MyNetworkTV, Univision, Telemundo and ION), and the decline of direct OTA reception. The number of major large-city independent stations has also declined as many have joined or formed new networks. Digital television See also DTV transition in the United States#VHF_frequencies_and_digital_television The majority of digital TV stations currently broadcast their over-the-air signals in the UHF band, both because VHF had been largely already filled with analog TV at the time the digital facilities were built and because of severe issues with impulse noise on digital low-VHF channels. While virtual channel numbering schemes routinely display channel numbers like "2.1" or "6.1" for individual North American terrestrial HDTV broadcasts, these are more often than not actually UHF signals. Many equipment vendors therefore use "HDTV antenna" or similar branding as all but synonymous to "UHF antenna". Terrestrial digital television is based on a forward error correction scheme, in which a channel is assumed to have a random bit error rate and additional data bits may be sent to allow these errors to be corrected at the receiver. While this error correction can work well in the UHF band where the interference consist largely of white noise, it has largely proven inadequate on lower VHF channels where bursts of impulse noise disrupt the entire channel for short lengths of time. A short impulse-noise burst might be a minor annoyance to analog TV viewers, but due to the fixed timing and repetitive nature of analog video synchronization is usually recoverable. The same interference can prove severe enough to prevent the reliable reception of the more fragile and more highly compressed ATSC digital television. Power limits are also lower on low-VHF; a digital UHF station may be licensed to transmit up to a megawatt of effective radiated power. Very few stations returned to VHF channels 2-6 after digital transition was completed in 2009. At least three quarters of all full-power digital broadcasts continued to use UHF transmitters, even after transition is complete, with most of the others located on the high-VHF channels. In some American markets, such as Syracuse, New York, there are no full-service VHF TV stations remaining after digital transition. The one remaining limitation of UHF, that of a greatly reduced ability for signals to travel great distances in the presence of obstacles due to terrain, continues to adversely affect digital UHF TV reception. Potentially, this limitation could be overcome by the use of DTS (Distributed Transmission Systems). Multiple digital UHF transmitters in carefully selected locations can be synchronized as a single-frequency network to produce a tailored coverage area pattern rivaling that of a single full-power VHF transmitter. While the Federal Communications Commission authorization to use DTS on anything more than an experimental basis came in November 2008, too late for sites to be acquired and transmitters built before the 2009 end of American digital transition, it is likely that more of these distributed UHF transmission systems will be constructed alongside conventional digital broadcast translator systems in the years to come as a means to get digital and high-definition television out to a wider audience. UHF islands One notable exception to historical patterns favoring VHF broadcasters has existed in mid-sized television markets within the United States which were too close to the outer fringe of the broadcast range of large-city VHF stations to qualify for their own stations on these frequencies. As no full-power VHF channels could be made available in these areas without encountering problems of interference from overlapping broadcast ranges, the Federal Communications Commission granted some mid-sized cities only UHF licenses. With all stations (including big-three network affiliates) on UHF, all-channel receivers and antennas became commonplace locally and UHF stations signing on as early as 1954 were often able to obtain the programming and viewership needed to remain viable into the modern era.[24] These communities, known as UHF islands, included cities like Springfield, Massachusetts; Fresno, California; South Bend, Indiana;Fort Wayne, Indiana; and Lexington, Kentucky. Other smaller cities found that only one VHF channel was open and any additional programming would need to be provided either by UHF, by distant stations or

45

Ultra high frequency by low-power broadcasting. Broadcast translators and low-power television A large number of very small UHF TV transmitters continue to operate with no programming or commercial identity of their own, merely retransmitting signals of existing full-power stations to a smaller area poorly covered by the main VHF signal. Such transmitters are called "translators" rather than “stations”. The smallest, owned by local municipal-level groups or the originating TV stations, are numbered sequentially - W or K, followed by the channel number, followed by two sequentially issued letters, yielding a "translator callsign" in a generic format which appears K14AA through W69ZZ. Translators and repeaters also exist on VHF channels, but infrequently and with stringently limited power as the VHF spectrum is already crowded with full-power network stations in most regions. The translator band, UHF TV channels 70-83, consisted mostly of these small repeaters; it was removed from television use in 1983 with the tiny repeaters moved primarily to lower UHF channels. The 804-890 MHz band segment is now primarily used by mobile phones. As improvements to originating stations signals lessen the need for these small translators in some areas, often the small transmitter facilities and their allocated frequencies would be repurposed for low-power broadcasting; instead of repeating a distant signal, the tiny transmitter would be used to originate programming for a small local area. Radio, mobile and non-broadcast applications The Family Radio Service and General Mobile Radio Service use the 462 and 467 MHz areas of the UHF spectrum. There is a considerable amount of lawful unlicensed activity (cordless phones, wireless networking) clustered around 900 MHz and 2.4 GHz. These ISM bands - open frequencies with a higher unlicensed power permitted for use originally by Industrial, Scientific, Medical apparatus - are now becoming some of the most crowded in the spectrum because they are open to everyone. The 2.45 GHz frequency, readily absorbed by water, is the standard for use by microwave ovens. The spectrum from 806 MHz to 890 MHz (UHF channels 70-83) was taken away from TV broadcast services in 1983, primarily for analogue mobile telephony. In 2009, as part of the transition from analog to digital over-the-air broadcast of television, the spectrum from 698 MHz to 806 MHz (UHF channels 52-69) was also no longer used for TV broadcasting. Channel 55, for instance, was sold to Qualcomm for their MediaFLO service, which is resold under various mobile telephone network brands. Some US broadcasters had been offered incentives to vacate this channel early, permitting its immediate mobile use. The FCC's scheduled auction for this newly available spectrum was completed in March 2008.[25]

Frequency allocation Australia • UHF CB Australia [26] - UHF CB News, Information & Repeater Locations. UHF CB Australia Supporting and expanding the UHF CB network • UHF Citizens Band: 300- 3000MHz

Canada • 470-806 MHz: Terrestrial television (with select channels in the 700 MHz band left vacant) • 1452-1492 MHz: Digital Audio Broadcasting (L band)[27] • Many other frequency assignments for Canada and Mexico are similar to their US counterparts

46

Ultra high frequency

United Kingdom • • • • •

380–395 MHz: Terrestrial Trunked Radio (TETRA) service for emergency use 430–440 MHz: Amateur radio (ham - 70 cm band) 457–464 MHz: Scanning telemetry and telecontrol, mostly assigned to the water, gas and electricity industries 606–614 MHz: Protected for radio-astronomy 470–862 MHz: TV channels 21–69 (channel 36 used for radar, channel 38 used for radio astronomy, channel 69 used for licenced and licence exempt wireless microphones, channels 31-40 and 63-68 to be released and may be made available for other uses by Ofcom. Public consultation due December 2006) • 1240–1316 MHz: Amateur radio (ham - 23 cm band) • 1880–1900 MHz: DECT Cordless telephone • 2310–2450 MHz: Amateur radio (ham - 13 cm band)

United States A brief summary of some UHF frequency use: • 225–420 MHz: Government use, including meteorology, military aviation, and federal two-way use[28] • 420–450 MHz: Government radiolocation and amateur radio (70 cm band) • 433 MHz: Short range consumer devices including automotive, alarm systems, home automation, temperature sensors • 450–470 MHz: UHF business band, General Mobile Radio Service, and Family Radio Service 2-way "walkie-talkies", public safety • 470–512 MHz: TV channels 14–20 (also shared for land mobile 2-way radio use in some areas) • 512–698 MHz: TV channels 21–51 (channel 37 used for radio astronomy) • 698–806 MHz: Was auctioned in March 2008; bidders got full use after the transition to digital TV was completed on June 12, 2009 (formerly UHF TV channels 52–69) • 806–824 MHz: Public safety and commercial 2-way (formerly TV channels 70–72) • 824–851 MHz: Cellular A & B franchises, terminal (mobile phone) (formerly TV channels 73–77) • 851–869 MHz: Public safety and commercial 2-way (formerly TV channels 77–80) • 869–896 MHz: Cellular A & B franchises, base station (formerly TV channels 80–83) • 902–928 MHz: ISM band, amateur radio (33 cm band), cordless phones and stereo, radio-frequency identification, datalinks • 929–930 MHz: Pagers • 931–932 MHz: Pagers • 935–941 MHz: Commercial 2-way radio • 941–960 MHz: Mixed studio-transmitter links, SCADA, other. • 960–1215 MHz: Aeronautical Radionavigation • 1240–1300 MHz: Amateur radio (23 cm band) • 1452–1492 MHz: Military use (therefore not available for Digital Audio Broadcasting, unlike Canada/Europe) • 1710–1755 MHz: AWS mobile phone uplink (UL) Operating Band • 1850–1910 MHz: PCS mobile phone—order is A, D, B, E, F, C blocks. A, B, C = 15 MHz; D, E, F = 5 MHz • 1920–1930 MHz: DECT Cordless telephone • 1930–1990 MHz: PCS base stations—order is A, D, B, E, F, C blocks. A, B, C = 15 MHz; D, E, F = 5 MHz • 2110–2155 MHz: AWS mobile phone downlink (DL) Operating Band • 2300–2310 MHz: Amateur radio (13 cm band, lower segment) • 2310–2360 MHz: Satellite radio (Sirius and XM) • 2390–2450 MHz: Amateur radio (13 cm band, upper segment)

47

Ultra high frequency • 2400–2483.5 MHz: ISM, IEEE 802.11, 802.11b, 802.11g, 802.11n Wireless LAN, IEEE 802.15.4-2006, Bluetooth, Radio-controlled aircraft, Microwave oven, ZigBee

References [1] [2] [3] [4] [5] [6] [7] [8] [9]

http:/ / www. ic. gc. ca/ eic/ site/ smt-gst. nsf/ vwapj/ DTV_PLAN_Dec08-e. pdf/ $file/ DTV_PLAN_Dec08-e. pdf About DAB - Canadian Association of Broadcasters (http:/ / www. cab-acr. ca/ drri/ index. shtm) http:/ / springfield375. org/ ?p=126 Fifties Television: The Industry and Its Critics, William Boddy, University of Illinois Press, 1992, ISBN 9780252062995 http:/ / www. nrcdxas. org/ articles/ 1945ass. txt Fifties Television: The Industry and Its Critics, William Boddy, University of Illinois Press, 1992, ISBN 9780252062995 Fifties Television: The Industry and Its Critics, William Boddy, University of Illinois Press, 1992, ISBN 9780252062995 Fifties Television: The Industry and Its Critics, William Boddy, University of Illinois Press, 1992, ISBN 9780252062995 Radio and Television Regulation: Broadcast Technology in the United States 1920-1960, Hugh Richard Slotten, JHU Press, 2000, ISBN 9780801864506 [10] Missed Opportunities: FCC Commissioner Frieda Hennock and the UHF Debacle, Susan L. Brinson, Journal of Broadcasting & Electronic Media • Spring, 2000 (http:/ / www. entrepreneur. com/ tradejournals/ article/ print/ 63018844. html) [11] VALVES AT UHF: A REVIEW OF RECENT DEVELOPMENTS, S. Simpson, Practical Television magazine, March 1954 (http:/ / www. thevalvepage. com/ valvetek/ uhfvalve/ uhfvalve. htm) [12] TV-technology.com The Superheterodyne Concept and Reception, Charles W. Rhodes, TV Technology, July 20, 2005 (http:/ / www. tv-technology. com/ pages/ s. 0072/ t. 1648. html) [13] Sterling, C. H., & Kittross, J. M. (1990). Stay Tuned: A concise history of American broadcasting (2nd ed.). Belmont, CA:Wadsworth. [14] Boddy,W.(1990) Fifties Television: The Industry and Its Critics. Urbana,IL: University of Illinois Press. [15] http:/ / tulsatvmemories. com/ tvthesi3. html [16] Stay Tuned: A History of American Broadcasting; pp 387-388; Christopher H. Sterling, John M. Kittross; Erlbaum 2002; ISBN 9780805826241 [17] UHF morgue (http:/ / radiodxer. bravehost. com/ morgue. html) [18] U-62 program schedule, July 1989 (http:/ / www. allthingsyank. com/ uhf15/ u-62. htm) [19] Buffalo Broadcasters: History - UHF (http:/ / www. buffalobroadcasters. com/ hist_uhf. asp) [20] Media Economics: Theory and Practice, Alison Alexander, Erlbaum Associates 2004 ISBN 9780805845808 [21] FCC order revoking secondary affiliation rule, 1995 (http:/ / www. fcc. gov/ Bureaus/ Mass_Media/ Orders/ 1995_Orders/ fcc95097. txt) [22] http:/ / www. wcl. american. edu/ journal/ lawrev/ 53/ rothenberger. pdf [23] The DuMont Television Network, Appendix 10/11: A Trail of Bleached Bones, C. Ingram (http:/ / www. dumonthistory. tv/ a10. html) [24] WSJV 28 South Bend, Indiana history (http:/ / www. fox28. com/ Global/ story. asp?S=8396164& nav=menu1356_11) indicates station founded 1954, still extant as no VHF channels available due to proximity to Chicago [25] Going once, twice, the 700MHz spectrum is sold, NY Times, Mar 18 2008 (http:/ / bits. blogs. nytimes. com/ 2008/ 03/ 18/ going-oncegoing-twicethe-700-mhz-spectrum-is-sold/ ?ref=technology) [26] http:/ / www. uhfcb. com. au [27] http:/ / www. broadcasting-history. ca/ stations/ radio/ Digital_Audio_Broadcasting. html [28] http:/ / www. raytheon. com/ capabilities/ rtnwcm/ groups/ ncs/ documents/ content/ rtn_ncs_products_arc164_pdf. pdf

External links • U.S. cable television channel frequencies (http://www.jneuhaus.com/fccindex/cablech.html) • TVTower.com - Commercial Television Frequencies (http://www.tvtower.com/Commercial Television Frequencies.html) • Tomislav Stimac, " Definition of frequency bands (VLF, ELF... etc.) (http://www.vlf.it/frequency/bands. html)". IK1QFK Home Page (vlf.it).

48

Super high frequency

49

Super high frequency Super high frequency Frequency range 3 to 30 GHz

ITU Radio Band Numbers

1 2 3 4 5 6 7 8 9 10 11 ITU Radio Band Symbols

ELF SLF ULF VLF LF MF HF VHF UHF SHF EHF NATO Radio bands

ABCDEFGHIJKLM IEEE Radar bands

HF VHF UHF L S C X Ku K Ka Q V W

Super high frequency (or SHF) refers to radio frequencies (RF) in the range of 3 GHz and 30 GHz. Also known as the centimeter band or centimeter wave as the wavelengths range from ten to one centimeters.

Description The International Telecommunication Union (ITU), an international civil organization established to standardized worldwide telecommunications, have stated that the superhigh frequency is encountered between 100 mm to 10 mm. Super high frequency electromagnetic waves are relatively short for radio waves. This frequency is used for microwave devices, WLAN, most modern radars. The commencing Wireless USB technology will be using approximately 1/3 of this spectrum.

Uses Some uses are IEEE 802.11a wireless LANs, satellite uplinks/downlinks and terrestrial high-speed data links which are sometimes referred to as "backhauls".

External links • Tomislav Stimac, " Definition of frequency bands (VLF, ELF... etc.) [1]". IK1QFK Home Page (vlf.it). • Inés Vidal Castiñeira, " Celeria: Wireless Access To Cable Networks [2]"

References [1] http:/ / www. vlf. it/ frequency/ bands. html [2] http:/ / www. broadbandhomecentral. com/ report/ backissues/ Report0308_3. html

Extremely high frequency

50

Extremely high frequency Extremely high frequency Frequency range 30 to 300 GHz

ITU Radio Band Numbers

1 2 3 4 5 6 7 8 9 10 11 ITU Radio Band Symbols

ELF SLF ULF VLF LF MF HF VHF UHF SHF EHF NATO Radio bands

ABCDEFGHIJKLM IEEE Radar bands

HF VHF UHF L S C X Ku K Ka Q V W

Extremely high frequency is the highest radio frequency band. EHF runs the range of frequencies from 30 to 300 gigahertz, above which electromagnetic radiation is considered to be low (or far) infrared light, also referred to as terahertz radiation. This band has a wavelength of ten to one millimetre, giving it the name millimeter band or millimetre wave, sometimes abbreviated MMW or mmW. Compared to lower bands, terrestrial radio signals in this band are extremely prone to atmospheric attenuation, making them of very little use over long distances. In particular, signals in the 57–64 GHz region are subject to a resonance of the oxygen molecule and are severely attenuated. Even over relatively short distances, rain fade is a serious problem, caused when absorption by rain reduces signal strength. In climates other than deserts absorption due to humidity also has an impact on propagation. While this absorption limits potential communications range, it also allows for smaller frequency reuse distances than lower frequencies. The small wavelength allows modest size antennas to have a small beam width, further increasing frequency reuse potential.

Applications Scientific research This band is commonly used in radio astronomy and remote sensing. Ground-based radio astronomy is limited to high altitude sites such as Kitt Peak and Atacama Large Millimeter Array (ALMA) due to atmospheric absorption issues. Satellite-based remote sensing near 60 GHz can determine temperature in the upper atmosphere by measuring radiation emitted from oxygen molecules that is a function of temperature and pressure. The ITU non-exclusive passive frequency allocation at 57-59.3 is used for atmospheric monitoring in meteorological and climate sensing applications, and is important for these purposes due to the properties of oxygen absorption and emission in Earth’s atmosphere. Currently operational U.S. satellite sensors such as the Advanced Microwave Sounding Unit (AMSU) on one NASA satellite (Aqua) and four NOAA (15-18) satellites and the Special Sensor Microwave Imager Sounder (SSMI/S) on Department of Defense satellite F-16 make use of this frequency range.[1]

Extremely high frequency

Telecommunications In the United States, the band 38.6 - 40.0 GHz is used for licensed high-speed microwave data links, and the 60 GHz band can be used for unlicensed short range (1.7 km) data links with data throughputs up to 2.5 Gbit/s. It is used commonly in flat terrain. The 71-76, 81-86 and 92–95 GHz bands are also used for point-to-point high-bandwidth communication links. These frequencies, as opposed to the 60 GHz frequency, require a transmitting license in the US from the Federal Communications Commission (FCC), though they do not suffer from the effects of oxygen absorption as the 60 GHz does. There are plans for 10 Gbit/s links using these frequencies as well. In the case of the 92–95 GHz band, a small 100 MHz range has been reserved for space-borne radios, making this reserved range limited to a transmission rate of under a few gigabits per second. [2] The band is essentially undeveloped and available for use in a broad range of new products and services, including high-speed, point-to-point wireless local area networks and broadband Internet access. WirelessHD is another recent technology that operates near the 60 GHz range. Highly directional, "pencil-beam" signal characteristics permit systems in these bands to be engineered in close proximity to one another without causing interference. Potential applications include radar systems with very high resolution. Uses of the millimeter wave bands include point-to-point communications, intersatellite links, and point-to-multipoint communications. Because of shorter wavelengths, the band permits the use of smaller antennas than would be required for similar circumstances in the lower bands, to achieve the same high directivity and high gain. The immediate consequence of this high directivity, coupled with the high free space loss at these frequencies, is the possibility of a more efficient use of the spectrum for point-to-multipoint applications. Since a greater number of highly directive antennas can be placed in a given area than less directive antennas, the net result is higher reuse of the spectrum, and higher density of users, as compared to lower frequencies. Furthermore, because one can place more voice channels or broadband information using a higher frequency to transmit the information, this spectrum could potentially be used as a replacement for or supplement to fiber optics. ASELSAN, a Turkish Armed Forces owned company, is currently working on an on-board processing EHF satellite transponder in conjunction with the Scientific and Technological Research Council of Turkey (TUBITAK) and Bilkent University. ASELSAN's aim is to produce an indegenous on-board processing EHF satellite transponder and its phased array antenna.[3] As reported in incisor.tv monthly magazine, the WiMedia Alliance is looking at using the 60 GHz range in their road map.[4]

Weapons systems The U.S. Air Force is reported to have developed a nonlethal weapon system called Active Denial System (ADS) which emits a beam of radiation with a wavelength of 3 mm.[5] The weapon is reportedly not dangerous and causes no physical harm, but is extremely painful and causes the target to feel an intense burning pain, as if his or her skin is going to catch fire.

Security screening A recent development has been imagers for security applications as clothing and other organic materials are translucent in some mm-wave atmospheric windows.[6] Privacy advocates are concerned about the use of this technology because it allows screeners to see airport passengers as if without clothing. The TSA has deployed a $170,000 machine, in the month of February 2009, for use in Tulsa International Airport according to USA Today. Machines will follow in Las Vegas, San Francisco, Albuquerque and Salt Lake City by May 2009.[7] Similar units have been deployed in Baltimore (BWI) and Raleigh (RDU) for some time. These

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Extremely high frequency machines were been deployed in the Jersey City PATH train system for two weeks in 2006.[8] Currently the technology does not mask any part of the bodies of the people who are being scanned. However, passengers' faces are deliberately masked by the system. The photos are screened by technicians in a closed room, then deleted immediately upon search completion. Currently, passengers can decline scanning and be screened via a metal detector and patted down. The machines do allow the screener to see detailed images of body parts. Privacy advocates are concerned. "We're getting closer and closer to a required strip-search to board an airplane," said Barry Steinhardt of the American Civil Liberties Union.[7] Three security scanners using millimeter waves were put into use at Schiphol Airport in Amsterdam on 15 May 2007, with more expected to be installed later. The passenger's head is masked from the view of the security personnel. According to Farran Technologies, a manufacturer of one model of the millimeter wave scanner, the technology exists to extend the search area to as far as 50 meters beyond the scanning area which would allow security workers to scan a large number of people without their awareness that they are being scanned.[9]

Medicine Most widely used in former USSR nations,[10] [11] low intensity (usually 10 mW/cm2 or less) electromagnetic radiation of extremely high frequency (especially in the range 40 - 70 GHz, which corresponds to wavelength of 7.5 - 4.3 mm) is used in human medicine for the treatment of many types of diseases.[11] [12] This type of therapy is called Millimeter Wave (MMW) Therapy or Extremely High Frequency (EHF) Therapy. More than 10 000 devices are used for Millimeter Wave Therapy worldwide [13] and more than a million people have been successfully treated with millimeter wave therapy during its documented history.[13] Established in 1992, the Russian Journal Millimeter waves in biology and medicine is dedicated to the scientific basis and clinical applications of Millimeter Wave Therapy.[14] More than 50 issues of it have been published.

References [1] FCC.gov (http:/ / gullfoss2. fcc. gov/ prod/ ecfs/ retrieve. cgi?native_or_pdf=pdf& id_document=6519741794), Comments of IEEE Geoscience and Remote Sensing Society, FCC RM-11104, 10/17/07 [2] Rfdesign.com (http:/ / rfdesign. com/ mag/ 605RFDF4. pdf), Multigigabit wireless technology at 70 GHz, 80 GHz and 90 GHz, RF Design, May 2006 [3] "Processed EHF Transponder And Antenna Design And Production" (http:/ / www. aselsan. com. tr/ urun. asp?urun_id=229& lang=en). . Retrieved 5 June 2009. [4] Incisor.tv (http:/ / www. incisor. tv/ download. php?file=124july2008. pdf) [5] "Slideshow: Say Hello to the Goodbye Weapon" (http:/ / www. wired. com/ news/ technology/ 0,72134-0. html). . Retrieved 4 June 2009. [6] Newscientisttech.com (http:/ / www. newscientisttech. com/ article. ns?id=dn10160& feedId=tech_rss20) [7] Frank, Thomas (18 February 2009). "Body scanners replace metal detectors in tryout at Tulsa airport." (http:/ / www. usatoday. com/ travel/ flights/ 2009-02-17-detectors_N. htm). USA Today. . Retrieved 2 May 2010. [8] "Mirror for Star Ledger Article "PATH riders to face anti-terror screening -- Program will begin at station in Jersey City" (http:/ / www. hudsoncity. net/ tubes/ securitytesting. html). 2006/07/12 Wed. p. 014. . [9] Scenta.co.uk (http:/ / www. scenta. co. uk/ scenta/ news. cfm?cit_id=289412& FAArea1=customWidgets. content_view_1) [10] M. Rojavin and M. Ziskin, Medical application of millimetre waves, QJM: An International Journal of Medicine, vol. 91, no. 1, p. 57, 1998, http:/ / qjmed. oxfordjournals. org/ content/ 91/ 1/ 57. full. pdf [11] Pakhomov, A.G., Murphy, P.R., Low-intensity millimeter waves as a novel therapeutic modality, IEEE Transactions on Plasma Science, 2000, vol. 28, no. 1, http:/ / dx. doi. org/ 10. 1109/ 27. 842821 [12] Betskii, O. V. , Devyatkov, N. D., Kislov, V., Low Intensity Millimeter Waves in Medicine and Biology, Critical Reviews in Biomedical Engineering, 2000, vol. 28 no. 1&2, p. 247-268 http:/ / www. begellhouse. com/ journals/ 4b27cbfc562e21b8. html [13] Betskii, O. V., Kotrovskaya T. I., Lebedeva, N. N., Millimeter Waves in Biology and Medicine, III Всероссийская конференция «Радиолокация и радиосвязь» – ИРЭ РАН, 26-30 октября 2009, http:/ / jre. cplire. ru/ jre/ library/ 3conference/ pdffiles/ b004. pdf [14] http:/ / www. benran. ru/ Magazin/ El/ 13/ N71320. HTM

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Extremely high frequency

External links • FCC bulletin on MMW propagation (http://www.fcc.gov/Bureaus/Engineering_Technology/Documents/ bulletins/oet70/oet70a.pdf) • Asyrmatos Millimeter Wave Communication System (http://www.asyrmatos.com) • L-3 Communications ProVision Body Screening System (http://www.dsxray.com/products/mmwave.htm) • FCC 70/80/90 GHz overview. (http://wireless.fcc.gov/services/millimeterwave) • FCC 57–64 GHz rules. (http://edocket.access.gpo.gov/cfr_2007/octqtr/47cfr15.255.htm) • Civil mm-wave Regulation in US (http://www.marcus-spectrum.com/MMW.htm) • Definition of frequency bands (VLF, ELF... etc.) (http://www.vlf.it/frequency/bands.html) • Millimetre-Wave Technology Group (http://www.mmt.rl.ac.uk/) at Rutherford-Appleton Laboratory • Overview of active methods for shielding spacecraft from energetic space radiation (http://www.physicamedica. com/VOLXVII_S1/17-TOWNSEND.pdf) • St. Andrews University mm-wave group (http://www.st-andrews.ac.uk/~mmwave/mmwave/index.shtml) • A Survey of University Capabilities for a New Canadian Radio Telescope (http://www.drao-ofr.hia-iha. nrc-cnrc.gc.ca/science/ska/univ_assessment.html) • US Patent 7220488 - Deflecting magnetic field shield (http://www.patentstorm.us/patents/7220488/ description.html)

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Modulation Modulation In electronics, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted. This is done in a similar fashion to a musician modulating a tone (a periodic waveform) from a musical instrument by varying its volume, timing and pitch. The purpose of modulation is usually to enable the carrier signal to transport the information in the modulation signal to some destination. At the destination, a process of demodulation extracts the modulation signal from the modulated carrier. The three key parameters of a periodic waveform are its amplitude ("volume"), its phase ("timing") and its frequency ("pitch"). Any of these properties can be modified in accordance with a low frequency signal to obtain the modulated signal. Typically a high-frequency sinusoid waveform is used as carrier signal, but a square wave pulse train may also be used. In telecommunications, modulation is the process of conveying a message signal, for example a digital bit stream or an analog audio signal, inside another signal that can be physically transmitted. Modulation of a sine waveform is used to transform a baseband message signal into a passband signal, for example low-frequency audio signal into a radio-frequency signal (RF signal). In radio communications, cable TV systems or the public switched telephone network for instance, electrical signals can only be transferred over a limited passband frequency spectrum, with specific (non-zero) lower and upper cutoff frequencies. Modulating a sine-wave carrier makes it possible to keep the frequency content of the transferred signal as close as possible to the centre frequency (typically the carrier frequency) of the passband. A device that performs modulation is known as a modulator and a device that performs the inverse operation of modulation is known as a demodulator (sometimes detector or demod). A device that can do both operations is a modem (modulator–demodulator).

Aim The aim of digital modulation is to transfer a digital bit stream over an analog bandpass channel, for example over the public switched telephone network (where a bandpass filter limits the frequency range to between 300 and 3400 Hz), or over a limited radio frequency band. The aim of analog modulation is to transfer an analog baseband (or lowpass) signal, for example an audio signal or TV signal, over an analog bandpass channel, for example a limited radio frequency band or a cable TV network channel. Analog and digital modulation facilitate frequency division multiplexing (FDM), where several low pass information signals are transferred simultaneously over the same shared physical medium, using separate passband channels. The aim of digital baseband modulation methods, also known as line coding, is to transfer a digital bit stream over a baseband channel, typically a non-filtered copper wire such as a serial bus or a wired local area network. The aim of pulse modulation methods is to transfer a narrowband analog signal, for example a phone call over a wideband baseband channel or, in some of the schemes, as a bit stream over another digital transmission system. In music synthesizers, modulation may be used to synthesise waveforms with a desired overtone spectrum. In this case the carrier frequency is typically in the same order or much lower than the modulating waveform. See for example frequency modulation synthesis or ring modulation.

Modulation

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Analog modulation methods In analog modulation, the modulation is applied continuously in response to the analog information signal. Common analog modulation techniques are:[1] • Amplitude modulation (AM) (here the amplitude of the carrier signal is varied in accordance to the instantaneous amplitude of the modulating signal) • Double-sideband modulation (DSB) • Double-sideband modulation with carrier (DSB-WC) (used on the AM radio broadcasting band) • Double-sideband suppressed-carrier transmission (DSB-SC)

A low-frequency message signal (top) may be carried by an AM or FM radio wave.

• Double-sideband reduced carrier transmission (DSB-RC) • Single-sideband modulation (SSB, or SSB-AM) • SSB with carrier (SSB-WC) • SSB suppressed carrier modulation (SSB-SC) • Vestigial sideband modulation (VSB, or VSB-AM) • Quadrature amplitude modulation (QAM) • Angle modulation • Frequency modulation (FM) (here the frequency of the carrier signal is varied in accordance to the instantaneous amplitude of the modulating signal) • Phase modulation (PM) (here the phase shift of the carrier signal is varied in accordance to the instantaneous amplitude of the modulating signal)

Digital modulation methods In digital modulation, an analog carrier signal is modulated by a discrete signal. Digital modulation methods can be considered as digital-to-analog conversion, and the corresponding demodulation or detection as analog-to-digital conversion. The changes in the carrier signal are chosen from a finite number of M alternative symbols (the modulation alphabet). A simple example: A telephone line is designed for transferring audible sounds, for example tones, and not digital bits (zeros and ones). Computers may however communicate over a telephone line by means of modems, which are representing the digital bits by tones, called symbols. If there are four alternative symbols (corresponding to a musical instrument that can generate four different tones, one at a time), the first symbol may represent the bit sequence 00, the second 01, the third 10 and the fourth 11. If the Schematic of 4 baud (8 bps) data link. modem plays a melody consisting of 1000 tones per second, the symbol rate is 1000 symbols/second, or baud. Since each tone (i.e., symbol) represents a message consisting of two digital bits in this example, the bit rate is twice the symbol rate, i.e. 2000 bits per second. This is similar to the technique used by dialup modems as opposed to DSL modems.

Modulation

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. According to one definition of digital signal, the modulated signal is a digital signal, and according to another definition, the modulation is a form of digital-to-analog conversion. Most textbooks would consider digital modulation schemes as a form of digital transmission, synonymous to data transmission; very few would consider it as analog transmission.

Fundamental digital modulation methods The most fundamental digital modulation techniques are based on keying: • • • •

In the case of PSK (phase-shift keying), a finite number of phases are used. In the case of FSK (frequency-shift keying), a finite number of frequencies are used. In the case of ASK (amplitude-shift keying), a finite number of amplitudes are used. In the case of QAM (quadrature amplitude modulation), a finite number of at least two phases, and at least two amplitudes are used.

In QAM, an inphase signal (the I signal, for example a cosine waveform) and a quadrature phase signal (the Q signal, for example a sine wave) are amplitude modulated with a finite number of amplitudes, and summed. It can be seen as a two-channel system, each channel using ASK. The resulting signal is equivalent to a combination of PSK and ASK. In all of the above methods, each of these phases, frequencies or amplitudes are assigned a unique pattern of binary bits. Usually, each phase, frequency or amplitude encodes an equal number of bits. This number of bits comprises the symbol that is represented by the particular phase, frequency or amplitude. If the alphabet consists of

alternative symbols, each symbol represents a message consisting of N bits. If

the symbol rate (also known as the baud rate) is

symbols/second (or baud), the data rate is

bit/second.

For example, with an alphabet consisting of 16 alternative symbols, each symbol represents 4 bits. Thus, the data rate is four times the baud rate. In the case of PSK, ASK or QAM, where the carrier frequency of the modulated signal is constant, the modulation alphabet is often conveniently represented on a constellation diagram, showing the amplitude of the I signal at the x-axis, and the amplitude of the Q signal at the y-axis, for each symbol.

Modulator and detector principles of operation PSK and ASK, and sometimes also FSK, are often generated and detected using the principle of QAM. The I and Q signals can be combined into a complex-valued signal I+jQ (where j is the imaginary unit). The resulting so called equivalent lowpass signal or equivalent baseband signal is a complex-valued representation of the real-valued modulated physical signal (the so called passband signal or RF signal). These are the general steps used by the modulator to transmit data: 1. Group the incoming data bits into codewords, one for each symbol that will be transmitted. 2. Map the codewords to attributes, for example amplitudes of the I and Q signals (the equivalent low pass signal), or frequency or phase values. 3. Adapt pulse shaping or some other filtering to limit the bandwidth and form the spectrum of the equivalent low pass signal, typically using digital signal processing. 4. Perform digital-to-analog conversion (DAC) of the I and Q signals (since today all of the above is normally achieved using digital signal processing, DSP). 5. Generate a high-frequency sine wave carrier waveform, and perhaps also a cosine quadrature component. Carry out the modulation, for example by multiplying the sine and cosine wave form with the I and Q signals, resulting in that the equivalent low pass signal is frequency shifted into a modulated passband signal or RF signal. Sometimes this is achieved using DSP technology, for example direct digital synthesis using a waveform table,

Modulation instead of analog signal processing. In that case the above DAC step should be done after this step. 6. Amplification and analog bandpass filtering to avoid harmonic distortion and periodic spectrum At the receiver side, the demodulator typically performs: 1. Bandpass filtering. 2. Automatic gain control, AGC (to compensate for attenuation, for example fading). 3. Frequency shifting of the RF signal to the equivalent baseband I and Q signals, or to an intermediate frequency (IF) signal, by multiplying the RF signal with a local oscillator sinewave and cosine wave frequency (see the superheterodyne receiver principle). 4. Sampling and analog-to-digital conversion (ADC) (Sometimes before or instead of the above point, for example by means of undersampling). 5. Equalization filtering, for example a matched filter, compensation for multipath propagation, time spreading, phase distortion and frequency selective fading, to avoid intersymbol interference and symbol distortion. 6. Detection of the amplitudes of the I and Q signals, or the frequency or phase of the IF signal. 7. Quantization of the amplitudes, frequencies or phases to the nearest allowed symbol values. 8. Mapping of the quantized amplitudes, frequencies or phases to codewords (bit groups). 9. Parallel-to-serial conversion of the codewords into a bit stream. 10. Pass the resultant bit stream on for further processing such as removal of any error-correcting codes. As is common to all digital communication systems, the design of both the modulator and demodulator must be done simultaneously. Digital modulation schemes are possible because the transmitter-receiver pair have prior knowledge of how data is encoded and represented in the communications system. In all digital communication systems, both the modulator at the transmitter and the demodulator at the receiver are structured so that they perform inverse operations. Non-coherent modulation methods do not require a receiver reference clock signal that is phase synchronized with the sender carrier wave. In this case, modulation symbols (rather than bits, characters, or data packets) are asynchronously transferred. The opposite is coherent modulation.

List of common digital modulation techniques The most common digital modulation techniques are: • Phase-shift keying (PSK): • Binary PSK (BPSK), using M=2 symbols • Quadrature PSK (QPSK), using M=4 symbols • 8PSK, using M=8 symbols • 16PSK, using M=16 symbols • Differential PSK (DPSK) • Differential QPSK (DQPSK) • Offset QPSK (OQPSK) • π/4–QPSK • Frequency-shift keying (FSK): • Audio frequency-shift keying (AFSK) • Multi-frequency shift keying (M-ary FSK or MFSK) • Dual-tone multi-frequency (DTMF) • Continuous-phase frequency-shift keying (CPFSK) • Amplitude-shift keying (ASK) • On-off keying (OOK), the most common ASK form • M-ary vestigial sideband modulation, for example 8VSB

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Modulation • Quadrature amplitude modulation (QAM) - a combination of PSK and ASK: • Polar modulation like QAM a combination of PSK and ASK. • Continuous phase modulation (CPM) methods: • Minimum-shift keying (MSK) • Gaussian minimum-shift keying (GMSK) • Orthogonal frequency-division multiplexing (OFDM) modulation: • discrete multitone (DMT) - including adaptive modulation and bit-loading. • Wavelet modulation • Trellis coded modulation (TCM), also known as trellis modulation • Spread-spectrum techniques: • Direct-sequence spread spectrum (DSSS) • Chirp spread spectrum (CSS) according to IEEE 802.15.4a CSS uses pseudo-stochastic coding • Frequency-hopping spread spectrum (FHSS) applies a special scheme for channel release MSK and GMSK are particular cases of continuous phase modulation. Indeed, MSK is a particular case of the sub-family of CPM known as continuous-phase frequency-shift keying (CPFSK) which is defined by a rectangular frequency pulse (i.e. a linearly increasing phase pulse) of one symbol-time duration (total response signaling). OFDM is based on the idea of frequency-division multiplexing (FDM), but is utilized as a digital modulation scheme. The bit stream is split into several parallel data streams, each transferred over its own sub-carrier using some conventional digital modulation scheme. The modulated sub-carriers are summed to form an OFDM signal. OFDM is considered as a modulation technique rather than a multiplex technique, since it transfers one bit stream over one communication channel using one sequence of so-called OFDM symbols. OFDM can be extended to multi-user channel access method in the orthogonal frequency-division multiple access (OFDMA) and multi-carrier code division multiple access (MC-CDMA) schemes, allowing several users to share the same physical medium by giving different sub-carriers or spreading codes to different users. Of the two kinds of RF power amplifier, switching amplifiers (Class C amplifiers) cost less and use less battery power than linear amplifiers of the same output power. However, they only work with relatively constant-amplitude-modulation signals such as angle modulation (FSK or PSK) and CDMA, but not with QAM and OFDM. Nevertheless, even though switching amplifiers are completely unsuitable for normal QAM constellations, often the QAM modulation principle are used to drive switching amplifiers with these FM and other waveforms, and sometimes QAM demodulators are used to receive the signals put out by these switching amplifiers.

Digital baseband modulation or line coding The term digital baseband modulation (or digital baseband transmission) is synonymous to line codes. These are methods to transfer a digital bit stream over an analog baseband channel (a.k.a. lowpass channel) using a pulse train, i.e. a discrete number of signal levels, by directly modulating the voltage or current on a cable. Common examples are unipolar, non-return-to-zero (NRZ), Manchester and alternate mark inversion (AMI) codings.[2]

Pulse modulation methods Pulse modulation schemes aim at transferring a narrowband analog signal over an analog baseband channel as a two-level signal by modulating a pulse wave. Some pulse modulation schemes also allow the narrowband analog signal to be transferred as a digital signal (i.e. as a quantized discrete-time signal) with a fixed bit rate, which can be transferred over an underlying digital transmission system, for example some line code. These are not modulation schemes in the conventional sense since they are not channel coding schemes, but should be considered as source coding schemes, and in some cases analog-to-digital conversion techniques. Analog-over-analog methods:

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Modulation • Pulse-amplitude modulation (PAM) • Pulse-width modulation (PWM) • Pulse-position modulation (PPM) Analog-over-digital methods: • Pulse-code modulation (PCM)

• • • •

• Differential PCM (DPCM) • Adaptive DPCM (ADPCM) Delta modulation (DM or Δ-modulation) Sigma-delta modulation (∑Δ) Continuously variable slope delta modulation (CVSDM), also called Adaptive-delta modulation (ADM) Pulse-density modulation (PDM)

Miscellaneous modulation techniques • The use of on-off keying to transmit Morse code at radio frequencies is known as continuous wave (CW) operation. • Adaptive modulation • Space modulation A method whereby signals are modulated within airspace, such as that used in Instrument landing systems.

References [1] Kundu Sudakshina (2010). Analog and Digital Communications (http:/ / books. google. com/ books?id=JKfTrRRHT5QC& pg=PA164). Pearson Education India. p. 163–184. ISBN 9788131731871. . [2] Ke-Lin Du and M. N. S. Swamy (2010). Wireless Communication Systems: From RF Subsystems to 4G Enabling Technologies (http:/ / books. google. com/ books?id=5dGjKLawsTkC& pg=PA188). Cambridge University Press. p. 188. ISBN 9780521114035. .

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Transmitter Transmitter In electronics and telecommunications a transmitter or radio transmitter is an electronic device which, with the aid of an antenna, produces radio waves. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating current, the antenna radiates radio waves. In addition to their use in broadcasting, transmitters are necessary component parts of many electronic devices that communicate by radio, such as cell phones, Wifi and Bluetooth enabled devices, garage door openers, two-way radios in aircraft, ships, and spacecraft, radar sets, and navigational beacons. The term transmitter is usually limited to equipment that generates radio waves for communication purposes; or radiolocation, such as radar and navigational transmitters. Generators of radio waves for heating or industrial purposes, such as microwave ovens or diathermy equipment, are not usually called transmitters even though they often have similar circuits. The term is popularly used more specifically to refer to transmitting equipment used for broadcasting, as in radio transmitter or television transmitter. This usage usually includes both the transmitter proper as described above, and the antenna, and often the building it is housed in. An unrelated use of the term is in industrial process control, where a "transmitter" is a device which converts measurements from a sensor into a signal, and sends it, usually via wires, to be received by some display or control device located a distance away.

Description

Antenna tower of Crystal Palace transmitter, London

A transmitter can be a separate piece of electronic equipment, or an electrical circuit within another electronic device. A transmitter and receiver combined in one unit is called a transceiver. The term transmitter is often abbreviated "XMTR" or "TX" in technical documents. The purpose of most transmitters is radio communication of information over a distance. The information is provided to the transmitter in the form of an electronic signal, such as an audio (sound) signal from a microphone, a video (TV) signal from a TV camera, or in wireless networking devices a digital signal from a computer. The transmitter combines the information signal to be carried with the radio frequency signal which generates the radio waves, which is often called the carrier. This process is called modulation. The information can be added to the carrier in several different ways, in different types of transmitter. In an amplitude modulation (AM) transmitter, the information is added to the radio signal by varying its amplitude (strength). In a frequency modulation (FM) transmitter, it is added by varying the radio signal's frequency slightly. Many other types of modulation are used.

Transmitter

Legal restrictions In most parts of the world, use of transmitters is strictly controlled by law because of the potential for dangerous interference with other radio transmissions (for example to emergency communications). Transmitters must be licensed by governments, under a variety of license classes depending on use: (broadcast, marine radio, Airband, Amateur etc.), and are restricted to certain frequencies and power levels. In some classes each transmitter is given a unique call sign consisting of a string of letters and numbers which must be used as an identifier in transmissions. The operator of the transmitter usually must hold a government license, such as a general radiotelephone operator license, which is obtained by passing a test demonstrating adequate technical and legal knowledge of safe radio operation. An exception is made allowing the unlicensed use of low-power short-range transmitters in devices such as wireless microphones, cordless telephones, walkie-talkies, Wifi and Bluetooth, garage door openers, and baby monitors. In the US, these fall under Part 15 of the Federal Communications Commission (FCC) regulations. Although they can be operated without a license, these devices still generally must be type-approved before sale.

How it works A radio transmitter is an electronic circuit which transforms electric power from a battery or electrical mains into a radio frequency alternating current, which reverses direction millions to billions of times per second. The energy in such a rapidly-reversing current can radiate off a conductor (the antenna) as electromagnetic waves (radio waves). The transmitter also "piggybacks" information, such as an audio or video signal, onto the radio frequency current to be carried by the radio waves. When they strike the antenna of a radio receiver, the waves excite similar (but less powerful) radio frequency currents in it. The radio receiver extracts the information from the received waves. A practical radio transmitter usually consists of these parts: • A power supply circuit to transform the input electrical power to the higher voltages needed to produce the required power output. • An electronic oscillator circuit to generate the radio frequency signal. This usually generates a sine wave of constant amplitude often called the carrier wave. In most modern transmitters this is a crystal oscillator in which the frequency is precisely controlled by the vibrations of a quartz crystal. • A modulator circuit to add the information to be transmitted to the carrier wave produced by the oscillator. This is done by varying some aspect of the carrier wave. The information is provided to the transmitter either in the form of an audio signal, which represents sound, a video signal, or for data in the form of a binary digital signal. • In an AM (amplitude modulation) transmitter the amplitude (strength) of the carrier wave is varied in proportion to the audio signal. • In an FM (frequency modulation) transmitter the frequency of the carrier is varied by the audio signal. • In an FSK (frequency-shift keying) transmitter, which transmits digital data, the frequency of the carrier is shifted between two frequencies which represent the two binary digits, 0 and 1. Many other types of modulation are also used. In large transmitters the oscillator and modulator together are often referred to as the exciter. • An RF power amplifier to increase the power of the signal, to increase the range of the radio waves. • An impedance matching (antenna tuner) circuit to match the impedance of the transmitter to the impedance of the antenna (or the transmission line to the antenna), to transfer power efficiently to the antenna. If these impedances are not equal, it causes a condition called standing waves, in which the power is reflected back from the antenna toward the transmitter, wasting power and sometimes overheating the transmitter. In higher frequency transmitters, in the UHF and microwave range, oscillators that operate stably at the output frequency cannot be built. In these transmitters the oscillator usually operates at a lower frequency, usually a submultiple of the output frequency, and this intermediate frequency (IF) is multiplied to get a signal at the output

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frequency by frequency multipliers.

History The first primitive radio transmitters (called Hertzian oscillators) were built by German physicist Heinrich Hertz in 1887 during his pioneering investigations of radio waves. These generated radio waves by a high voltage spark between two conductors. These spark-gap transmitters were used during the first three decades of radio (1887-1917), called the wireless telegraphy era. Short-lived competing techniques came into use after the turn of the century, such as the Alexanderson alternator and Poulsen Arc transmitters. But all these early technologies were replaced by vacuum tube transmitters in the 1920s, because they were inexpensive and produced continuous waves, which could be modulated to transmit audio (sound) using amplitude modulation (AM) and frequency modulation (FM). This made possible commercial radio broadcasting, which began about 1920. The development of radar before and during World War 2 was a great stimulus to the evolution of high frequency transmitters in the UHF and microwave ranges, using new devices such as the magnetron and traveling wave tube. In recent years, the need to conserve crowded radio spectrum bandwidth has driven the development of new types of transmitters such as spread spectrum.

Broadcast transmitters Power output In broadcasting and telecommunication, the part which contains the oscillator, modulator, and sometimes audio processor, is called the "exciter". Most transmitters use heterodyne principle, so they also have a frequency conversion units. Confusingly, the high-power amplifier which the exciter then feeds into is often called the "transmitter" by broadcast engineers. The final output is given as transmitter power output (TPO), although this is not what most stations are rated by. Effective radiated power (ERP) is used when calculating station coverage, even for most non-broadcast stations. It is the TPO, minus Commercial FM broadcasting transmitter at radio any attenuation or radiated loss in the line to the antenna, multiplied by station WDET-FM, Wayne State University, the gain (magnification) which the antenna provides toward the Detroit, USA. It broadcasts at 101.9 MHz with a radiated power of 48 kW. horizon. This antenna gain is important, because achieving a desired signal strength without it would result in an enormous electric utility bill for the transmitter, and a prohibitively expensive transmitter. For most large stations in the VHF- and UHF-range, the transmitter power is no more than 20% of the ERP. For VLF, LF, MF and HF the ERP is typically not determined separately. In most cases the transmission power found in lists of transmitters is the value for the output of the transmitter. This is only correct for omnidirectional aerials with a length of a quarter wavelength or shorter. For other aerial types there are gain factors, which can reach values until 50 for shortwave directional beams in the direction of maximum beam intensity. Since some authors take account of gain factors of aerials of transmitters for frequencies below 30 MHz and others not, there are often discrepancies of the values of transmitted powers.

Transmitter

Power supply Transmitters are sometimes fed from a higher voltage level of the power supply grid than necessary in order to improve security of supply. For example, the Allouis, Konstantynow and Roumoules transmitters are fed from the high-voltage network (110 kV in Alouis and Konstantynow, 150 kV in Roumoules) even though a power supply from the medium-voltage level of the power grid (about 20 kV) would be able to deliver enough power. [1] [2]

Cooling of final stages Low-power transmitters do not require special cooling equipment. Modern transmitters can be incredibly efficient, with efficiencies exceeding 98 percent. However, a broadcast transmitter with a megawatt power stage transferring 98% of that into the antenna can also be viewed as a 20 kilowatt electric heater. For medium-power transmitters, up to a few hundred watts, air cooling with fans is used. At power levels over a few kilowatts, the output stage is cooled by a forced liquid cooling system analogous to an automobile cooling system. Since the coolant directly touches the high-voltage anodes of the tubes, only distilled, deionised water or a special dielectric coolant can be used in the cooling circuit. This high-purity coolant is in turn cooled by a heat exchanger, where the second cooling circuit can use water of ordinary quality because it is not in contact with energized parts. Very-high-power tubes of small physical size may use evaporative cooling by water in contact with the anode. The production of steam allows a high heat flow in a small space.

Protection equipment The high voltages used in high power transmitters (up to 40 kV) require extensive protection equipment. Also, transmitters are exposed to damage from lightning. Transmitters may be damaged if operated without an antenna, so protection circuits must detect the loss of the antenna and switch off the transmitter immediately. Tube-based transmitters must have power applied in the proper sequence, with the filament voltage applied before the anode voltage, otherwise the tubes can be damaged. The output stage must be monitored for standing waves, which indicate that generated power is not being radiated but instead is being reflected back into the transmitter. Lightning protection is required between the transmitter and antenna. This consists of spark gaps and gas-filled surge arresters to limit the voltage that appears on the transmitter terminals. The control instrument that measures the voltage standing-wave ratio switches the transmitter off briefly if a higher voltage standing-wave ratio is detected after a lightning strike, as the reflections are probably due to lightning damage. If this does not succeed after several attempts, the antenna may be damaged and the transmitter should remain switched off. In some transmitting plants UV detectors are fitted in critical places, to switch off the transmitter if an arc is detected. The operating voltages, modulation factor, frequency and other transmitter parameters are monitored for protection and diagnostic purposes, and may be displayed locally and/or at a remote control room.

Building A commercial transmitter site will usually have a control building to shelter the transmitter components and control devices. This is usually a purely functional building, which may contain apparatus for both radio and television transmitters. To reduce transmission line loss the transmitter building is usually immediately adjacent to the antenna for VHF and UHF sites, but for lower frequencies it may be desirable to have a distance of a few score or several hundred metres between the building and the antenna. Some transmitting towers have enclosures built into the tower to house radio relay link transmitters or other, relatively low-power transmitters. A few transmitter buildings may include limited broadcasting facilities to allow a station to use the building as a backup studio in case of incapacitation of the main facility.

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Legal and regulatory aspects Since radio waves go over borders, international agreements control radio transmissions. In European countries like Germany often the national Post Office is the regulating authority. In the United States broadcast and industrial transmitters are regulated by the Federal Communications Commission (FCC). In Canada technical aspects of broadcast and radio transmitters are controlled by Industry Canada, but broadcast content is regulated separately by the Canadian Radio-television and Telecommunications Commission (CRTC). In Australia transmitters, spectrum, and content are controlled by the Australian Communications and Media Authority (ACMA). The International Telecommunication Union (ITU) helps managing the radio-frequency spectrum internationally.

Planning As in any costly project, the planning of a high power transmitter site requires great care. This begins with the location. A minimum distance, which depends on the transmitter frequency, transmitter power, and the design of the transmitting antennas, is required to protect people from the radio frequency energy. Antenna towers are often very tall and therefore flight paths must be evaluated. Sufficient electric power must be available for high power transmitters. Transmitters for long and medium wave require good grounding and soil of high electrical conductivity. Locations at the sea or in river valleys are ideal, but the flood danger must be considered. Transmitters for UHF are best on high mountains to improve the range (see radio propagation). The antenna pattern must be considered because it is costly to change the pattern of a long-wave or medium-wave antenna. Transmitting antennas for long and medium wave are usually implemented as a mast radiator. Similar antennas with smaller dimensions are used also for short wave transmitters, if these send in the round spray enterprise. For arranging radiation at free standing steel towers fastened planar arrays are used. Radio towers for UHF and TV transmitters can be implemented in principle as grounded constructions. Towers may be steel lattice masts or reinforced concrete towers with antennas mounted at the top. Some transmitting towers for UHF have high-altitude operating rooms and/or facilities such as restaurants and observation platforms, which are accessible by elevator. Such towers are usually called TV tower. For microwaves one frequently uses parabolic antennas. These can be set up for applications of radio relay links on transmitting towers for FM to special platforms. For example, large parabolic antennas ranging from 3 to 100 meters in diameter are necessary to pass on signals to television satellites and space vehicles. These plants, which can be used if necessary also as radio telescope, are established on free standing constructions, whereby there are also numerous special designs, like the radio telescope in Arecibo. Just as important as the planning of the construction and location of the transmitter is Antenna guyed tower how its output fits in with existing transmissions. Two transmitters cannot broadcast on the same frequency in the same area as this would cause co-channel interference. For a good example of how the channel planners have dovetailed different transmitters' outputs see Crystal Palace UHF TV channel allocations [3]. This reference also provides a good example of a grouped transmitter, in this case an A group. That is, all of its output is within the bottom third of the UK UHF television broadcast band. The other two groups (B and C/D) utilise the middle and top third of the band, see graph [4]. By replicating this grouping across the country (using different groups for adjacent transmitters), co-channel interference can be minimised, and in addition, those in marginal reception areas can use more efficient grouped receiving antennas. Unfortunately, in the UK, this carefully planned system has had to be compromised with the advent of digital broadcasting which (during the changeover period at least) requires yet more channel space, and consequently the additional digital broadcast

Transmitter channels cannot always be fitted within the transmitter's existing group. Thus many UK transmitters have become "wideband" with the consequent need for replacement of receiving antennas (see external links). Once the Digital Switch Over (DSO) occurs the plan is that most transmitters will revert to their original groups, source Ofcom July 2007 [5]. Further complication arises when adjacent transmitters have to transmit on the same frequency and under these circumstances the broadcast radiation patterns are attenuated in the relevant direction(s). A good example of this is in the United Kingdom, where the Waltham transmitting station broadcasts at high power on the same frequencies as the Sandy Heath transmitting station's high power transmissions, with the two being only 50 miles apart. Thus Waltham's antenna array [6] does not broadcast these two channels in the direction of Sandy Heath and vice versa. Where a particular service needs to have wide coverage, this is usually achieved by using multiple transmitters at different locations. Usually, these transmitters will operate at different frequencies to avoid interference where coverage overlaps. Examples include national broadcasting networks and cellular networks. In the latter, frequency switching is automatically done by the receiver as necessary, in the former, manual retuning is more common (though the Radio Data System is an example of automatic frequency switching in broadcast networks). Another system for extending coverage using multiple transmitters is quasi-synchronous transmission, but this is rarely used nowadays.

Main and relay (repeater) transmitters Transmitting stations are usually either classified as main stations or relay stations (also known as repeaters, translators or sometimes "transposers".) Main stations are defined as those that generate their own modulated output signal from a baseband (unmodulated) input. Usually main stations operate at high power and cover large areas. Relay stations (translators) take an already modulated input signal, usually by direct reception of a parent station off the air, and simply rebroadcast it on another frequency. Usually relay stations operate at medium or low power, and are used to fill in pockets of poor reception within, or at the fringe of, the service area of a parent main station. Note that a main station may also take its input signal directly off-air from another station, however this signal would be fully demodulated to baseband first, processed, and then remodulated for transmission.

Transmitters in culture Some cities in Europe, like Mühlacker, Ismaning, Langenberg, Kalundborg, Hörby and Allouis became famous as sites of powerful transmitters. For example, Goliath transmitter was a VLF transmitter of the German Navy during World War II located near Kalbe an der Milde in Saxony-Anhalt, Germany. Some transmitting towers like the radio tower Berlin or the TV tower Stuttgart have become landmarks of cities. Many transmitting plants have very high radio towers that are masterpieces of engineering. Having the tallest building in the world, the nation, the state/province/prefecture, city, etc., has often been considered something to brag about. Often, builders of high-rise buildings have used transmitter antennas to lay claim to having the tallest building. A historic example was the "tallest building" feud between the Chrysler Building and the Empire State Building in New York, New York. Some towers have an observation deck accessible to tourists. An example is the Ostankino Tower in Moscow, which was completed in 1967 on the 50th anniversary of the October Revolution to demonstrate the technical abilities of the Soviet Union. As very tall radio towers of any construction type are prominent landmarks, requiring careful planning and construction, and high-power transmitters especially in the long- and medium-wave ranges can be received over long distances, such facilities were often mentioned in propaganda. Other examples were the Deutschlandsender Herzberg/Elster and the Warsaw Radio Mast.

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Transmitter KVLY-TV's tower located near Blanchard, North Dakota was the tallest artificial structure in the world when it was completed in 1963. It was surpassed in 1974 by the Warszawa radio mast, but regained its title when the latter collapsed in 1991. It was surpassed by the Burj Khalifa skyscraper in early 2009, but the KVLY-TV mast is still the tallest transmitter.

Records • Tallest radio/television mast: • 1974–1991: Konstantynow for 2000 kW longwave transmitter, 646.38 m (2120 ft 8 in) • 1963–1974 and since 1991: KVLY Tower, 2,063 ft (628.8 m) • Highest power: • Longwave, Taldom transmitter, 2500 kW • Medium wave, Bolshakovo transmitter, 2500 kW • Highest transmission sites (Europe): • FM Pic du Aigu in Chamonix • MW Pic Blanc in Andorra

References [1] [2] [3] [4] [5] [6]

Long Waves, MCR, Roumoules station radio (http:/ / perso. orange. fr/ monte-carlo-radiodiffusion/ anglais/ olan. htm) Allouis - France Inter (http:/ / perso. orange. fr/ tvignaud/ am/ allouis/ allouis4. htm) http:/ / www. aerialsandtv. com/ crystalpalacetx. html#crystalpalaceschannels http:/ / www. aerialsandtv. com/ aerials. html#AerialGainCurves http:/ / www. ofcom. org. uk/ tv/ ifi/ tech/ dsodetails/ http:/ / www. aerialsandtv. com/ walthamtx. html#WalthamsTransmittingArray

External links • • • •

International Telecommunication Union (http://www.itu.int/net/home/index.aspx) Jim Hawkins' Radio and Broadcast Technology Page (http://hawkins.pair.com/radio.html) WCOV-TV's Transmitter Technical Website (http://www.wcov.com/technical/transmitter.html) Major UK television transmitters including change of group information, see Transmitter Planning section. (http:/ /www.aerialsandtv.com/digitalnationwide.html) • Details of UK digital television transmitters (http://www.wolfbane.com/ukdtt.htm) • Richard Moore's Anorak Zone Photo Gallery of UK TV and Radio transmission sites (http://www.the-moores. co.uk/MediaGallery/Default.aspx?directory=56)

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Antenna Antenna (radio) An antenna (or aerial) is an electrical device which couples radio waves in free space to an electrical current used by a radio receiver or transmitter. In reception, the antenna intercepts some of the power of an electromagnetic wave in order to produce a tiny voltage that the radio receiver can amplify. Alternatively, a radio transmitter will produce a large radio frequency current that may be applied to the terminals of the same antenna in order to convert it into an electromagnetic wave (radio wave) radiated into free space. Antennas are thus essential to the operation of all radio equipment, both transmitters and receivers. They are used in systems such as radio and television broadcasting, two-way radio, wireless LAN, mobile telephony, radar, and satellite communications. Typically an antenna consists of an arrangement of metallic conductors (or "elements") with an electrical connection (often through a transmission line) to the receiver or transmitter. A current forced through such a conductor by a radio transmitter will create an alternating magnetic field according to Ampère's law. Or the alternating magnetic field due to a distant radio transmitter will induce a voltage at the antenna terminals, according to Faraday's law, which is connected to the input of a receiver. In the so-called far field, at a considerable distance away from the antenna, the oscillating magnetic field is coupled with a similarly oscillating electric field; together these define an electromagnetic wave which is capable of propagating great distances.

Whip antenna on car

Half-wave dipole antenna

Light is one example of electromagnetic radiation, along with infrared and x-rays, while radio waves differ only in their much lower frequency (and much longer wavelength). Electronic circuits can operate at these lower frequencies, processing radio signals conducted through wires. But it is only through antennas that those radio frequency electrical signals are converted to (and from) propagating radio waves. Depending on the design of the antenna, radio waves can be sent toward and received from all directions ("omnidirectional"), whereas a directional or beam antenna is designed to operate in a particular direction. The first antennas were built in 1888 by Heinrich Hertz (1857–1894) in his pioneering experiments to prove the existence of electromagnetic waves Large parabolic antenna for predicted by the theory of James Clerk Maxwell. Hertz placed dipole communicating with spacecraft antennas at the focal point of parabolic reflectors for both transmitting and receiving. He published his work and installation drawings in Annalen der Physik und Chemie (vol. 36, 1889).

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Terminology The words antenna (plural: antennas[1] ) and aerial are used interchangeably; but usually a rigid metallic structure is termed an antenna and a wire format is called an aerial. In the United Kingdom and other British English speaking areas the term aerial is more common, even for rigid types. The noun aerial is occasionally written with a diaeresis mark—aërial—in recognition of the original spelling of the adjective aërial from which the noun is derived. The origin of the word antenna relative to wireless apparatus is attributed to Guglielmo Marconi. In 1895, while testing early radio apparatuses in the Swiss Alps at Salvan, Switzerland in the Mont Blanc region, Marconi experimented with early wireless equipment. A 2.5 meter long pole, along which was carried a wire, was used as a radiating and receiving aerial Rooftop directional antennas, typical for element. In Italian a tent pole is known as l'antenna centrale, and the pole use at VHF and UHF frequencies with a wire alongside it used as an aerial was simply called l'antenna. Until then wireless radiating transmitting and receiving elements were known simply as aerials or terminals. Marconi's use of the word antenna (Italian for pole) would become a popular term for what today is uniformly known as the antenna.[2] In common usage, the word antenna may refer broadly to an entire assembly including support structure, enclosure (if any), etc. in addition to the actual functional components. Especially at microwave frequencies, a receiving antenna may include not only the actual electrical antenna but an integrated preamplifier and/or mixer.

"Rabbit ears" dipole antenna for television reception

Cell phone base station antennas

Satellite link antenna used by Himalaya Television Nepal

Yagi antenna used for mobile military communications station, Dresden, Germany, 1955

Antenna (radio)

"Super Turnstile" type transmitting antenna for VHF low band television broadcasting station, Germany.

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Folded dipole antenna

Large Yagi antenna used by amateur radio hobbyist

A vertical mast radiator, Chapel Hill, North Carolina

Overview Antennas are required by any radio receiver or transmitter in order to couple its electrical connection to the electromagnetic field. Radio waves are electromagnetic waves which carry signals through the air (or through space) at the speed of light with almost no transmission loss. Radio transmitters and receivers are used to convey signals (information) in systems including broadcast (audio) radio, television, mobile telephones, wi-fi (WLAN) data networks, trunk lines and point-to-point communications links (telephone, data networks), satellite links, many remote controlled devices such as garage door openers, and wireless remote sensors, among many others. Radio waves are also used directly for measurements in technologies including RADAR, GPS, and radio astronomy. In each and every case, the transmitters and receivers involved require antennas, although these are sometimes hidden (such as the antenna inside an AM radio or inside a laptop computer equipped with wi-fi). According to their applications and technology available, antennas generally fall in one of two categories: 1. Omnidirectional or only weakly directional antennas which receive or radiate more or less in all directions. These are employed when the relative position of the other station is unknown or arbitrary. They are also used at lower frequencies where a directional antenna would be too large, or simply to cut costs in applications where a directional antenna isn't required. 2. Directional or beam antennas which are intended to preferentially radiate or receive in a particular direction or directional pattern. In common usage "omnidirectional" usually refers to all horizontal directions, typically with reduced performance in the direction of the sky or the ground (a truly isotropic radiator is not even possible). A "directional" antenna usually is intended to maximize its coupling to the electromagnetic field in the direction of the other station, or sometimes to cover a particular sector such as a 120° horizontal fan pattern in the case of a panel antenna at a cell site. One example of omnidirectional antennas is the very common vertical antenna or whip antenna consisting of a metal rod (often, but not always, a quarter of a wavelength long). A dipole antenna is similar but consists of two such conductors extending in opposite directions, with a total length that is often, but not always, a half of a wavelength long. Dipoles are typically oriented horizontally in which case they are weakly directional: signals are reasonably well radiated toward or received from all directions with the exception of the direction along the conductor itself; this region is called the antenna blind cone or null. Both the vertical and dipole antennas are simple in construction and relatively inexpensive. The dipole antenna, which is the basis for most antenna designs, is a balanced component, with equal but opposite voltages and currents

Antenna (radio) applied at its two terminals through a balanced transmission line (or to a coaxial transmission line through a so-called balun). The vertical antenna, on the other hand, is a monopole antenna. It is typically connected to the inner conductor of a coaxial transmission line (or a matching network); the shield of the transmission line is connected to ground. In this way, the ground (or any large conductive surface) plays the role of the second conductor of a dipole, thereby forming a complete circuit.[3] Since monopole antennas rely on a conductive ground, a so-called grounding structure may be employed in order to provide a better ground contact to the earth or which itself acts as a ground plane to perform that function regardless of (or in absence of) an actual contact with the earth. Antennas fancier than the dipole or vertical designs are usually intended to increase the directivity and consequently the gain of the antenna. This can be accomplished in many different ways leading to a plethora of antenna designs. The vast majority of designs are fed with a balanced line (unlike a monopole antenna) and are based on the dipole antenna with additional components (or elements) which increase its directionality. For instance, a phased array consists of two or more simple antennas which are connected together through an electrical network. This often involves a number of parallel dipole antennas with a certain spacing. Depending on the relative phase introduced by the network, the same combination of dipole antennas can operate as a "broadside array" (directional normal to a line connecting the elements) or as an "end-fire array" (directional along the line connecting the elements). Antenna arrays may employ any basic (omnidirectional or weakly directional) antenna type, such as dipole, loop or slot antennas. These elements are often identical. However a log-periodic dipole array consists of a number of dipole elements of different lengths in order to obtain a somewhat directional antenna having an extremely wide bandwidth: these are frequently used for television reception in fringe areas. The dipole antennas composing it are all considered "active elements" since they are all electrically connected together (and to the transmission line). On the other hand, a superficially similar dipole array, the Yagi-Uda Antenna (or simply "Yagi"), has only one dipole element with an electrical connection; the other so-called parasitic elements interact with the electromagnetic field in order to realize a fairly directional antenna but one which is limited to a rather narrow bandwidth. The Yagi antenna has similar looking parasitic dipole elements but which act differently due to their somewhat different lengths. There may be a number of so-called "directors" in front of the active element in the direction of propagation, and usually a single (but possibly more) "reflector" on the opposite side of the active element. Greater directionality can be obtained using beam-forming techniques such as a parabolic reflector or a horn. Since the size of a directional antenna depends on it being large compared to the wavelength, very directional antennas of this sort are mainly feasible at UHF and microwave frequencies. On the other hand, at low frequencies (such as AM broadcast) where a practical antenna must be much smaller than a wavelength, significant directionality isn't even possible. A vertical antenna or loop antenna small compared to the wavelength is typically used, with the main design challenge being that of impedance matching. With a vertical antenna a loading coil at the base of the antenna may be employed to cancel the reactive component of impedance; small loop antennas are tuned with parallel capacitors for this purpose. An antenna lead-in is the transmission line (or feed line) which connects the antenna to a transmitter or receiver. The antenna feed may refer to all components connecting the antenna to the transmitter or receiver, such as an impedance matching network in addition to the transmission line. In a so-called aperture antenna, such as a horn or parabolic dish, the "feed" may also refer to a basic antenna inside the entire system (normally at the focus of the parabolic dish or at the throat of a horn) which could be considered the one active element in that antenna system. A microwave antenna may also be fed directly from a waveguide in lieu of a (conductive) transmission line. An antenna counterpoise or ground plane is a structure of conductive material which improves or substitutes for the ground. It may be connected to or insulated from the natural ground. In a monopole antenna, this aids in the function of the natural ground, particularly where variations (or limitations) of the characteristics of the natural ground interfere with its proper function. Such a structure is normally connected to the return connection of an unbalanced transmission line such as the shield of a coaxial cable.

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Antenna (radio) An electromagnetic wave refractor in some aperture antennas is a component which due to its shape and position functions to selectively delay or advance portions of the electromagnetic wavefront passing through it. The refractor alters the spatial characteristics of the wave on one side relative to the other side. It can, for instance, bring the wave to a focus or alter the wave front in other ways, generally in order to maximize the directivity of the antenna system. This is the radio equivalent of an optical lens. An antenna coupling network is a passive network (generally a combination of inductive and capacitive circuit elements) used for impedance matching in between the antenna and the transmitter or receiver. This may be used to improve the standing wave ratio in order to minimize losses in the transmission line (especially at higher frequencies and/or over longer distances) and to present the transmitter or receiver with a standard resistive impedance (such as 75 ohms) that it expects to see for optimum operation.

Reciprocity It is a fundamental property of antennas that the characteristics of an antenna described in the next section, such as gain, radiation pattern, impedance, bandwidth, resonant frequency and polarization, are the same whether the antenna is transmitting or receiving. For example, the "receiving pattern" (sensitivity as a function of direction) of an antenna when used for reception is identical to the radiation pattern of the antenna when it is driven and functions as a radiator. This is a consequence of the reciprocity theorem of electromagnetics. Therefore in discussions of antenna properties no distinction is usually made between receiving and transmitting terminology, and the antenna can be viewed as either transmitting or receiving, whichever is more convenient. A necessary condition for the above reciprocity property is that the materials in the antenna and transmission medium are linear and reciprocal. Reciprocal (or bilateral) means that the material has the same response to an electric or magnetic field, or a current, in one direction, as it has to the field or current in the opposite direction. Most materials used in antennas meet these conditions, but some microwave antennas use high-tech components such as isolators and circulators, made of nonreciprocal materials such as ferrite or garnet. These can be used to give the antenna a different behavior on receiving than it has on transmitting, which can be useful in applications like radar.

Parameters Antennas are characterized by a number of performance measures which a user would be concerned with in selecting or designing an antenna for a particular application. Chief among these relate to the directional characteristics (as depicted in the antenna's radiation pattern) and the resulting gain. Even in omnidirectional (or weakly directional) antennas, the gain can often be increased by concentrating more of its power in the horizontal directions, sacrificing power radiated toward the sky and ground. The antenna's power gain (or simply "gain") also takes into account the antenna's efficiency, and is often the primary figure of merit. Resonant antennas are expected to be used around a particular resonant frequency; an antenna must therefore be built or ordered to match the frequency range of the intended application. A particular antenna design will present a particular feedpoint impedance. While this may affect the choice of an antenna, an antenna's impedance can also be adapted to the desired impedance level of a system using an matching network while maintaining the other characteristics (except for a possible loss of efficiency). Although these parameters can be measured in principle, such measurements are difficult and require very specialized equipment. Beyond tuning a transmitting antenna using an SWR meter, the typical user will depend on theoretical predictions based on the antenna design and/or on claims of a vendor. An antenna transmits and receives radio waves with a particular polarization which can be reoriented by tilting the axis of the antenna in many (but not all) cases. The physical size of an antenna is often a practical issue, particularly at lower frequencies (longer wavelengths). Highly directional antennas need to be significantly larger than the wavelength. Resonant antennas use a conductor, or a pair of conductors, each of which is about one quarter of the

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Antenna (radio) wavelength in length. Antennas that are required to be very small compared to the wavelength sacrifice efficiency and cannot be very directional. Fortunately at higher frequencies (UHF, microwaves) trading off performance to obtain a smaller physical size is usually not required.

Resonant antennas While there are broadband designs for antennas, the vast majority of antennas are based on the half-wave dipole which has a particular resonant frequency. At its resonant frequency, the wavelength (given by the speed of light divided by the resonant frequency) is slightly over twice the length of the half-wave dipole (thus the name). The quarter-wave vertical antenna consists of one arm of a half-wave dipole, with the other arm replaced by a connection to ground or an equivalent ground plane (or counterpoise). A Yagi-Uda array consists of a number of resonant dipole elements, only one of which is directly connected to the transmission line. The quarter-wave elements of a dipole or vertical antenna imitate a series-resonant electrical element, since if they are driven at the resonant frequency a standing wave is created with the peak current at the feedpoint and the peak voltage at the far end. A common misconception is that the ability of a resonant antenna to transmit (or receive) fails at frequencies far from the resonant frequency. The reason a dipole antenna needs to be used at the resonant frequency has to do with the impedance match between the antenna and the transmitter or receiver (and its transmission line). For instance, a dipole using a fairly thin conductor[4] will have a purely resistive feedpoint impedance of about 63 ohms at its design frequency. Feeding that antenna with a current of 1 ampere will require 63 volts of RF, and the antenna will radiate 63 watts (ignoring losses) of radio frequency power. If that antenna is driven with 1 ampere at a frequency 20% higher, it will still radiate as efficiently but in order to do that about 200 volts would be required due to the change in the antenna's impedance which is now largely reactive (voltage out of phase with the current). A typical transmitter would not find that impedance acceptable and would deliver much less than 63 watts to it; the transmission line would be operating at a high (poor) standing wave ratio. But using an appropriate matching network, that large reactive impedance could be converted to a resistive impedance satisfying the transmitter and accepting the available power of the transmitter. This principle is used to construct vertical antennas substantially shorter than the 1/4 wavelength at which the antenna is resonant. By adding an inductance in series with the vertical antenna (a so-called loading coil) the capacitative reactance of this antenna can be cancelled leaving a pure resistance which can then be matched to the transmission line. Sometimes the resulting resonant frequency of such a system (antenna plus matching network) is described using the construct of "electrical length" and the use of a shorter antenna at a lower frequency than its resonant frequency is termed "electrical lengthening". For example, at 30 MHz (wavelength = 10 meters) a true resonant monopole would be almost 2.5 meters (1/4 wavelength) long, and using an antenna only 1.5 meters tall would require the addition of a loading coil. Then it may be said that the coil has "lengthened" the antenna to achieve an "electrical length" of 2.5 meters, that is, 1/4 wavelength at 30 MHz where the combined system now resonates. However, the resulting resistive impedance achieved will be quite a bit lower than the impedance of a resonant monopole, likely requiring further impedance matching. Current and voltage distribution The antenna conductors have the lowest feedpoint impedance at the resonant frequency where they are just under 1/4 wavelength long; two such conductors in line fed differentially thus realizes the familiar "half-wave dipole". When fed with an RF current at the resonant frequency, the quarter wave element contains a standing wave with the voltage and current largely (but not exactly) in phase quadrature, as would be obtained using a quarter wave stub of transmission line. The current reaches a minimum at the end of the element (where it has nowhere to go!) and is maximum at the feedpoint. The voltage, on the other hand, is the greatest at the end of the conductor and reaches a minimum (but not zero) at the feedpoint. Making the conductor shorter or longer than 1/4 wavelength means that the voltage pattern reaches its minimum somewhere beyond the feedpoint, so that the feedpoint has a higher voltage and thus sees a higher impedance, as we have noted. Since that voltage pattern is almost in phase quadrature with the

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Antenna (radio) current, the impedance seen at the feedpoint is not only much higher but mainly reactive. It can be seen that if such an element is resonant at f0 to produce such a standing wave pattern, then feeding that element with 3f0 (whose wavelength is 1/3 that of f0) will lead to a standing wave pattern in which the voltage is likewise a minimum at the feedpoint (and the current at a maximum there). Thus, an antenna element is also resonant when its length is 3/4 of a wavelength (3/2 wavelength for a complete dipole). This is true for all odd multiples of 1/4 wavelength, where the feedpoint impedance is purely resistive, though larger than the resistive impedance of the 1/4 wave element. Although such an antenna is resonant and works perfectly well at the higher frequency, the antenna radiation pattern is also altered compared to the half-wave dipole. The use of a monopole or dipole at odd multiples of the fundamental resonant frequency, however, does not extend to even multiples (thus a 1/2 wavelength monopole or 1 wavelength dipole). Now the voltage standing wave is at its peak at the feedpoint, while that of the current (which must be zero at the end of the conductor) is at a minimum (but not exactly zero). The antenna is anti-resonant at this frequency. Although the reactance at the feedpoint can be cancelled using such an element length, the feedpoint impedance is very high, and is highly dependent on the diameter of the conductor (which makes only a small difference at the actual resonant frequency). Such an antenna does not match the much lower characteristic impedance of available transmission lines, and is generally not used. However some equipment where transmission lines are not involved which desire a high driving point impedance may take advantage of this anti-resonance. Bandwidth Although a resonant antenna has a purely resistive feedpoint impedance at a particular frequency, many (if not most) applications require using an antenna over a range of frequencies. An antenna's bandwidth specifies the range of frequencies over which its performance does not suffer due a poor impedance match. Also in the case of a Yagi-Uda array, the use of the antenna very far away from its design frequency reduces the antenna's directivity, thus reducing the usable bandwidth regardless of impedance matching. Except for the latter concern, the resonant frequency of a resonant antenna can always be altered by adjusting a suitable matching network. To do this efficiently one would require remotely adjusting a matching network at the site of the antenna, since simply adjusting a matching network at the transmitter (or receiver) would leave the transmission line with a poor standing wave ratio. Instead, it is often desired to have an antenna whose impedance does not vary so greatly over a certain bandwidth. It turns out that the amount of reactance seen at the terminals of a resonant antenna when the frequency is shifted, say, by 5%, depends very much on the diameter of the conductor used. A long thin wire used as a half-wave dipole (or quarter wave monopole) will have a reactance significantly greater than the resistive impedance it has at resonance, leading to a poor match and generally unacceptable performance. Making the element using a tube of a diameter perhaps 1/50 of its length, however, results in a reactance at this altered frequency which is not so great, and a much less serious mismatch which will only modestly damage the antenna's net performance. Thus rather thick tubes are typically used for the solid elements of such antennas, including Yagi-Uda arrays. Rather than just using a thick tube, there are similar techniques used to the same effect such as replacing thin wire elements with cages to simulate a thicker element. This widens the bandwidth of the resonance. On the other hand, amateur radio antennas need to operate over several bands which are widely separated from each other. This can often be accomplished simply by connecting resonant elements for the different bands in parallel. Most of the transmitter's power will flow into the resonant element while the others present a high (reactive) impedance and draw little current from the same voltage. A popular solution uses so-called traps consisting of parallel resonant circuits which are strategically placed in breaks along each antenna element. When used at one particular frequency band the trap presents a very high impedance (parallel resonance) effectively truncating the element at that length, making it a proper resonant antenna. At a lower frequency the trap allows the full length of the element to be employed, albeit with a shifted resonant frequency due to the inclusion of the trap's net reactance at that lower frequency.

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The bandwidth characteristics of a resonant antenna element can be characterized according to its Q, just as one uses to characterize the sharpness of an L-C resonant circuit. However it is often assumed that there is an advantage in an antenna having a high Q. After all, Q is short for "quality factor" and a low Q typically signifies excessive loss (due to unwanted resistance) in a resonant L-C circuit. However this understanding does not apply to resonant antennas where the resistance involved is the radiation resistance, a desired quantity which removes energy from the resonant element in order to radiate it (the purpose of an antenna, after all!). The Q is a measure of the ratio of reactance to resistance, so with a fixed radiation resistance (an element's radiation resistance is almost independent of its diameter) a greater reactance off-resonance corresponds to the poorer bandwidth of a very thin conductor. The Q of such a narrowband antenna can be as high as 15. On the other hand a thick element presents less reactance at an off-resonant frequency, and consequently a Q as low as 5. These two antennas will perform equivalently at the resonant frequency, but the second antenna will perform over a bandwidth 3 times as wide as the "hi-Q" antenna consisting of a thin conductor.

Gain Gain is a parameter which measures the degree of directivity of the antenna's radiation pattern. A high-gain antenna will preferentially radiate in a particular direction. Specifically, the antenna gain, or power gain of an antenna is defined as the ratio of the intensity (power per unit surface) radiated by the antenna in the direction of its maximum output, at an arbitrary distance, divided by the intensity radiated at the same distance by a hypothetical isotropic antenna. The gain of an antenna is a passive phenomenon - power is not added by the antenna, but simply redistributed to provide more radiated power in a certain direction than would be transmitted by an isotropic antenna. An antenna designer must take into account the application for the antenna when determining the gain. High-gain antennas have the advantage of longer range and better signal quality, but must be aimed carefully in a particular direction. Low-gain antennas have shorter range, but the orientation of the antenna is relatively inconsequential. For example, a dish antenna on a spacecraft is a high-gain device that must be pointed at the planet to be effective, whereas a typical Wi-Fi antenna in a laptop computer is low-gain, and as long as the base station is within range, the antenna can be in any orientation in space. It makes sense to improve horizontal range at the expense of reception above or below the antenna. Thus most antennas labelled "omnidirectional" really have some gain.[5] In practice, the half-wave dipole is taken as a reference instead of the isotropic radiator. The gain is then given in dBd (decibels over dipole): NOTE: 0 dBd = 2.15 dBi. It is vital in expressing gain values that the reference point be included. Failure to do so can lead to confusion and error.

Effective area or aperture The effective area or effective aperture of a receiving antenna expresses the portion of the power of a passing electromagnetic wave which it delivers to its terminals, expressed in terms of an equivalent area. For instance, if a radio wave passing a given location has a flux of 1 pW / m2 (10−12 watts per square meter) and an antenna has an effective area of 12 m2, then the antenna would deliver 12 pW of RF power to the receiver (30 microvolts rms at 75 ohms). Since the receiving antenna is not equally sensitive to signals received from all directions, the effective area is a function of the direction to the source. Due to reciprocity (discussed above) the gain of an antenna used for transmitting must be proportional to its effective area when used for receiving. Consider an antenna with no loss, that is, one whose electrical efficiency is 100%. It can be shown that its effective area averaged over all directions must be equal to λ2/4π, the wavelength squared divided by 4π. Gain is defined such that the average gain over all directions for an antenna with 100% electrical efficiency is equal to 1. Therefore the effective area Aeff in terms of the gain G in a given direction is given by:

Antenna (radio)

For an antenna with an efficiency of less than 100%, both the effective area and gain are reduced by that same amount. Therefore the above relationship between gain and effective area still holds. These are thus two different ways of expressing the same quantity. Aeff is especially convenient when computing the power that would be received by an antenna of a specified gain, as illustrated by the above example.

Radiation pattern The radiation pattern of an antenna is a plot of the relative field strength of the radio waves emitted by the antenna at different angles. It is typically represented by a three dimensional graph, or polar plots of the horizontal and vertical cross sections. The pattern of an ideal isotropic antenna, which radiates equally in all directions, would look like a sphere. Many nondirectional antennas, such as monopoles and dipoles, emit equal power in all horizontal directions, with the power dropping off at higher and lower angles; this is called an omnidirectional pattern and when plotted looks like a torus or donut. The radiation of many antennas shows a pattern of maxima or "lobes" at various angles, separated by "nulls", angles where the radiation falls polar plots of the horizontal cross sections of a to zero. This is because the radio waves emitted by different parts of (virtual) Yagi-Uda-antenna. Outline connects the antenna typically interfere, causing maxima at angles where the points with 3db field power compared to an ISO radio waves arrive at distant points in phase, and zero radiation at other emitter. angles where the radio waves arrive out of phase. In a directional antenna designed to project radio waves in a particular direction, the lobe in that direction is designed larger than the others and is called the "main lobe". The other lobes usually represent unwanted radiation and are called "sidelobes". The axis through the main lobe is called the "principle axis" or "boresight axis".

Impedance As an electro-magnetic wave travels through the different parts of the antenna system (radio, feed line, antenna, free space) it may encounter differences in impedance (E/H, V/I, etc.). At each interface, depending on the impedance match, some fraction of the wave's energy will reflect back to the source,[6] forming a standing wave in the feed line. The ratio of maximum power to minimum power in the wave can be measured and is called the standing wave ratio (SWR). A SWR of 1:1 is ideal. A SWR of 1.5:1 is considered to be marginally acceptable in low power applications where power loss is more critical, although an SWR as high as 6:1 may still be usable with the right equipment. Minimizing impedance differences at each interface (impedance matching) will reduce SWR and maximize power transfer through each part of the antenna system. Complex impedance of an antenna is related to the electrical length of the antenna at the wavelength in use. The impedance of an antenna can be matched to the feed line and radio by adjusting the impedance of the feed line, using the feed line as an impedance transformer. More commonly, the impedance is adjusted at the load (see below) with an antenna tuner, a balun, a matching transformer, matching networks composed of inductors and capacitors, or matching sections such as the gamma match.

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Efficiency Efficiency of a transmitting antenna is the ratio of power actually radiated (in all directions) to the power absorbed by the antenna terminals. The power supplied to the antenna terminals which is not radiated is converted into heat. This is usually through loss resistance in the antenna's conductors, but can also be due to dielectric or magnetic core losses in antennas (or antenna systems) using such components. Such loss effectively robs power from the transmitter, requiring a stronger transmitter in order to transmit a signal of a given strength. For instance, if a transmitter delivers 100 W into an antenna having an efficiency of 80%, then the antenna will radiate 80 W as radio waves and produce 20 W of heat. In order to radiate 100 W of power, one would need to use a transmitter capable of supplying 125 W to the antenna. Note that antenna efficiency is a separate issue from impedance matching, which may also reduce the amount of power radiated using a given transmitter. If an SWR meter reads 150 W of incident power and 50 W of reflected power, that means that 100 W have actually been absorbed by the antenna (ignoring transmission line losses). How much of that power has actually been radiated cannot be directly determined through electrical measurements at (or before) the antenna terminals, but would require (for instance) careful measurement of field strength. Fortunately the loss resistance of antenna conductors such as aluminum rods can be calculated and the efficiency of an antenna using such materials predicted. However loss resistance will generally affect the feedpoint impedance, adding to its resistive (real) component. That resistance will consist of the sum of the radiation resistance Rr and the loss resistance Rloss. If an rms current I is delivered to the terminals of an antenna, then a power of I2Rr will be radiated and a power of I2Rloss will be lost as heat. Therefore the efficiency of an antenna is equal to Rr / (Rr + Rloss). Of course only the total resistance Rr + Rloss can be directly measured. According to reciprocity, the efficiency of an antenna, when used as a receiving antenna, is identical to the efficiency as defined above. The power that an antenna will deliver to a receiver (with a proper impedance match) is reduced by the same amount. However often in a receiving application, inefficiency of an antenna may be of lesser consequence or even of no consequence, notably at lower frequencies or when used to receive signals in "crowded" bands. That is true in cases where the received signal competes not against receiver noise, but against atmospheric noise or Amount of atmospheric noise at various elevation angles versus frequency according interference received by the antenna CCIR 322 itself. The loss within the antenna will affect the intended signal and the noise/interference identically, leading to no reduction in signal to noise ratio (SNR). According to the graph shown illustrating the frequency dependence of atmospheric and man-made noise, one can see that using a receiving antenna with an efficiency of only 10% at frequencies below 10 MHz will still supply a signal to the receiver which includes noise well above the thermal limit. A decent RF amplifier in the receiver will not significantly add to this noise level or reduce the resulting SNR. This is fortunate, since antennas at lower frequencies which are not rather large (a good fraction of a wavelength in size) are inevitably inefficient (due to the small radiation resistance Rr of small antennas). Most AM broadcast radios

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Antenna (radio) (except for car radios) take advantage of this principle by including a small loop antenna for reception which has an extremely poor efficiency. Using such an inefficient antenna at this low frequency (530–1650 kHz) thus has little effect on the receiver's net performance, but simply requires greater amplification by the receiver's electronics. Contrast this tiny component to the massive and very tall towers used at AM broadcast stations for transmitting at the very same frequency, where every percentage point of reduced antenna efficiency entails a substantial cost. The definition of antenna gain or power gain already includes the effect of the antenna's efficiency. Therefore if one is trying to radiate a signal toward a receiver using a transmitter of a given power, one need only compare the gain of various antennas rather than considering the efficiency as well. This is likewise true for a receiving antenna at very high (especially microwave) frequencies, where the point is to receive a signal which is strong compared to the receiver's noise temperature. However in the case of a directional antenna used for receiving signals with the intention of rejecting interference from different directions, one is no longer concerned with the antenna efficiency, as discussed above. In this case, rather than quoting the antenna gain, one would be more concerned with the directive gain which does not include the effect of antenna (in)efficiency. The directive gain of an antenna can be computed from the published gain divided by the antenna's efficiency.

Polarization The polarization of an antenna is the orientation of the electric field (E-plane) of the radio wave with respect to the Earth's surface and is determined by the physical structure of the antenna and by its orientation. It has nothing in common with antenna directionality terms: "horizontal", "vertical" and "circular". Thus, a simple straight wire antenna will have one polarization when mounted vertically, and a different polarization when mounted horizontally. "Electromagnetic wave polarization filters" are structures which can be employed to act directly on the electromagnetic wave to filter out wave energy of an undesired polarization and to pass wave energy of a desired polarization. Reflections generally affect polarization. For radio waves the most important reflector is the ionosphere - signals which reflect from it will have their polarization changed unpredictably. For signals which are reflected by the ionosphere, polarization cannot be relied upon. For line-of-sight communications for which polarization can be relied upon, it can make a large difference in signal quality to have the transmitter and receiver using the same polarization; many tens of dB difference are commonly seen and this is more than enough to make the difference between reasonable communication and a broken link. Polarization is largely predictable from antenna construction but, especially in directional antennas, the polarization of side lobes can be quite different from that of the main propagation lobe. For radio antennas, polarization corresponds to the orientation of the radiating element in an antenna. A vertical omnidirectional WiFi antenna will have vertical polarization (the most common type). An exception is a class of elongated waveguide antennas in which vertically placed antennas are horizontally polarized. Many commercial antennas are marked as to the polarization of their emitted signals. Polarization is the sum of the E-plane orientations over time projected onto an imaginary plane perpendicular to the direction of motion of the radio wave. In the most general case, polarization is elliptical, meaning that the polarization of the radio waves varies over time. Two special cases are linear polarization (the ellipse collapses into a line) and circular polarization (in which the two axes of the ellipse are equal). In linear polarization the antenna compels the electric field of the emitted radio wave to a particular orientation. Depending on the orientation of the antenna mounting, the usual linear cases are horizontal and vertical polarization. In circular polarization, the antenna continuously varies the electric field of the radio wave through all possible values of its orientation with regard to the Earth's surface. Circular polarizations, like elliptical ones, are classified as right-hand polarized or left-hand polarized using a "thumb in the direction of the propagation" rule. Optical researchers use the same rule of thumb, but pointing it in the direction of the emitter, not in the direction of propagation, and so are opposite to radio engineers' use.

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Antenna (radio) In practice, regardless of confusing terminology, it is important that linearly polarized antennas be matched, lest the received signal strength be greatly reduced. So horizontal should be used with horizontal and vertical with vertical. Intermediate matchings will lose some signal strength, but not as much as a complete mismatch. Transmitters mounted on vehicles with large motional freedom commonly use circularly polarized antennas so that there will never be a complete mismatch with signals from other sources.

Impedance matching Maximum power transfer requires matching the impedance of an antenna system (as seen looking into the transmission line) to the complex conjugate of the impedance of the receiver or transmitter. In the case of a transmitter, however, the desired matching impedance might not correspond to the dynamic output impedance of the transmitter as analyzed as a source impedance but rather the design value (typically 50 ohms) required for efficient and safe operation of the transmitting circuitry. The intended impedance is normally resistive but a transmitter (and some receivers) may have additional adjustments to cancel a certain amount of reactance in order to "tweak" the match. When a transmission line is used in between the antenna and the transmitter (or receiver) one generally would like an antenna system whose impedance is resistive and near the characteristic impedance of that transmission line in order to minimize the standing wave ratio (SWR) and the increase in transmission line losses it entails, in addition to supplying a good match at the transmitter or receiver itself. Antenna tuning generally refers to cancellation of any reactance seen at the antenna terminals, leaving only a resistive impedance which might or might not be exactly the desired impedance (that of the transmission line). Although an antenna may be designed to have a purely resistive feedpoint impedance (such as a dipole 97% of a half wavelength long) this might not be exactly true at the frequency that it is eventually used at. In some cases the physical length of the antenna can be "trimmed" to obtain a pure resistance. On the other hand, the addition of a series inductance or parallel capacitance can be used to cancel a residual capacitative or inductive reactance, respectively. In some cases this is done in a more extreme manner, not simply to cancel a small amount of residual reactance, but to resonate an antenna whose resonance frequency is quite different than the intended frequency of operation. For instance, a "whip antenna" can be made significantly shorter than 1/4 wavelength long, for practical reasons, and then resonated using a so-called loading coil. This physically large inductor at the base of the antenna has an inductive reactance which is the opposite of the capacitative reactance that such a vertical antenna has at the desired operating frequency. The result is a pure resistance seen at feedpoint of the loading coil; unfortunately that resistance is somewhat lower than would be desired to match commercial coax. So an additional problem beyond canceling the unwanted reactance is of matching the remaining resistive impedance to the characteristic impedance of the transmission line. In principle this can always be done with a transformer, however the turns ratio of a transformer is not adjustable. A general matching network with at least two adjustments can be made to correct both components of impedance. Matching networks using discrete inductors and capacitors will have losses associated with those components, and will have power restrictions when used for transmitting. Avoiding these difficulties, commercial antennas are generally designed with fixed matching elements and/or feeding strategies to get an approximate match to standard coax, such as 50 or 75 Ohms. Antennas based on the dipole (rather than vertical antennas) should include a balun in between the transmission line and antenna element, which may be integrated into any such matching network. Another extreme case of impedance matching occurs when using a small loop antenna (usually, but not always, for receiving) at a relatively low frequency where it appears almost as a pure inductor. Resonating such an inductor with a capacitor at the frequency of operation not only cancels the reactance but greatly magnifies the very small radiation resistance of such a loop. This is implemented in most AM broadcast receivers, with a small ferrite loop antenna resonated by a capacitor which is varied along with the receiver tuning in order to maintain resonance over the AM broadcast band

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Basic antenna models There are many variations of antennas. Below are a few basic models. More can be found in Category:Radio frequency antenna types. • The isotropic radiator is a purely theoretical antenna that radiates equally in all directions. It is considered to be a point in space with no dimensions and no mass. This antenna cannot physically exist, but is useful as a theoretical model for comparison with all other antennas. Most antennas' gains are measured with reference to an isotropic radiator, and are rated in dBi (decibels with respect to an isotropic radiator).

Typical US multiband TV antenna (aerial)

• The dipole antenna is simply two wires pointed in opposite directions arranged either horizontally or vertically, with one end of each wire connected to the radio and the other end hanging free in space. Since this is the simplest practical antenna, it is also used as a reference model for other antennas; gain with respect to a dipole is labeled as dBd. Generally, the dipole is considered to be omnidirectional in the plane perpendicular to the axis of the antenna, but it has deep nulls in the directions of the axis. Variations of the dipole include the folded dipole, the half wave antenna, the ground plane antenna, the whip, and the J-pole. • The Yagi-Uda antenna is a directional variation of the dipole with parasitic elements added which are functionality similar to adding a reflector and lenses (directors) to focus a filament light bulb. • The random wire antenna is simply a very long (at least one quarter wavelength) wire with one end connected to the radio and the other in free space, arranged in any way most convenient for the space available. Folding will reduce effectiveness and make theoretical analysis extremely difficult. (The added length helps more than the folding typically hurts.) Typically, a random wire antenna will also require an antenna tuner, as it might have a random impedance that varies non-linearly with frequency. • The horn antenna is used where high gain is needed, the wavelength is short (microwave) and space is not an issue. Horns can be narrow band or wide band, depending on their shape. A horn can be built for any frequency, but horns for lower frequencies are typically impractical. Horns are also frequently used as reference antennas. • The parabolic antenna consists of an active element at the focus of a parabolic reflector to reflect the waves into a plane wave. Like the horn it is used for high gain, microwave applications, such as satellite dishes. • The patch antenna consists mainly of a square conductor mounted over a groundplane. Another example of a planar antenna is the tapered slot antenna (TSA), as the Vivaldi-antenna.

Practical antennas Although any circuit can radiate if driven with a signal of high enough frequency, most practical antennas are specially designed to radiate efficiently at a particular frequency. An example of an inefficient antenna is the simple Hertzian dipole antenna, which radiates over wide range of frequencies and is useful for its small size. A more efficient variation of this is the half-wave dipole, which radiates with high efficiency when the signal wavelength is twice the electrical length of the antenna. One of the goals of antenna design is to minimize the reactance of the device so that it appears as a resistive load. An "antenna

"Rabbit ears" set-top antenna

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inherent reactance" includes not only the distributed reactance of the active antenna but also the natural reactance due to its location and surroundings (as for example, the capacity relation inherent in the position of the active antenna relative to ground). Reactance can be eliminated by operating the antenna at its resonant frequency, when its capacitive and inductive reactances are equal and opposite, resulting in a net zero reactive current. If this is not possible, compensating inductors or capacitors can instead be added to the antenna to cancel its reactance as far as the source is concerned. Once the reactance has been eliminated, what remains is a pure resistance, which is the sum of two parts: the ohmic resistance of the conductors, and the radiation resistance. Power absorbed by the ohmic resistance becomes waste heat, and that absorbed by the radiation resistance becomes radiated electromagnetic energy. The greater the ratio of radiation resistance to ohmic resistance, the more efficient the antenna.

Effect of ground Antennas are typically used in an environment where other objects are present that may have an effect on their performance. Height above ground has a very significant effect on the radiation pattern of some antenna types. At frequencies used in antennas, the ground behaves mainly as a dielectric. The conductivity of ground at these frequencies is negligible. When an electromagnetic wave arrives at the surface of an object, two waves are created: one enters the dielectric and the other is reflected. If the object is a conductor, the transmitted wave is negligible and the reflected wave has almost the same amplitude as the incident one. When the object is a dielectric, the fraction reflected depends (among others things) on the angle of incidence. When the angle of incidence is small (that is, the wave arrives almost perpendicularly) most of the energy traverses the surface and very little is reflected. When the angle of incidence is near 90° (grazing incidence) almost all the wave is reflected. Most of the electromagnetic waves emitted by an antenna to the ground below the antenna at moderate (say < 60°) angles of incidence enter the earth and are absorbed (lost). But waves emitted to the ground at grazing angles, far from the antenna, are almost totally reflected. At grazing angles, the ground behaves as a mirror. Quality of reflection depends on the nature of the surface. When the irregularities of the surface are smaller than the wavelength reflection is good. This means that the receptor "sees" the real antenna and, under the ground, the image of the antenna reflected by the ground. If the ground has irregularities, the image will appear fuzzy. If the receiver is placed at some height above the ground, waves reflected by ground will travel a little longer distance to arrive to the receiver than direct waves. The distance will be the same only if the receiver is close to ground. The wave reflected by earth can be considered as emitted by the image antenna

In the drawing at right, we have drawn the angle image is .

far bigger than in reality. Distance between the antenna and its

The situation is a bit more complex because the reflection of electromagnetic waves depends on the polarization of the incident wave. As the refractive index of the ground (average value ) is bigger than the refractive index of the air ( ), the direction of the component of the electric field parallel to the ground inverses at the reflection. This is equivalent to a phase shift of radians or 180°. The vertical component of the electric field reflects without changing direction. This sign inversion of the parallel component and the non-inversion of the perpendicular component would also happen if the ground were a good electrical conductor.

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This means that a receiving antenna "sees" the image antenna with the current in the same direction if the antenna is vertical or with the current inverted if the antenna is horizontal. For a vertical polarized emission antenna the far electric field of the electromagnetic wave produced by the direct ray plus the reflected ray is: The vertical component of the current reflects without changing sign. The horizontal component reverses sign at reflection.

The sign inversion for the parallel field case just changes a cosine to a sine:

In these two equations: • • • •

is the electrical field radiated by the antenna if there were no ground. is the wave number. is the wave length. is the distance between antenna and its image (twice the height of the center of the antenna).

For emitting and receiving antenna situated near the ground (in a building or on a mast) far from each other, distances traveled by direct and reflected rays are nearly the same. There is no induced phase shift. If the emission is polarized vertically the two fields (direct and Radiation patterns of antennas and their images reflected) add and there is maximum of received signal. If the emission reflected by the ground. At left the polarization is is polarized horizontally the two signals subtracts and the received vertical and there is always a maximum for signal is minimum. This is depicted in the image at right. In the case of . If the polarization is horizontal as at right, there is always a zero for . vertical polarization, there is always a maximum at earth level (left pattern). For horizontal polarization, there is always a minimum at earth level. Note that in these drawings the ground is considered as a perfect mirror, even for low angles of incidence. In these drawings the distance between the antenna and its image is just a few wavelengths. For greater distances, the number of lobes increases. Note that the situation is different–and more complex–if reflections in the ionosphere occur. This happens over very long distances (thousands of kilometers). There is not a direct ray but several reflected rays that add with different phase shifts. This is the reason why almost all public address radio emissions have vertical polarization. As public users are near ground, horizontal polarized emissions would be poorly received. Observe household and automobile radio receivers. They all have vertical antennas or horizontal ferrite antennas for vertical polarized emissions. In cases where the receiving antenna must work in any position, as in mobile phones, the emitter and receivers in base stations use circular polarized electromagnetic waves. Classical (analog) television emissions are an exception. They are almost always horizontally polarized, because the presence of buildings makes it unlikely that a good emitter antenna image will appear. However, these same buildings reflect the electromagnetic waves and can create ghost images. Using horizontal polarization, reflections are attenuated because of the low reflection of electromagnetic waves whose magnetic field is parallel to the dielectric surface near the Brewster's angle. Vertically polarized analog television has been used in some rural areas. In digital terrestrial television reflections are less obtrusive, due to the inherent robustness of digital signalling and

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built-in error correction.

Mutual impedance and interaction between antennas

Mutual impedance between parallel

dipoles

not staggered. Curves Re and Im are the resistive and reactive parts of the impedance.

Current circulating in any antenna induces currents in all others. One can postulate a mutual impedance between two antennas that has the same significance as the in ordinary coupled inductors. The mutual impedance between two antennas is defined as:

where

is the current flowing in antenna 1 and

is the voltage that would have to be applied to antenna 2–with

antenna 1 removed–to produce the current in the antenna 2 that was produced by antenna 1. From this definition, the currents and voltages applied in a set of coupled antennas are:

where: • • •

is the voltage applied to the antenna is the impedance of antenna is the mutual impedance between antennas

and

Note that, as is the case for mutual inductances,

This is a consequence of Lorentz reciprocity. If some of the elements are not fed (there is a short circuit instead a feeder cable), as is the case in television antennas (Yagi-Uda antennas), the corresponding are zero. Those elements are called parasitic elements. Parasitic elements are unpowered elements that either reflect or absorb and reradiate RF energy. In some geometrical settings, the mutual impedance between antennas can be zero. This is the case for crossed dipoles used in circular polarization antennas.

Antenna (radio)

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Antenna gallery Antennas and antenna arrays

A Yagi-Uda beam antenna.

A multi-band rotary directional antenna for amateur radio use.

Rooftop TV antenna. It is actually three Yagi antennas. The longest elements are for the low band, while the medium and short elements are for the high and UHF band.

A terrestrial microwave radio antenna array.

Examples of US 136-174 MHz base station antennas.

Low cost LF time signal receiver, antenna (left) and receiver (right).

Rotatable log-periodic array for VHF and UHF.

Shortwave antennas in Delano, California.

An old VHF-band Yagi-type television antenna.

A T2FD broadband antenna, covering the 5-30MHz band.

A US multiband "aerial" TV antenna.

"Rabbit ears" antenna

AM loop antenna

Antenna (radio)

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Antennas and supporting structures

A building rooftop supporting numerous dish and sectored mobile telecommunications antennas (Doncaster, Victoria, Australia).

A water tower in Palmerston, Northern Territory with radio broadcasting and communications antennas.

A three-sector telephone site in Mexico City.

Telephone site concealed as a palm tree.

Diagrams as part of a system

Antennas may be connected through a multiplexing arrangement in some applications like this trunked two-way radio example.

Antenna network for an emergency medical services base station.

Antenna (radio)

Notes [1] In the context of engineering and physics, the plural of antenna is antennas, and it has been this way since about 1950 (or earlier), when a cornerstone textbook in this field, Antennas, was published by John D. Kraus of the Ohio State University. Besides the title, Dr. Kraus noted this in a footnote on the first page of his book. Insects may have "antennae", but this form is not used in the context of electronics. [2] "Salvan: Cradle of Wireless, How Marconi Conducted Early Wireless Experiments in the Swiss Alps", Fred Gardiol & Yves Fournier, Microwave Journal, February 2006, pp. 124-136. [3] Tesla said during the development of radio that "One of the terminals of the source would be connected to Earth [as a electric ground connection ...] the other to an insulated body of large surface. For more information, see " On Light and Other High Frequency Phenomena (http:/ / www. tfcbooks. com/ tesla/ 1893-02-24. htm)". Delivered before the Franklin Institute, Philadelphia, February 1893, and before the National Electric Light Association, St. Louis, Missouri, March 1893. [4] This example assumes a length to diameter ratio of 1000. [5] "Guide to Wi-Fi Wireless Network Antenna Selection." (http:/ / networkbits. net/ wireless-printing/ wireless-network-antenna-guide/ ). NetworkBits.net. . Retrieved April 8, 2008. [6] Impedance is caused by the same physics as refractive index in optics, although impedance effects are typically one dimensional, where effects of refractive index is three dimensional.

References General references • • • •

Antenna Theory (3rd edition), by C. Balanis, Wiley, 2005, ISBN 0-471-66782-X; Antenna Theory and Design (2nd edition), by W. Stutzman and G. Thiele, Wiley, 1997, ISBN 0-471-02590-9; Antennas (3rd edition), by J. Kraus and R. Marhefka, McGraw-Hill, 2001, ISBN 0-072-32103-2; Antennenbuch, by Karl Rothammel, publ. Franck'sche Verlagshandlung Stuttgart, 1991, ISBN 3-440-05853-0; other editions (http://www.worldcat.org/oclc/65969707?tab=editions) (in German) • Antennas for portable Devices (http://www1.i2r.a-star.edu.sg/~chenzn), Zhi Ning Chen (edited), John Wiley & Sons in March 2007 • Broadband Planar Antennas: Design and Applications, Zhi Ning Chen and M. Y. W. Chia, John Wiley & Sons in February 2006 • The ARRL Antenna Book (15th edition), ARRL, 1988, ISBN 0-87259-207-5

"Practical antenna" references • • • • • • • •

Antenna Theory antenna-theory.com (http://www.antenna-theory.com) Patch Antenna: From Simulation to Realization EM Talk (http://www.emtalk.com/mwt_mpa.htm) Why an Antenna Radiates at ARRL (http://www.arrl.org/tis/info/whyantradiates.html) Why Antennas Radiate, Stuart G. Downs, WY6EE (http://www.arrl.org/files/file/QEX Binaries/0105downs. pdf) (PDF) Understanding electromagnetic fields and antenna radiation takes (almost) no math, Ron Schmitt, EDN Magazine, March 2 2000 (http://www.classictesla.com/download/emfields.pdf) (PDF) Tests of FM/VHF receiving antennas. (http://www.aerialsandtv.com/fmanddabradio. html#FMandDABaerialTests) http://www.tvantennasperth.com.au/Diyantennas.html :"Antenna Gain" Antennas: Generalities, Principle of operation, As electronic component, Hertz Marconi and Other types Antennas etc etc (http://www.ilmondodelletelecomunicazioni.it/english/antennas/index.htm)

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Antenna (radio)

Theory and simulations • http://www.dipoleanimator.com • EM Talk, " Microstrip Patch Antenna (http://www.emtalk.com/tut_1.htm)", (Theory and simulation of microstrip patch antenna) • " Online Calculations and Conversions (http://www.jampro.com/index. php?page=technical-documents-and-calculators)" Formulas for simulating and optimizing Antenna specs and placement • " Microwave Antenna Design Calculator (http://www.q-par.com/capabilities/software/ microwave-antenna-design-calculator)" Provides quick estimation of antenna size required for a given gain and frequency. 3 dB and 10 dB beamwidths are also derived; the calculator additionally gives the far-field range required for a given antenna. • Sophocles J. Orfanidis, " Electromagnetic Waves and Antennas (http://www.ece.rutgers.edu/~orfanidi/ewa/ )", Rutgers University (20 PDF Chaps. Basic theory, definitions and reference) • Hans Lohninger, "Learning by Simulations: Physics: Coupled Radiators (http://www.vias.org/simulations/ simusoft_twoaerials.html)". vias.org, 2005. (ed. Interactive simulation of two coupled antennas) • NEC Lab (http://www.ingenierias.ugto.mx/profesores/sledesma/documentos/index.htm) - NEC Lab is a tool that uses Numerical Electromagnetics Code and Artificial Intelligence to design and simulate antennas. • Justin Smith " Aerials (http://www.aerialsandtv.com/aerials.html)". A.T.V (Aerials and Television), 2009. (ed. Article on the (basic) theory and use of FM, DAB & TV aerials) • Antennas Research Group, " Virtual (Reality) Antennas (http://www.antennas.gr)". Democritus University of Thrace, 2005. • "Support > Knowledgebase > RF Basics > Antennas / Cables > dBi vs. dBd detail (http://www.maxstream.net/ helpdesk/article-27)". MaxStream, Inc., 2005. (ed. How to measure antenna gain) • Yagis and Log Periodics, Astrosurf article. (http://www.astrosurf.com/luxorion/qsl-antenna4.htm) • Raines, J. K., "Virtual Outer Conductor for Linear Antennas," Microwave Journal, Vol. 52, No. 1, January, 2009, pp. 76–86 • Tests of FM/VHF receiving antennas. (http://www.aerialsandtv.com/fmanddabradio. html#FMandDABaerialTests) Effect of ground references • Electronic Radio and Engineering. F.E. Terman. McGraw-Hill • Lectures on physics. Feynman, Leighton and Sands. Addison-Wesley • Classical Electricity and Magnetism. W. Panofsky and M. Phillips. Addison-Wesley

Patents and USPTO • CLASS 343 (http://www.uspto.gov/go/classification/uspc343/defs343.htm), Communication: Radio Wave Antenna

Further reading • Antennas for Base Stations in Wireless Communications, edited by Zhi Ning Chen and Kwai-Man Luk, McGraw-Hill Companies, Inc, USA in May 2009

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Reciever Receiver (radio) A radio receiver is an electronic circuit that receives its input from an antenna, uses electronic filters to separate a wanted radio signal from all other signals picked up by this antenna, amplifies it to a level suitable for further processing, and finally converts through demodulation and decoding the signal into a form usable for the consumer, such as sound, pictures, digital data, measurement values, navigational positions, etc.[1] In consumer electronics, the terms radio and radio receiver are often used specifically for receivers designed for the sound signals transmitted by radio broadcasting services – historically the first mass-market radio application.

Types of radio receivers Various types of radio receivers may include: • Consumer audio and high fidelity audio receivers and AV receivers used by home stereo listeners and audio and home theatre system enthusiasts. • Communications receivers, used as a component of a radio communication link, characterized by high stability and reliability of performance. • Simple crystal radio receivers (also known as a crystal set) which operate using the power received from radio waves.

Early broadcast radio receiver--wireless Truetone model from about 1940

• Satellite television receivers, used to receive television programming from communication satellites in geosynchronous orbit. • Specialized-use receivers such as telemetry receivers that allow the remote measurement and reporting of information. • Measuring receivers (also: measurement receivers) are calibrated laboratory-grade devices that are used to measure the signal strength of broadcasting stations, the electromagnetic interference radiation emitted by electrical products, as well as to calibrate RF attenuators and signal generators. • Scanners are specialized receivers that can automatically scan two or more discrete frequencies, stopping when they find a signal on one of them and then continuing to scan other frequencies when the initial transmission ceases. They are mainly used for monitoring VHF and UHF radio systems. • Internet radio device

Receiver (radio)

Consumer audio receivers In the context of home audio systems, the term "receiver" often refers to a combination of a tuner, a preamplifier, and a power amplifier all on the same chassis. Audiophiles will refer to such a device as an integrated receiver, while a single chassis that implements only one of the three component functions is called a discrete component. Some audio purists still prefer three discreet units - tuner, preamplifier and power amplifier - but the integrated receiver has, for some years, been the mainstream choice for music listening. The first integrated stereo receiver was made by the Harman Kardon company, and came onto the market in 1958. It had undistinguished performance, but it represented a breakthrough to the "all in one" concept of a receiver, and rapidly improving designs gradually made the receiver the mainstay of the marketplace. Many radio receivers also include a loudspeaker.

Hi-Fi / Home theater Today AV receivers are a common component in a high-fidelity or home-theatre system. The receiver is generally the nerve centre of a sophisticated home-theatre system providing selectable inputs for a number of different audio components like turntables, compact-disc players and recorders, and tape decks ( like video-cassette recorders) and video components (DVD players and recorders, video-game systems, and televisions). With the decline of vinyl discs, modern receivers tend to omit inputs for turntables, which have separate requirements of their own. All other common audio/visual components can use any of the identical line-level inputs on the receiver for playback, regardless of how they are marked (the "name" on each input is mostly for the convenience of the user.) For instance, a second CD player can be plugged into an "Aux" input, and will work the same as it will in the "CD" input jacks. Some receivers can also provide signal processors to give a more realistic illusion of listening in a concert hall. Digital audio S/PDIF and USB connections are also common today. The home theater receiver, in the vocabulary of consumer electronics, comprises both the 'radio receiver' and other functions, such as control, sound processing, and power amplification. The standalone radio receiver is usually known in consumer electronics as a tuner. Some modern integrated receivers can send audio out to seven loudspeakers and an additional channel for a subwoofer and often include connections for headphones. Receivers vary greatly in price, and support stereophonic or surround sound. A high-quality receiver for dedicated audio-only listening (two channel stereo) can be relatively inexpensive; excellent ones can be purchased for $300 United States or less. Because modern receivers are purely electronic devices with no moving parts unlike electromechanical devices like turntables and cassette decks, they tend to offer many years of trouble-free service. In recent years, the home theater in a box has become common, which often integrates a surround-capable receiver with a DVD player. The user simply connects it to a television, perhaps other components, and a set of loudspeakers.

Portable radios Portable radios include simple transistor radios that are typically monoaural and receive the AM, FM, and/or short wave broadcast bands. FM, and often AM, radios are sometimes included as a feature of portable DVD/CD, MP3 CD, and USB key players, as well as cassette player/recorders. AM/FM stereo car radios can be a separate dashboard mounted component or a feature of in car entertainment systems. A Boombox (or Boom-box)—also sometimes known as a Ghettoblaster or a Jambox, or (in parts of Europe) as a "radio-cassette"—is a name given to larger portable stereo systems capable of playing radio stations and recorded music, often at a high level of volume. Self-powered portable radios, such as clockwork radios are used in developing nations or as part of an emergency preparedness kit.[2]

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Receiver (radio)

History of radio receivers Early development While James Clerk Maxwell was the first person to prove electromagnetic waves existed, in 1887 a German named Heinrich Hertz demonstrated these new waves by using spark gap equipment to transmit and receive radio or "Hertzian waves", as they were first called. The experiments were not followed up by Hertz. The practical applications of the wireless communication and remote control technology were implemented by Nikola Tesla. The world's first radio receiver (thunderstorm register) was designed by Alexander Stepanovich Popov, and it was first seen at the All-Russia Exhibition 1896. He was the first to demonstrate the practical application of electromagnetic (radio) waves,[3] although he did not care to apply for a patent for his invention. A device called a coherer became the basis for receiving radio signals. The first person to use the device to detect radio waves was a Frenchman named Edouard Branly, and Oliver Lodge popularised it when he gave a lecture in 1898 in honour of Hertz. Lodge also made improvements to the coherer. Many experimenters at the time believed that these new waves could be used to communicate over great distances and made significant improvements to both radio receiving and transmitting apparatus. In 1895 Marconi demonstrated the first viable radio system, leading to transatlantic radio communication in December 1901. The honor was later contested as he was found to be using equipment and designs of other experimenters that held the patents at that time. John Ambrose Fleming's development of an early thermionic valve to help detect radio waves was based upon a discovery of Thomas Edison's (called "The Edison effect", which essentially modified an early light bulb). Fleming called it his "oscillation valve" because it acted in the same way as water valve in only allowing flow in one direction. While Fleming's valve was a great stride forward it would take some years before thermionic, or vacuum tube technology was fully adopted. Around this time work on other types of detectors started to be undertaken and it resulted in what was later known as the cat's whisker. It consisted of a crystal of a material such as galena with a small springy piece of wire brought up against it. The detector was constructed so that the wire contact could be moved to different points on the crystal, and thereby obtain the best point for rectifying the signal and the best detection. They were never very reliable as the "whisker" needed to be moved periodically to enable it to detect the signal properly.[4]

Valves (Tubes) An American named Lee de Forest, a competitor to Marconi, set about to develop receiver technology that did not infringe any patents to which Marconi had access. He took out a number of patents in the period between 1905 and 1907 covering a variety of developments that culminated in the form of the triode valve in which there was a third electrode called a grid. He called this an audion tube. One of the first areas in which valves were used was in the manufacture of telephone repeaters, and although the performance was poor, they gave significant improvement in long distance telephone receiving circuits. With the discovery that triode valves could amplify signals it was soon noticed that they would also oscillate, a fact that was exploited in generating signals. Once the triode was established as an amplifier it made a tremendous difference to radio receiver performance as it allowed the incoming signals to be amplified. One way that proved very successful was introduced in 1913 and involved the use of positive feedback in the form of a regenerative detector. This gave significant improvements in the levels of gain that could be achieved, greatly increasing selectivity, enabling this type of receiver to outperform all other types of the era. With the outbreak of the First World War, there was a great impetus to develop radio receiving technology further. An American named Irving Langmuir helped introduce a new generation of totally air-evacuated "hard" valves. H. J. Round undertook some work on this and in 1916 he produced a number of valves with the grid connection taken out of the top of the envelope away from the anode connection.[4]

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Receiver (radio)

Autodyne and superheterodyne By the 1920s, the tuned radio frequency receiver (TRF) represented a major improvement in performance over what had been available before, it still fell short of the needs for some of the new applications. To enable receiver technology to meet the needs placed upon it a number of new ideas started to surface. One of these was a new form of direct conversion receiver. Here an internal or local oscillator was used to beat with the incoming signal to produce an audible signal that could be amplified by an audio amplifier. H. J. Round developed a receiver he called an autodyne in which the same valve was used as a mixer and an oscillator, Whilst the set used fewer valves it was difficult to optimise the circuit for both the mixer and oscillator functions. The next leap forward in receiver technology was a new type of receiver known as the superheterodyne, or supersonic heterodyne receiver. A Frenchman named Lucien Levy was investigating ways in which receiver selectivity could be improved and in doing this he devised a system whereby the signals were converted down to a lower frequency where the filter bandwidths could be made narrower. A further advantage was that the gain of valves was considerably greater at the lower frequencies used after the frequency conversion, and there were fewer problems with the circuits bursting into oscillation. The idea for developing a receiver with a fixed intermediate frequency amplifier and filter is credited to Edwin Armstrong of the United States. Working for the American Expeditionary Force in Europe during 1918, Armstrong thought that if the incoming signals was mixed with a variable frequency oscillator (the "local oscillator"), a lower-frequency fixed tuned amplifier could be used. Armstrong's original receiver consisted of a total of eight vacuum tubes. Several tuned circuits could be cascaded to improve selectivity, and being set on a fixed frequency they did not all need to be changed in line with one another. The filters could be preset and left correctly tuned. Armstrong was not the only person working on the idea of a superheterodyne receiver. Alexander Meissner in Germany had taken out a patent for the idea six months before Armstrong, but since Meissner did not prove the idea in practice, and he did not build a superheterodyne radio, the invention is credited to Armstrong. The need for the increased performance of the superheterodyne receiver was first experienced in North America, and by the late 1920s most radio sets were superheterodyne receivers. However, in Europe the number of broadcast stations did not start to rise as rapidly until later. Even so, by the mid-1930s virtually all receiving sets in Europe as well were using the superheterodyne principle. In 1926, the tetrode valve was introduced, and enabled further improvements in performance.[4]

War and postwar developments In 1939 the outbreak of war gave a new impetus to receiver development. During this time a number of classic communications receivers were designed. Some like the National HRO are still sought by enthusiasts today and although they are relatively large by today's standards, they can still give a good account of themselves under current crowded band conditions. In the late 1940s the transistor was discovered. Initially the devices were not widely used because of their expense, and the fact that valves were being made smaller, and performed better. However by the early 1960s portable transistor Military HF receiver, type BC-224-D (1942) broadcast receivers (transistor radios) were hitting the market place. These radios were ideal for broadcast reception on the long and medium wave bands. They were much smaller than their valve equivalents, they were portable and could be powered from batteries. Although some valve portable receivers were available, batteries for these were expensive and did not

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Receiver (radio) last for long. The power requirements for transistor radios were very much less, resulting in batteries lasting for much longer and being considerably cheaper.[4]

Semiconductors Further developments in semiconductor technology led to the introduction of the integrated circuit in the late 1950s.[5] This enabled radio receiver technology to move forward even further. Integrated circuits enabled high performance circuits to be built for less cost, and significant amounts of space could be saved. As a result of these developments new techniques could be introduced. One of these was the frequency synthesizer that was used to generate the local oscillator signal for the receiver. By using a synthesizer it was possible to generate a very accurate and stable local oscillator signal. Also the ability of synthesizers to be controlled by microprocessors meant that many new facilities could be introduced apart from the significant performance improvements offered by synthesizers.[4]

Digital technologies Receiver technology had been advancing gradually and regularly. Many of the functions performed by a analogue electronics can be performed by software instead. The benefit is that software is not affected by temperature, physical variables, electronic noise and manufacturing defects. While today's radios are amazing pieces of modern technology, filled with low- power, high performance, integrated circuits crammed into the smallest spaces, the basic principle of the radio receiver is practically always the superheterodyne one, the same idea which was developed by Edwin Armstrong back in 1918.[4] For really high-performance receivers, such as satellite communications receivers and military/naval receivers, two-stage ("double conversion") and even three-stage ("triple conversion") superheterodyne processing is frequently used. Single-conversion receivers are rather simple-minded in their nature.

DSP technology DSP technology, short for digital signal processing, is the use of digital means to process signals and is coming into wide use in modern shortwave receivers. It is the basis of many areas of modern technology including cell phones, CD players, video recorders and computers. A digital signal is essentially a stream or sequence of numbers that relay a message through some sort of medium such as a wire. The primary benefit of DSP hardware in shortwave receivers is the ability to tailor the bandwidth of the receiver to current reception conditions and to the type of signal being listened to. A typical analog only receiver may have a limited number of fixed bandwidths, or only one, but a DSP receiver may have 40 or more individually selectable filters.

PC controlled radio receivers "PC radios", or radios that are designed to be controlled by a standard PC are controlled by specialized PC software using a serial port connected to the radio. A "PC radio" may not have a front-panel at all, and may be designed exclusively for computer control, which reduces cost. Some PC radios have the great advantage of being field upgradable by the owner. New versions of the DSP firmware can be downloaded from the manufacturer's web site and uploaded into the flash memory of the radio. The manufacturer can then in effect add new features to the radio over time, such as adding new filters, DSP noise reduction, or simply to correct bugs. A full-featured radio control program allows for scanning and a host of other functions and, in particular, integration of databases in real-time, like a "TV-Guide" type capability. This is particularly helpful in locating all transmissions on all frequencies of a particular broadcaster, at any given time. Some control software designers have even integrated Google Earth to the shortwave databases, so it is possible to "fly" to a given transmitter site location with

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Receiver (radio) a click of a mouse. In many cases the user is able to see the transmitting antennas where the signal is originating from.

Radio control software The field of software control of PC radios has grown rapidly in the last several years, with developers making a number of advances. Since the Graphical User Interface or GUI interface PC to the radio has unlimited flexibility, any number of new features can be added by the software designer.Features that can be found in advanced control software programs today include a band table, GUI controls corresponding to traditional radio controls, local time clock and a UTC clock, signal strength meter, an ILG database for shortwave listening with lookup capability, scanning capability, text-to-speech interface, and integrated Conference Server.

Software-defined radios The next level in radio / software integration are so-called pure "software defined radios". The distinction here is that all filtering, modulation and signal manipulation is done in software, usually by a PC soundcard or by a dedicated piece of DSP hardware. There may be a minimal RF front-end or traditional radio that supplies an IF to the SDR. SDR's can go far beyond the usual demodulation capability of typical, and even high-end DSP shortwave radios. They can for example, record large swaths of the radio spectrum to a hard drive for "playback" at a later date. The same SDR that one minute is demodulating a simple AM broadcast may also be able to decode an HDTV broadcast in the next. A well known open-source project called GNU Radio is dedicated to evolving a high-performance SDR. All the source code for this SDR is freely downloadable and modifiable by anyone.

Notes [1] http:/ / www. radio-electronics. com/ info/ rf-technology-design/ index. php Radio-Electronics, Radio Receiver Technology [2] http:/ / radio. electrical-guide. info/ types/ The Radio Guide, Types of Portable Radios [3] "Early Radio Transmission Recognized as Milestone" (http:/ / www. ieee. org/ portal/ site/ tionline/ menuitem. 130a3558587d56e8fb2275875bac26c8/ index. jsp?& pName=institute_level1_article& TheCat=1008& article=tionline/ legacy/ inst2005/ may05/ 5w. fhistory. xml& ). IEEE. . Retrieved 16 July 2006. [4] "History of the Radio Receiver" (http:/ / www. radio-electronics. com/ info/ radio_history/ radiohist/ hstrx. php#top). Radio-Electronics.Com. . Retrieved 2007-11-23. [5] http:/ / www. ti. com/ corp/ docs/ kilbyctr/ jackbuilt. shtml Texas Instruments, The Chip That Jack Built

References • Communications Receivers, Third Edition, Ulrich L. Rohde, Jerry Whitaker, McGraw Hill, New York, 2001, ISBN 0-07-136121-9

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Tuned radio frequency receiver

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Tuned radio frequency receiver A tuned radio frequency receiver (TRF receiver) is a radio receiver that is usually composed of several tuned radio frequency amplifiers followed by circuits to detect and amplify the audio signal. Prevalent in the early 20th century, it can be difficult to operate because each stage must be individually tuned to the station's frequency. It was replaced by the Superheterodyne receiver invented by Edwin Armstrong.

Background The TRF receiver was patented in 1916 by Ernst Alexanderson. His concept was that each stage would amplify the desired signal while reducing the interfering ones.

This 1920s TRF radio manufactured by Signal is constructed on a breadboard

The significance of the term "tuned radio frequency" is best understood when compared to the Superheterodyne receiver. A tuned radio frequency receiver actually tunes the receiver on the true radio frequency whereas the Superheterodyne receiver, tunes the desired signal after conversion to an intermediate frequency. Many homemade radios constructed by enthusiasts today, are tuned radio receivers, and these can range from single stage to multi-stage receivers. Antique TRF receivers can often be identified by their cabinets. They typically have a long, low appearance, with a flip-up lid for access to the vacuum tubes and tuned circuits. On their front panels there are typically two or three large dials, each controlling the tuning for one stage. Inside, along with several vacuum tubes, there will be a series of large coils. These will sometimes be tilted slightly to reduce interaction between their magnetic fields. A problem with the TRF receiver in the time of triode vacuum tubes was that interelectrode capacitance (the so-called Miller capacitance) can cause instability and oscillation. In 1922, Louis Alan Hazeltine invented the technique of neutralization which uses an additional winding on the output or input tuned circuit to introduce an opposing signal which can cancel that capacitance, when properly adjusted. This was used in the popular Neutrodyne series of TRF receivers. The later adoption of the tetrode vacuum tube eliminated the Miller capacitance and the need for this touchy circuitry.

Tuned radio frequency receiver

How it works A 3 stage TRF receiver includes a RF stage, a detector stage and an audio stage:

This schematic diagram shows a typical TRF receiver. This particular radio uses a six tube design utilizing triode tubes. It has two radio frequency amplifiers, one grid-leak detector/amplifier and three class ‘A’ audio amplifiers. Generally, 2 or 3 RF amplifiers are required to filter and amplify the received signal to a level sufficient to drive the detector stage. The detector converts RF signals directly to information, and the audio stage amplifies the information signal to a usable level. The final stage was often simply a grid-leak detector.

Disadvantages of TRF receiver Terman (1943, p. 658) characterizes the TRF's disadvantages as "poor selectivity and low sensitivity in proportion to the number of tubes employed. They are accordingly practically obsolete." Selectivity requires narrow bandwidth, and narrow bandwidth at a high radio frequency implies high Q or many filter sections. For contrast, a superheterodyne receiver can translate the incoming high radio frequency to a lower intermediate frequency where selectivity is easier to achieve. An additional problem for the TRF receiver is tuning different frequencies. All the tuned circuits need to track to keep the narrow bandwidth tuning. Keeping several tuned circuits aligned is difficult. For contrast, a superheterodyne receiver only needs to track the RF and LO stages; the onerous selectivity requirements are confined to the IF amplifier which is fixed-tuned. Although a TRF receiver can not be engineered for a high degree of selectivity relative to its carrier frequency, there is no reason it cannot reach the same level of sensitivity as other designs. The 1930's era BC-AN-229/429 military receiver was a six-valve design covering 201 to 398 KHz and 2.5 to 7.7 MHz (requiring several sets of plug-in coils to cover those ranges). This equipment probably exemplifies the limit of T.R.F. performance. Although the receiver bandwidth does vary, as noted above, the sensitivity of the set was around 8 microvolts for 10 milliwatts of audio output, comparable to that of the famous AN/ARC-5 superhet receiver that superseded it.

Modern Usage Although the TRF design heralds from the early years of radio and has been largely superseded by superheterodyne and other circuits, it was 'resurrected' in 1972 in silicon as the ZN414 TRF radio integrated circuit from Ferranti, thus affording the design a new lease of life in hobbyist radio projects, kits and some commercial products.

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Tuned radio frequency receiver

References • Terman, Frederick E. (1943), Radio Engineers' Handbook, McGraw-Hill • Tomasi, Wayne (2004), Electronic Communications Systems: Fundamentals Through Advanced (5th ed.), Pearson Education

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Radar Radar Radar is an object-detection system which uses electromagnetic waves—specifically radio waves—to determine the range, altitude, direction, or speed of both moving and fixed objects such as aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish, or antenna, transmits pulses of radio waves or microwaves which bounce off any object in their path. The object returns a tiny part of the wave's energy to a dish or antenna which is usually located at the same site as the transmitter. Practical radar was developed in secrecy during World War II by Britain and other nations. The term RADAR was coined in 1940 by the U.S. Navy as an acronym for radio detection and ranging.[1] [2] The term radar has since entered the English and other languages as the common noun radar, losing all capitalization. In the United Kingdom, the technology was initially called RDF (range and direction finding), using the same initials used for radio direction finding to conceal its ranging capability. The modern uses of radar are highly diverse, including air traffic control, radar astronomy, air-defense systems, antimissile systems; nautical radars to locate landmarks and other ships; aircraft anticollision systems; ocean-surveillance systems, outer-space surveillance and rendezvous systems; meteorological precipitation monitoring; altimetry and flight-control systems; guided-missile target-locating systems; and ground-penetrating radar for geological observations. High tech radar systems are associated with digital signal processing and are capable of extracting objects from very high noise levels. Other systems similar to radar have been used in other parts of the electromagnetic spectrum. One example is "lidar", which uses visible light from lasers rather than radio waves.

History Several inventors, scientists, and engineers contributed to the development of radar.

A long-range radar antenna, known as ALTAIR, used to detect and track space objects in conjunction with ABM testing at the Ronald Reagan Test Site on Kwajalein Atoll.

Israeli military radar is typical of the type of radar used for air traffic control. The antenna rotates at a steady rate, sweeping the local airspace with a narrow vertical fan-shaped beam, to detect aircraft at all altitudes.

As early as 1886, Heinrich Hertz showed that radio waves could be reflected from solid objects. In 1895 Alexander Popov, a physics instructor at the Imperial Russian Navy school in Kronstadt, developed

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an apparatus using a coherer tube for detecting distant lightning strikes. The next year, he added a spark-gap transmitter. During 1897, while testing this in communicating between two ships in the Baltic Sea, he took note of an interference beat caused by the passage of a third vessel. In his report, Popov wrote that this phenomenon might be used for detecting objects, but he did nothing more with this observation.[3] The German Christian Huelsmeyer was the first to use radio waves to detect "the presence of distant metallic objects". In 1904 he demonstrated the feasibility of detecting a ship in dense fog, but not its distance.[4] He received Reichspatent Nr. 165546[5] for his detection device in April 1904, and later patent 169154[6] for a related amendment for also determining the distance to the ship. He also received a British patent on September 23, 1904[7] for the first full Radar application, which he called telemobiloscope. This Melbourne base Primary and secondary radar is used for air traffic control and terminal area intrusion detection by local domestic aircraft.

In August 1917 Nikola Tesla outlined a concept for primitive radar units.[8] He stated, "[...] by their [standing electromagnetic waves] use we may produce at will, from a sending station, an electrical effect in any particular region of the globe; [with which] we may determine the relative position or course of a moving object, such as a vessel at sea, the distance traversed by the same, or its speed." In 1922 A. Hoyt Taylor and Leo C. Young, researchers working with the U.S. Navy, discovered that when radio waves were broadcast at 60 MHz it was possible to determine the range and bearing of nearby ships in the Potomac River. Despite Taylor's suggestion that this method could be used in darkness and low visibility, the Navy did not immediately continue the work.[9] Serious investigation began eight years later after the discovery that radar could be used to track airplanes.[10] Before the Second World War, researchers in France, Germany, Italy, Japan, the Netherlands, the Soviet Union, the United Kingdom, and the United States, independently and in great secrecy, developed technologies that led to the modern version of radar. Australia, Canada, New Zealand, and South Africa followed prewar Great Britain, and Hungary had similar developments during the war.[11]

A Chain Home tower in Great Baddow, United Kingdom

In 1934 the Frenchman Émile Girardeau stated he was building an obstacle-locating radio apparatus "conceived according to the principles stated by Tesla" and obtained a patent (French Patent n° 788795 in 1934) for a working system, a part of which was installed on the Normandie liner in 1935.[12] [13] [14] During the same year, the Soviet military engineer P.K.Oschepkov, in collaboration with Leningrad Electrophysical Institute, produced an experimental apparatus, RAPID, capable of detecting an aircraft within 3 km of a receiver.[15] The French and Soviet systems, however, had continuous-wave operation and could not give the full performance that was ultimately at the center of modern radar.

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Full radar evolved as a pulsed system, and the first such elementary apparatus was demonstrated in December 1934 by the American Robert M. Page, working at the Naval Research Laboratory.[16] The year after the US Army successfully tested a primitive surface to surface radar to aim coastal battery search lights at night. [17] This was followed by a pulsed system demonstrated in May 1935 by Rudolf Kühnhold and the firm GEMA in Germany and then one in June 1935 by an Air Ministry team led by Robert A. Watson Watt in Great Britain. Later, in 1943, Page greatly improved radar with the monopulse technique that was then used for many years in most radar applications.[18] The British were the first to fully exploit radar as a defence against aircraft attack. This was spurred on by fears that the Germans were developing death rays[19] . The Air Ministry asked British scientists in 1934 to investigate the possibility of propagating electromagnetic energy and the likely effect. Following a study, they concluded that a death ray was impractical but that detection of aircraft appeared feasible.[19] Robert Watson Watt's team demonstrated to his superiors the capabilities of a working prototype and then patented the device (British Patent GB593017).[14] [20] [21] It served as the basis for the Chain Home network of radars to defend Great Britain. In April 1940, Popular Science showed an example of a radar unit using the Watson-Watt patent in an article on air defence, but not knowing that the U.S. Army and U.S. Navy were working on radars with the same principle, stated under the illustration, "This is not U.S. Army equipment."[22] The war precipitated research to find better resolution, more portability, and more features for radar, including complementary navigation systems like Oboe used by the RAF's Pathfinder. The postwar years have seen the use of radar in fields as diverse as air traffic control, weather monitoring, astrometry, and road speed control.

Applications of radar The information provided by radar includes the bearing and range (and therefore position) of the object from the radar scanner. It is thus used in many different fields where the need for such positioning is crucial. The first use of radar was for military purposes: to locate air, ground and sea targets. This evolved in the civilian field into applications for aircraft, ships, and roads. In aviation, aircraft are equipped with radar devices that warn of obstacles in or approaching their path and give accurate altitude readings. They can land in fog at airports equipped with radar-assisted ground-controlled approach (GCA) systems, in which the plane's flight is observed on radar screens while operators radio landing directions to the pilot. Commercial marine radar antenna. The rotating antenna radiates a vertical fan-shaped beam.

Marine radars are used to measure the bearing and distance of ships to prevent collision with other ships, to navigate and to fix their position at sea when within range of shore or other fixed references such as islands, buoys, and lightships. In port or in harbour, vessel traffic service radar systems are used to monitor and regulate ship movements in busy waters. Police forces use radar guns to monitor vehicle speeds on the roads. Meteorologists use radar to monitor precipitation. It has become the primary tool for short-term weather forecasting and to watch for severe weather such as thunderstorms, tornadoes, winter storms, precipitation types, etc. Geologists use specialised ground-penetrating radars to map the composition of the Earth's crust.

Radar

Principles A radar system has a transmitter that emits radio waves called radar signals in predetermined directions. When these come into contact with an object they are usually reflected and/or scattered in many directions. Radar signals are reflected especially well by materials of considerable electrical conductivity—especially by most metals, by seawater, by wet land, and by wetlands. Some of these make the use of radar altimeters possible. The radar signals that are reflected back towards the transmitter are the desirable ones that make radar work. If the object is moving either closer or farther away, there is a slight change in the frequency of the radio waves, due to the Doppler effect. Radar receivers are usually, but not always, in the same location as the transmitter. Although the reflected radar signals captured by the receiving antenna are usually very weak, these signals can be strengthened by the electronic amplifiers that all radar sets contain. More sophisticated methods of signal processing are also nearly always used in order to recover useful radar signals. The weak absorption of radio waves by the medium through which it passes is what enables radar sets to detect objects at relatively-long ranges—ranges at which other electromagnetic wavelengths, such as visible light, infrared light, and ultraviolet light, are too strongly attenuated. Such things as fog, clouds, rain, falling snow, and sleet that block visible light are usually transparent to radio waves. Certain, specific radio frequencies that are absorbed or scattered by water vapor, raindrops, or atmospheric gases (especially oxygen) are avoided in designing radars except when detection of these is intended. Finally, radar relies on its own transmissions, rather than light from the Sun or the Moon, or from electromagnetic waves emitted by the objects themselves, such as infrared wavelengths (heat). This process of directing artificial radio waves towards objects is called illumination, regardless of the fact that radio waves are completely invisible to the human eye or cameras.

Reflection Electromagnetic waves reflect (scatter) from any large change in the dielectric constant or diamagnetic constants. This means that a solid object in air or a vacuum, or other significant change in atomic density between the object and what is surrounding it, will usually scatter radar (radio) waves. This is particularly true for electrically conductive materials, such as metal and carbon fiber, making radar particularly well suited to the detection of aircraft and ships. Radar absorbing material, containing resistive and sometimes magnetic substances, is used on military vehicles to reduce radar reflection. This is the radio equivalent of painting something a dark color so that it cannot be seen through normal means (see stealth technology). Radar waves scatter in a variety of ways depending on the size Brightness can indicate reflectivity as in this 1960 (wavelength) of the radio wave and the shape of the target. If the weather radar image (of Hurricane Abby). The wavelength is much shorter than the target's size, the wave will bounce radar's frequency, pulse form, polarization, signal off in a way similar to the way light is reflected by a mirror. If the processing, and antenna determine what it can observe. wavelength is much longer than the size of the target, the target may not be visible due to poor reflection. Low Frequency radar technology is dependent on resonances for detection, but not identification, of targets. This is described by Rayleigh scattering, an effect that creates the Earth's blue sky and red sunsets. When the two length scales are comparable, there may be resonances. Early radars used very long wavelengths that were larger than the targets and received a vague signal, whereas some modern systems use shorter wavelengths (a few centimeters or shorter) that can image objects as small as a loaf of bread.

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Short radio waves reflect from curves and corners, in a way similar to glint from a rounded piece of glass. The most reflective targets for short wavelengths have 90° angles between the reflective surfaces. A structure consisting of three flat surfaces meeting at a single corner, like the corner on a box, will always reflect waves entering its opening directly back at the source. These so-called corner reflectors are commonly used as radar reflectors to make otherwise difficult-to-detect objects easier to detect, and are often found on boats in order to improve their detection in a rescue situation and to reduce collisions. For similar reasons, objects attempting to avoid detection will angle their surfaces in a way to eliminate inside corners and avoid surfaces and edges perpendicular to likely detection directions, which leads to "odd" looking stealth aircraft. These precautions do not completely eliminate reflection because of diffraction, especially at longer wavelengths. Half wavelength long wires or strips of conducting material, such as chaff, are very reflective but do not direct the scattered energy back toward the source. The extent to which an object reflects or scatters radio waves is called its radar cross section.

Radar equation The power Pr returning to the receiving antenna is given by the radar equation:

where • • • • • • •

Pt = transmitter power Gt = gain of the transmitting antenna Ar = effective aperture (area) of the receiving antenna σ = radar cross section, or scattering coefficient, of the target F = pattern propagation factor Rt = distance from the transmitter to the target Rr = distance from the target to the receiver.

In the common case where the transmitter and the receiver are at the same location, Rt = Rr and the term Rt² Rr² can be replaced by R4, where R is the range. This yields:

This shows that the received power declines as the fourth power of the range, which means that the reflected power from distant targets is very, very small. The equation above with F = 1 is a simplification for vacuum without interference. The propagation factor accounts for the effects of multipath and shadowing and depends on the details of the environment. In a real-world situation, pathloss effects should also be considered.

Radar

Doppler effect Ground-based radar systems used for detecting speeds rely on the Doppler effect. The apparent frequency (f) of the wave changes with the relative position of the target. The doppler equation is stated as follows for (the radial speed of the observer) and (the radial speed of the target) and frequency of wave :

However, the change in phase of the return signal is often used instead of the change in frequency. It is to be noted that only the radial component of the speed is available. Hence when a target is moving at right angle to the radar beam, it has no velocity while one parallel to it has maximum recorded speed even if both might have the same real absolute motion.

Polarization In the transmitted radar signal, the electric field is perpendicular to the direction of propagation, and this direction of the electric field is the polarization of the wave. Radars use horizontal, vertical, linear and circular polarization to detect different types of reflections. For example, circular polarization is used to minimize the interference caused by rain. Linear polarization returns usually indicate metal surfaces. Random polarization returns usually indicate a fractal surface, such as rocks or soil, and are used by navigation radars.

Limiting factors Beam path and range The radar beam would follow a linear path in vacuum but it really follows a somewhat curved path in the atmosphere due to the variation of the refractive index of air. Even when the beam is emitted parallel to the ground, it will raise above it as the Earth curvature sink below the horizon. Furthermore, the signal is attenuated by the medium it crosses and the beam disperse as its not a perfect pencil shape. The maximum range of a conventional radar can either be limited by a number of factors: 1. Line of sight, which depends on height above ground. Echo heights above ground 2. The maximum non-ambiguous range (MUR) which is determined by the Pulse repetition frequency (PRF). Simply put, MUR is the distance the pulse could travel and return before the next pulse is emitted. 3. Radar sensitivity and power of the return signal as computed in the radar equation. This includes factors such as environmentals and the size (or radar cross section) of the target.

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Radar Noise Signal noise is an internal source of random variations in the signal, which is generated by all electronic components. Noise typically appears as random variations superimposed on the desired echo signal received in the radar receiver. The lower the power of the desired signal, the more difficult it is to discern it from the noise (similar to trying to hear a whisper while standing near a busy road). Noise figure is a measure of the noise produced by a receiver compared to an ideal receiver, and this needs to be minimized. Noise is also generated by external sources, most importantly the natural thermal radiation of the background scene surrounding the target of interest. In modern radar systems, due to the high performance of their receivers, the internal noise is typically about equal to or lower than the external scene noise. An exception is if the radar is aimed upwards at clear sky, where the scene is so "cold" that it generates very little thermal noise. There will be also flicker noise due to electrons transit, but depending on 1/f, will be much lower than thermal noise when the frequency is high. Hence, in pulse radar, the system will be always heterodyne. See intermediate frequency. Interference Radar systems must overcome unwanted signals in order to focus only on the actual targets of interest. These unwanted signals may originate from internal and external sources, both passive and active. The ability of the radar system to overcome these unwanted signals defines its signal-to-noise ratio (SNR). SNR is defined as the ratio of a signal power to the noise power within the desired signal. In less technical terms, SNR compares the level of a desired signal (such as targets) to the level of background noise. The higher a system's SNR, the better it is in isolating actual targets from the surrounding noise signals. Clutter Clutter refers to radio frequency (RF) echoes returned from targets which are uninteresting to the radar operators. Such targets include natural objects such as ground, sea, precipitation (such as rain, snow or hail), sand storms, animals (especially birds), atmospheric turbulence, and other atmospheric effects, such as ionosphere reflections, meteor trails, and three body scatter spike. Clutter may also be returned from man-made objects such as buildings and, intentionally, by radar countermeasures such as chaff. Some clutter may also be caused by a long radar waveguide between the radar transceiver and the antenna. In a typical plan position indicator (PPI) radar with a rotating antenna, this will usually be seen as a "sun" or "sunburst" in the centre of the display as the receiver responds to echoes from dust particles and misguided RF in the waveguide. Adjusting the timing between when the transmitter sends a pulse and when the receiver stage is enabled will generally reduce the sunburst without affecting the accuracy of the range, since most sunburst is caused by a diffused transmit pulse reflected before it leaves the antenna. While some clutter sources may be undesirable for some radar applications (such as storm clouds for air-defence radars), they may be desirable for others (meteorological radars in this example). Clutter is considered a passive interference source, since it only appears in response to radar signals sent by the radar. There are several methods of detecting and neutralizing clutter. Many of these methods rely on the fact that clutter tends to appear static between radar scans. Therefore, when comparing subsequent scans echoes, desirable targets will appear to move and all stationary echoes can be eliminated. Sea clutter can be reduced by using horizontal polarization, while rain is reduced with circular polarization (note that meteorological radars wish for the opposite effect, therefore using linear polarization the better to detect precipitation). Other methods attempt to increase the signal-to-clutter ratio. Constant False Alarm Rate (CFAR, a form of Automatic Gain Control, or AGC) is a method relying on the fact that clutter returns far outnumber echoes from targets of interest. The receiver's gain is automatically adjusted to maintain a constant level of overall visible clutter. While this does not help detect targets masked by stronger surrounding

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Radar clutter, it does help to distinguish strong target sources. In the past, radar AGC was electronically controlled and affected the gain of the entire radar receiver. As radars evolved, AGC became computer-software controlled, and affected the gain with greater granularity, in specific detection cells. Clutter may also originate from multipath echoes from valid targets due to ground reflection, atmospheric ducting or ionospheric reflection/refraction (e.g. Anomalous propagation). This clutter type is especially bothersome, since it appears to move and behave like other normal (point) targets of interest, thereby creating a ghost. In a typical scenario, an aircraft echo is multipath-reflected from the ground below, appearing to the receiver as an identical target below the correct one. The radar may try to unify the targets, reporting the target at an incorrect height, or—worse—eliminating it on the basis of jitter or a Radar multipath echoes from a target cause ghosts to appear. physical impossibility. These problems can be overcome by incorporating a ground map of the radar's surroundings and eliminating all echoes which appear to originate below ground or above a certain height. In newer Air Traffic Control (ATC) radar equipment, algorithms are used to identify the false targets by comparing the current pulse returns, to those adjacent, as well as calculating return improbabilities due to calculated height, distance, and radar timing. Jamming Radar jamming refers to radio frequency signals originating from sources outside the radar, transmitting in the radar's frequency and thereby masking targets of interest. Jamming may be intentional, as with an electronic warfare (EW) tactic, or unintentional, as with friendly forces operating equipment that transmits using the same frequency range. Jamming is considered an active interference source, since it is initiated by elements outside the radar and in general unrelated to the radar signals. Jamming is problematic to radar since the jamming signal only needs to travel one-way (from the jammer to the radar receiver) whereas the radar echoes travel two-ways (radar-target-radar) and are therefore significantly reduced in power by the time they return to the radar receiver. Jammers therefore can be much less powerful than their jammed radars and still effectively mask targets along the line of sight from the jammer to the radar (Mainlobe Jamming). Jammers have an added effect of affecting radars along other lines of sight, due to the radar receiver's sidelobes (Sidelobe Jamming). Mainlobe jamming can generally only be reduced by narrowing the mainlobe solid angle, and can never fully be eliminated when directly facing a jammer which uses the same frequency and polarization as the radar. Sidelobe jamming can be overcome by reducing receiving sidelobes in the radar antenna design and by using an omnidirectional antenna to detect and disregard non-mainlobe signals. Other anti-jamming techniques are frequency hopping and polarization. See Electronic counter-counter-measures for details. Interference has recently become a problem for C-band (5.66 GHz) meteorological radars with the proliferation of 5.4 GHz band WiFi equipment.

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Radar signal processing Distance measurement Transit time One way to measure the distance to an object is to transmit a short pulse of radio signal (electromagnetic radiation), and measure the time it takes for the reflection to return. The distance is one-half the product of the round trip time (because the signal has to travel to the target and then back to the receiver) and the speed of the signal. Since radio waves travel at the speed of light (186,000 miles per second or 300,000,000 meters per second), accurate distance measurement requires high-performance electronics.

Pulse radar: The round-trip time for the radar pulse to get to the target and return is measured. The distance is proportional to this time.

In most cases, the receiver does not detect the return while the signal is being transmitted. Through the use of a device called a duplexer, the radar switches between transmitting and receiving at a predetermined rate. The minimum range is calculated by measuring the length of the pulse multiplied by the speed of light, divided by two. In order to detect closer targets one must use a shorter pulse length. A similar effect imposes a maximum range as well. If the return from Continuous wave (CW) radar the target comes in when the next pulse is being sent out, once again the receiver cannot tell the difference. In order to maximize range, longer times between pulses should be used, referred to as a pulse repetition time (PRT), or its reciprocal, pulse repetition frequency (PRF). These two effects tend to be at odds with each other, and it is not easy to combine both good short range and good long range in a single radar. This is because the short pulses needed for a good minimum range broadcast have less total energy, making the returns much smaller and the target harder to detect. This could be offset by using more pulses, but this would shorten the maximum range again. So each radar uses a particular type of signal. Long-range radars tend to use long pulses with long delays between them, and short range radars use smaller pulses with less time between them. This pattern of pulses and pauses is known as the pulse repetition frequency (or PRF), and is one of the main ways to characterize a radar. As electronics have improved many radars now can change their PRF thereby changing their range. The newest radars fire 2 pulses during one cell, one for short range 10 km / 6 miles and a separate signal for longer ranges 100 km /60 miles. The distance resolution and the characteristics of the received signal as compared to noise depends heavily on the shape of the pulse. The pulse is often modulated to achieve better performance using a technique known as pulse compression. Distance may also be measured as a function of time. The radar mile is the amount of time it takes for a radar pulse to travel one nautical mile, reflect off a target, and return to the radar antenna. Since a nautical mile is defined as exactly 1,852 meters, then dividing this distance by the speed of light (exactly 299,792,458 meters per second), and then multiplying the result by 2 (round trip = twice the distance), yields a result of approximately 12.36 microseconds in duration.

Radar Frequency modulation Another form of distance measuring radar is based on frequency modulation. Frequency comparison between two signals is considerably more accurate, even with older electronics, than timing the signal. By measuring the frequency of the returned signal and comparing that with the original, the difference can be easily measured. This technique can be used in continuous wave radar, and is often found in aircraft radar altimeters. In these systems a "carrier" radar signal is frequency modulated in a predictable way, typically varying up and down with a sine wave or sawtooth pattern at audio frequencies. The signal is then sent out from one antenna and received on another, typically located on the bottom of the aircraft, and the signal can be continuously compared using a simple beat frequency modulator that produces an audio frequency tone from the returned signal and a portion of the transmitted signal. Since the signal frequency is changing, by the time the signal returns to the aircraft the broadcast has shifted to some other frequency. The amount of that shift is greater over longer times, so greater frequency differences mean a longer distance, the exact amount being the "ramp speed" selected by the electronics. The amount of shift is therefore directly related to the distance traveled, and can be displayed on an instrument. This signal processing is similar to that used in speed detecting Doppler radar. Example systems using this approach are AZUSA, MISTRAM, and UDOP. A further advantage is that the radar can operate effectively at relatively low frequencies, comparable to that used by UHF television. This was important in the early development of this type when high frequency signal generation was difficult or expensive. A new terrestrial radar uses low-power FM signals that cover a larger frequency range. The multiple reflections are analyzed mathematically for pattern changes with multiple passes creating a computerized synthetic image. Doppler effects are not used which allows slow moving objects to be detected as well as largely eliminating "noise" from the surfaces of bodies of water. Used primarily for detection of intruders approaching in small boats or intruders crawling on the ground toward an objective.

Speed measurement Speed is the change in distance to an object with respect to time. Thus the existing system for measuring distance, combined with a memory capacity to see where the target last was, is enough to measure speed. At one time the memory consisted of a user making grease-pencil marks on the radar screen, and then calculating the speed using a slide rule. Modern radar systems perform the equivalent operation faster and more accurately using computers. However, if the transmitter's output is coherent (phase synchronized), there is another effect that can be used to make almost instant speed measurements (no memory is required), known as the Doppler effect. Most modern radar systems use this principle in the pulse-doppler radar system. Return signals from targets are shifted away from this base frequency via the Doppler effect enabling the calculation of the speed of the object relative to the radar. The Doppler effect is only able to determine the relative speed of the target along the line of sight from the radar to the target. Any component of target velocity perpendicular to the line of sight cannot be determined by using the Doppler effect alone, but it can be determined by tracking the target's azimuth over time. Additional information of the nature of the Doppler returns may be found in the radar signal characteristics article. It is also possible to make a radar without any pulsing, known as a continuous-wave radar (CW radar), by sending out a very pure signal of a known frequency. CW radar is ideal for determining the radial component of a target's velocity, but it cannot determine the target's range. CW radar is typically used by traffic enforcement to measure vehicle speed quickly and accurately where range is not important. Other mathematical developments in radar signal processing include time-frequency analysis (Weyl Heisenberg or wavelet), as well as the chirplet transform which makes use of the fact that radar returns from moving targets typically "chirp" (change their frequency as a function of time, as does the sound of a bird or bat).

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Reduction of interference effects Signal processing is employed in radar systems to reduce the radar interference effects. Signal processing techniques include moving target indication (MTI), pulse doppler, moving target detection (MTD) processors, correlation with secondary surveillance radar (SSR) targets, space-time adaptive processing (STAP), and track-before-detect (TBD). Constant false alarm rate (CFAR) and digital terrain model (DTM) processing are also used in clutter environments.

Plot and track extraction Radar video returns on aircraft can be subjected to a plot extraction process whereby spurious and interfering signals are discarded. A sequence of target returns can be monitored through a device known as a plot extractor. The non relevant real time returns can be removed from the displayed information and a single plot displayed. In some radar systems, or alternatively in the command and control system to which the radar is connected, a radar tracker is used to associate the sequence of plots belonging to individual targets and estimate the targets' headings and speeds.

Radar engineering A radars components are: • A transmitter that generates the radio signal with an oscillator such as a klystron or a magnetron and controls its duration by a modulator. • A waveguide that links the transmitter and the antenna. • A duplexer that serves as a switch between the antenna and the transmitter or the receiver for the signal when the antenna is used in both situations. • A receiver. Knowing the shape of the desired received signal (a pulse), an optimal receiver can be designed using a matched filter.

Radar components

• An electronic section that controls all those devices and the antenna to perform the radar scan ordered by a software. • A link to end users.

Antenna design Radio signals broadcast from a single antenna will spread out in all directions, and likewise a single antenna will receive signals equally from all directions. This leaves the radar with the problem of deciding where the target object is located. Early systems tended to use omni-directional broadcast antennas, with directional receiver antennas which were pointed in various directions. For instance the first system to be deployed, Chain Home, used two straight antennas at right angles for reception, each on a different display. The maximum return would be detected with an antenna at right angles to the target, and a minimum with the antenna pointed directly at it (end on). The operator could determine the direction to a target by rotating the antenna so one display showed a maximum while the other shows a minimum. One serious limitation with this type of solution is that the broadcast is sent out in all directions, so the amount of energy in the direction being examined is a small part of that transmitted. To get a reasonable amount of power on the "target", the transmitting aerial should also be directional.

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Parabolic reflector More modern systems use a steerable parabolic "dish" to create a tight broadcast beam, typically using the same dish as the receiver. Such systems often combine two radar frequencies in the same antenna in order to allow automatic steering, or radar lock. Parabolic reflectors can be either symmetric parabolas or spoiled parabolas: • Symmetric parabolic antennas produce a narrow "pencil" beam in both the X and Y dimensions and consequently have a higher gain. The NEXRAD Pulse-Doppler weather radar uses a symmetric antenna to perform detailed volumetric scans of the atmosphere. • Spoiled parabolic antennas produce a narrow beam in one dimension and a relatively wide beam in the other. This feature is useful if target detection over a wide range of angles is more important than target location in three dimensions. Most 2D surveillance radars use a spoiled parabolic antenna with a narrow azimuthal beamwidth and wide vertical beamwidth. This beam configuration allows the radar operator to detect an aircraft at a specific azimuth but at an indeterminate height. Conversely, so-called "nodder" height finding radars use a dish with a narrow vertical beamwidth and wide azimuthal beamwidth to detect an aircraft at a specific height but with low azimuthal precision.

Surveillance radar antenna

Types of scan • Primary Scan: A scanning technique where the main antenna aerial is moved to produce a scanning beam, examples include circular scan, sector scan etc. • Secondary Scan: A scanning technique where the antenna feed is moved to produce a scanning beam, examples include conical scan, unidirectional sector scan, lobe switching etc. • Palmer Scan: A scanning technique that produces a scanning beam by moving the main antenna and its feed. A Palmer Scan is a combination of a Primary Scan and a Secondary Scan. Slotted waveguide Applied similarly to the parabolic reflector, the slotted waveguide is moved mechanically to scan and is particularly suitable for non-tracking surface scan systems, where the vertical pattern may remain constant. Owing to its lower cost and less wind exposure, shipboard, airport surface, and harbour surveillance radars now use this in preference to the parabolic antenna.

Slotted waveguide antenna

Radar Phased array Another method of steering is used in a phased array radar. This uses an array of similar aerials suitably spaced, the phase of the signal to each individual aerial being controlled so that the signal is reinforced in the desired direction and cancels in other directions. If the individual aerials are in one plane and the signal is fed to each aerial in phase with all others then the signal will reinforce in a direction perpendicular to that plane. By altering the relative phase of the signal fed to each aerial the direction of the beam can be moved because the direction of constructive interference will move. Because phased array radars require no physical movement the beam can scan at thousands of degrees per second, fast enough to irradiate and track many individual Phased array: Not all radar antennas must rotate to scan the sky. targets, and still run a wide-ranging search periodically. By simply turning some of the antennas on or off, the beam can be spread for searching, narrowed for tracking, or even split into two or more virtual radars. However, the beam cannot be effectively steered at small angles to the plane of the array, so for full coverage multiple arrays are required, typically disposed on the faces of a triangular pyramid (see picture). Phased array radars have been in use since the earliest years of radar use in World War II, but limitations of the electronics led to fairly poor accuracy. Phased array radars were originally used for missile defense. They are the heart of the ship-borne Aegis combat system, and the Patriot Missile System, and are increasingly used in other areas because the lack of moving parts makes them more reliable, and sometimes permits a much larger effective antenna, useful in fighter aircraft applications that offer only confined space for mechanical scanning. As the price of electronics has fallen, phased array radars have become more and more common. Almost all modern military radar systems are based on phased arrays, where the small additional cost is far offset by the improved reliability of a system with no moving parts. Traditional moving-antenna designs are still widely used in roles where cost is a significant factor such as air traffic surveillance, weather radars and similar systems. Phased array radars are also valued for use in aircraft, since they can track multiple targets. The first aircraft to use a phased array radar was the B-1B Lancer. The first aircraft fighter to use phased array radar was the Mikoyan MiG-31. The MiG-31M's SBI-16 Zaslon phased array radar is considered to be the world's most powerful fighter radar [23]. Phased-array interferometry or, aperture synthesis techniques, using an array of separate dishes that are phased into a single effective aperture, are not typically used for radar applications, although they are widely used in radio astronomy. Because of the Thinned array curse, such arrays of multiple apertures, when used in transmitters, result in narrow beams at the expense of reducing the total power transmitted to the target. In principle, such techniques used could increase the spatial resolution, but the lower power means that this is generally not effective. Aperture synthesis by post-processing of motion data from a single moving source, on the other hand, is widely used in space and airborne radar systems (see Synthetic aperture radar).

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Frequency bands The traditional band names originated as code-names during World War II and are still in military and aviation use throughout the world in the 21st century. They have been adopted in the United States by the IEEE, and internationally by the ITU. Most countries have additional regulations to control which parts of each band are available for civilian or military use. Other users of the radio spectrum, such as the broadcasting and electronic countermeasures (ECM) industries, have replaced the traditional military designations with their own systems.

Radar frequency bands Band name

Frequency range

Wavelength range

Notes

HF

3–30 MHz

10–100 m

coastal radar systems, over-the-horizon radar (OTH) radars; 'high frequency'

P

< 300 MHz

1 m+

'P' for 'previous', applied retrospectively to early radar systems

VHF

30–300 MHz

1–10 m

Very long range, ground penetrating; 'very high frequency'

UHF

300–1000 MHz

0.3–1 m

Very long range (e.g. ballistic missile early warning), ground penetrating, foliage penetrating; 'ultra high frequency'

L

1–2 GHz

15–30 cm

Long range air traffic control and surveillance; 'L' for 'long'

S

2–4 GHz

7.5–15 cm

Moderate range surveillance, Terminal air traffic control, long-range weather, marine radar; 'S' for 'short'

C

4–8 GHz

3.75–7.5 cm

Satellite transponders; a compromise (hence 'C') between X and S bands; weather; long range tracking

X

8–12 GHz

2.5–3.75 cm

Missile guidance, marine radar, weather, medium-resolution mapping and ground surveillance; in the USA the narrow range 10.525 GHz ±25 MHz is used for airport radar; short range tracking. Named X band because the frequency was a secret during WW2.

Ku

12–18 GHz

1.67–2.5 cm

high-resolution

K

18–24 GHz

1.11–1.67 cm

from German kurz, meaning 'short'; limited use due to absorption by water vapour, so Ku and Ka were used instead for surveillance. K-band is used for detecting clouds by meteorologists, and by police for detecting speeding motorists. K-band radar guns operate at 24.150 ± 0.100 GHz.

Ka

24–40 GHz

0.75–1.11 cm

mapping, short range, airport surveillance; frequency just above K band (hence 'a') Photo radar, used to trigger cameras which take pictures of license plates of cars running red lights, operates at 34.300 ± 0.100 GHz.

mm

40–300 GHz

7.5 mm – 1 mm

millimetre band, subdivided as below. The frequency ranges depend on waveguide size. Multiple letters are assigned to these bands by different groups. These are from Baytron, a now defunct company that made test equipment.

V

40–75 GHz

4.0–7.5 mm

Very strongly absorbed by atmospheric oxygen, which resonates at 60 GHz.

W

75–110 GHz

2.7–4.0 mm

used as a visual sensor for experimental autonomous vehicles, high-resolution meteorological observation, and imaging.

UWB

1.6–10.5 GHz

18.75 cm – 2.8 cm

used for through-the-wall radar and imaging systems.

Radar

Radar modulators Modulators act to provide the waveform of the RF-pulse. There are two different radar modulator designs: • high voltage switch for non-coherent keyed power-oscillators[24] These modulators consist of a high voltage pulse generator formed from a high voltage supply, a pulse forming network, and a high voltage switch such as a thyratron. They generate short pulses of power to feed the e.g. magnetron, a special type of vacuum tube that converts DC (usually pulsed) into microwaves. This technology is known as Pulsed power. In this way, the transmitted pulse of RF radiation is kept to a defined, and usually, very short duration. • hybrid mixers,[25] fed by a waveform generator and an exciter for a complex but coherent waveform. This waveform can be generated by low power/low-voltage input signals. In this case the radar transmitter must be a power-amplifier, e.g. a klystron tube or a solid state transmitter. In this way, the transmitted pulse is intrapulsemodulated and the radar receiver must use pulse compression technique mostly.

Radar coolant Coolanol and PAO (poly-alpha olefin) are the two main coolants used to cool airborne radar equipment today. Coolanol (silicate ester) was used in several military radars in the 1970s, for example the AN/APG-63 in the F-15. However, it is hygroscopic, leading to formation of highly flammable alcohol. The loss of a U.S. Navy aircraft in 1978 was attributed to a silicate ester fire.[26] Coolanol is also expensive and toxic. The U.S. Navy has instituted a program named Pollution Prevention (P2) to reduce or eliminate the volume and toxicity of waste, air emissions, and effluent discharges. Because of this Coolanol is used less often today. PAO is a synthetic lubricant blend of a polyol ester admixed with effective amounts of an antioxidant, yellow metal pacifier and rust inhibitors. The polyol ester blend includes a major proportion of poly (neopentyl polyol) ester blend formed by reacting poly(pentaerythritol) partial esters with at least one C7 to C12 carboxylic acid mixed with an ester formed by reacting a polyol having at least two hydroxyl groups and at least one C8-C10 carboxylic acid. Preferably, the acids are linear and avoid those which can cause odours during use. Effective additives include secondary arylamine antioxidants, triazole derivative yellow metal pacifier and an amino acid derivative and substituted primary and secondary amine and/or diamine rust inhibitor. A synthetic coolant/lubricant composition, comprising an ester mixture of 50 to 80 weight percent of poly (neopentyl polyol) ester formed by reacting a poly (neopentyl polyol) partial ester and at least one linear monocarboxylic acid having from 6 to 12 carbon atoms, and 20 to 50 weight percent of a polyol ester formed by reacting a polyol having 5 to 8 carbon atoms and at least two hydroxyl groups with at least one linear monocarboxylic acid having from 7 to 12 carbon atoms, the weight percents based on the total weight of the composition.

Radar configurations and types Radars configurations include Monopulse radar, Bistatic radar, Doppler radar, Continuous-wave radar, etc.. depending on the types of hardware and software used. It is used in aviation (Primary and secondary radar), sea vessels, law enforcement, weather surveillance, ground mapping, geophysical surveys, and biological research.

Notes [1] NASA. "RADAR means: Radio Detection and Ranging" (http:/ / web. archive. org/ web/ 20071014061010/ http:/ / nasaexplores. com/ show_k4_teacher_st. php?id=030703122033). Nasa Explores. Archived from the original (http:/ / www. nasaexplores. com/ show_k4_teacher_st. php?id=030703122033) on 2007-10-14. . [2] "Radar definition in multiple dictionaries" (http:/ / www. answers. com/ topic/ radar). Answers.com. . Retrieved 2008-10-09. [3] Kostenko, A. A., A. I. Nosich, and I. A. Tishchenko, "Radar Prehistory, Soviet Side," Proc. of IEEE APS International Symposium 2001, vol.4. p. 44, 2003 [4] Christian Hülsmeyer by Radar World (http:/ / www. radarworld. org/ huelsmeyer. html)

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Radar [5] Patent DE165546; Verfahren, um metallische Gegenstände mittels elektrischer Wellen einem Beobachter zu melden. (http:/ / upload. wikimedia. org/ wikipedia/ commons/ 1/ 11/ DE165546. pdf) [6] Verfahren zur Bestimmung der Entfernung von metallischen Gegenständen (Schiffen o. dgl.), deren Gegenwart durch das Verfahren nach Patent 16556 festgestellt wird. (http:/ / upload. wikimedia. org/ wikipedia/ commons/ e/ e9/ DE169154. pdf) [7] GB 13170 (http:/ / v3. espacenet. com/ textdoc?DB=EPODOC& IDX=GB13170) Telemobiloscope [8] The Electrical Experimenter, 1917 [9] Post-War Research and Development of Radio Communication Equipment (http:/ / earlyradiohistory. us/ 1963hw28. htm) [10] Radar (http:/ / earlyradiohistory. us/ 1963hw38. htm) [11] Watson, Raymond C., Jr., Radar Origins Worldwide: History of Its Evolution in 13 Nations Through World War II, Trafford Publishing, 2009, ISBN 978-1-4269-2111-7 [12] (http:/ / www. teslasociety. com/ time. jpg) [13] FR 788795 (http:/ / v3. espacenet. com/ textdoc?DB=EPODOC& IDX=FR788795) Nouveau système de repérage d'obstacles et ses applications [14] (French) Copy of Patents for the invention of radar (http:/ / www. radar-france. fr/ brevet radar1934. htm) on www.radar-france.fr [15] John Erickson. Radio-Location and the Air Defence Problem: The Design and Development of Soviet Radar. Science Studies, Vol. 2, No. 3 (Jul., 1972), pp. 241-263 [16] Page, Robert Morris, The Origin of Radar, Doubleday Anchor, New York, 1962, p. 66 [17] "Mystery Ray Locates Enemy" (http:/ / books. google. com/ books?id=bygDAAAAMBAJ& pg=PA29& dq=Popular+ Science+ 1932+ plane& hl=en& ei=Ku9QTcb4A425tgf6nbmeCQ& sa=X& oi=book_result& ct=result& resnum=5& ved=0CDoQ6AEwBDgy#v=onepage& q& f=true) Popular Mechanics, October 1935 [18] Goebel, Greg (2007-01-01). "The Wizard War: WW2 & The Origins Of Radar" (http:/ / www. vectorsite. net/ ttwiz_01. html). . Retrieved 2007-03-24. [19] Alan Dower Blumlein (2002). "The story of RADAR Development" (http:/ / www. doramusic. com/ Radar. htm). . Retrieved 2011-05-06. [20] British man first to patent radar (http:/ / www. patent. gov. uk/ media/ pressrelease/ 2001/ 1009. htm) official site of the Patent Office [21] GB 593017 (http:/ / v3. espacenet. com/ textdoc?DB=EPODOC& IDX=GB593017) Improvements in or relating to wireless systems [22] illustration bottom page 56 Popular Mechanics April 1940 (http:/ / books. google. com/ books?id=hCcDAAAAMBAJ& pg=PA56& dq=popular+ science+ April+ 1940& hl=en& ei=SHqMTJ6CK8fanAeroIC0Cw& sa=X& oi=book_result& ct=result& resnum=5& ved=0CEUQ6AEwBA#v=onepage& q& f=true) [23] http:/ / www. globalsecurity. org/ military/ world/ russia/ mig-31. htm [24] Radartutorial (http:/ / www. radartutorial. eu/ / 08. transmitters/ tx06. en. html) [25] Radartutorial (http:/ / www. radartutorial. eu/ / 08. transmitters/ tx10. en. html) [26] Stropki, Michael A. (1992). "POLYALPHAOLEFINS: A NEW IMPROVED COST EFFECTIVE AIRCRAFT RADAR COOLANT" (http:/ / www. dtic. mil/ cgi-bin/ GetTRDoc?AD=ADA250517& Location=U2& doc=GetTRDoc. pdf). Melbourne, Australia: Aeronautical Research Laboratory, Defense Science and Technology Organisation, Department of Defense. . Retrieved 2010-03-18.

References • Barrett, Dick, " All you ever wanted to know about British air defence radar (http://www.radarpages.co.uk/ index.htm)". The Radar Pages. (History and details of various British radar systems) • Buderi, " Telephone History: Radar History (http://www.privateline.com/TelephoneHistory3/ radarhistorybuderi.html)". Privateline.com. (Anecdotal account of the carriage of the world's first high power cavity magnetron from Britain to the US during WW2.) • Ekco Radar WW2 Shadow Factory (http://www.ekco-radar.co.uk/) The secret development of British radar. • ES310 " Introduction to Naval Weapons Engineering.". (Radar fundamentals section) (http://www.fas.org/ man/dod-101/navy/docs/es310/syllabus.htm) • Hollmann, Martin, " Radar Family Tree (http://www.radarworld.org/index.html)". Radar World (http:// www.radarworld.org/). • Penley, Bill, and Jonathan Penley, " Early Radar History (http://www.penleyradararchives.org.uk/history/ introduction.htm)—an Introduction". 2002. • Pub 1310 Radar Navigation and Maneuvering Board Manual, National Imagery and Mapping Agency, Bethesda, MD 2001 (US govt publication '...intended to be used primarily as a manual of instruction in navigation schools and by naval and merchant marine personnel.') • Swords, Seán S., Technical History of the Beginnings of Radar, IEE History of Technology Series, Vol. 6, London: Peter Peregrinus, 1986

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Further reading • Batt, Reg, "The Radar Army: Winning the War of the Airwaves", Robert Hale Ltd. 1991 ISBN 0-7090-4508-5 • Bowen, E.G., Radar Days, Institute of Physics Publishing, Bristol, 1987., ISBN 0-7503-0586-X • Bragg, Michael., RDF1 The Location of Aircraft by Radio Methods 1935–1945, Hawkhead Publishing, Paisley 1988 ISBN 0-9531544-0-8 The history of ground radar in the UK during World War II • Brown, Louis., A Radar History of World War II, Institute of Physics Publishing, Bristol, 1999., ISBN 0-7503-0659-9 • Buderi, Robert, The Invention That Changed the World: How a Small Group of Radar Pioneers Won the Second World War and Launched a Technological Revolution, Simon & Schuster, 1996. ISBN 0-684-81021-2 • Burch, David F., Radar For Mariners, McGraw Hill, 2005, ISBN 0-07-139867-8. • Hall, P.S., T.K. Garland-Collins, R.S. Picton and R.G. Lee, Radar, Brassey's (UK) Ltd., 1991, Land Warfare Series: Vol 9, ISBN 0-08-037711-4. • Howse, Derek, Radar At Sea The Royal Navy in World War 2, Naval Institute Press, Annapolis, Maryland, USA, 1993, ISBN 1-55750-704-X • Jones, R.V., Most Secret War, ISBN 1-85326-699-X. R.V. Jones' account of his part in British Scientific Intelligence between 1939 and 1945, working to anticipate the German's radar, radio navigation and V1/V2 developments. • Kaiser, Gerald, Chapter 10 in "A Friendly Guide to Wavelets", Birkhauser, Boston, 1994. • Kouemou, Guy (Ed.): Radar Technology. InTech, 2010, ISBN 978-953-307-029-2, ( (http://www.intechopen. com/books/show/title/radar-technology)). • Latham, Colin & Stobbs, Anne., Radar A Wartime Miracle, Sutton Publishing Ltd, Stroud 1996 ISBN 0-7509-1643-5 A history of radar in the UK during World War II told by the men and women who worked on it. • Le Chevalier, François, Principles of Radar and Sonar Signal Processing, Artech House, Boston, London, 2002. ISBN 1-58053-338-8. • Pritchard, David., The Radar War Germany's Pioneering Achievement 1904–1945 Patrick Stephens Ltd, Wellingborough 1989., ISBN 1-85260-246-5 • Skolnik, Merrill I., Introduction to Radar Systems, McGraw-Hill (1st ed., 1962; 2nd ed., 1980; 3rd ed., 2001), ISBN 0-07-066572-9. The de-facto radar introduction bible. • Skolnik, Merrill I., Radar Handbook. ISBN 0-07-057913-X widely used in the US since the 1970s. New 3rd Edition, February 2008, ISBN 0-07-148547-3; 978-0-07-148547-0 • Stimson, George W., Introduction to Airborne Radar, SciTech Publishing (2nd edition, 1998), ISBN 1-891121-01-4. Written for the non-specialist. The first half of the book on radar fundamentals is also applicable to ground- and sea-based radar. • Younghusband, Eileen., Not an Ordinary Life. How Changing Times Brought Historical Events into my Life, Cardiff Centre for Lifelong Learning, Cardiff, 2009., ISBN 987-0-9561156-9-0 (Pages 36–67 contain the experiences of a WAAF radar plotter in WWII.) • Younghusband, Eileen., One Woman's War. Cardiff. Candy Jar Books. 2011. ISBN 978-0-9566826-2-8 • Zimmerman, David., Britain's Shield Radar and the Defeat of the Luftwaffe, Sutton Publishing Ltd, Stroud, 2001, ISBN 0-7509-1799-7

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External links • MIT Video Course: Introduction to Radar Systems (http://ocw.mit.edu/resources/ res-ll-001-introduction-to-radar-systems-spring-2007/) A set of 10 video lectures developed at Lincoln Laboratory to develop an understanding of radar systems and technologies. • Popular Science, August 1943, What Are the Facts About RADAR (http://books.google.com/ books?id=_yYDAAAAMBAJ&pg=PA66&dq=popular+science+june+1941&hl=en& ei=cT2TTNqUB9Ofnwfn49ywCA&sa=X&oi=book_result&ct=result&resnum=4& ved=0CDwQ6AEwAw#v=onepage&q&f=true) one of the first detailed factual articles on radar history, principles and operation published in the US • "The Great Detective", 1946. Story of the development of radar by the Chrysler Corporation (http:// imperialclub.com/Yr/1945/46Radar/Cover.htm) • Christian Hülsmeyer and the early days of radar (http://www.xs4all.nl/~aobauer/Huelspart1def.pdf) • Radar: The Canadian History of Radar - Canadian War Museum (http://www.warmuseum.ca/cwm/ exhibitions/radar/index_e.shtml) • Radar technology principles (http://www.radartutorial.eu/index.en.html) • History of radar (http://math.la.asu.edu/~kuang/LM/030902-Radar_History10.pdf) • Radar invisibility with metamaterials (http://www.metamaterials.net) • Radar Research Center-Italy (http://crr.sesm.it) • Early radar development in the UK (http://www.purbeckradar.org.uk/) • Principles of radar target acquisition and weapon guidance systems (http://ourworld.compuserve.com/ homepages/edperry/ewtutor1.htm) • Cloaking and radar invisibility (http://www.radartechnology.eu/) • The Secrets of Radar Museum (http://www.secretsofradar.com) • 84th Radar Evaluation Squadron (http://www.rades.hill.af.mil/) • Radar (http://www.skybrary.aero/index.php/Radar) • EKCO WW II ASV radar units (http://www.ekco-radar.co.uk/ASV19/asv.php) • RAF Air Defence Radar Museum (http://www.radarmuseum.co.uk/) • Radar - A case study highlighting the vital contribution physics research has made to major technological development (http://www.iop.org/publications/iop/2011/page_47522.html)

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Applications Transistor radio A transistor radio is a small portable radio receiver using transistor-based circuitry. Following their development in 1954 they became the most popular electronic communication device in history, with billions manufactured during the 1960s and 1970s. Their pocket size sparked a change in popular music listening habits, for the first time allowing people to listen to music anywhere they went. In the 1970s their popularity declined as other portable media players such as boom boxes and portable cassette players took over.

Background Bell Laboratories demonstrated the first transistor on December 23, 1947.[1] The scientific team at Bell Laboratories responsible for the solid-state amplifier included William Shockley, Walter Houser Brattain, and John Bardeen.[2] After obtaining patent protection, the company held a news conference on June 30, 1948, at which a prototype transistor radio was demonstrated.[3] There are many claimants to the title of the first company to produce practical transistor radios, often incorrectly attributed to Sony (originally Tokyo Tsushin Kogyo). Texas Instruments had demonstrated all-transistor AM (amplitude modulation) radios as early as 1952, but their performance was well below that of equivalent vacuum tube models. A workable all-transistor radio was demonstrated in August 1953 at the Düsseldorf A five-transistor radio with back open, showing Radio Fair by the German firm Intermetall. It was built with four of parts. Intermetall's hand-made transistors, based upon the 1948 invention of Herbert Mataré and Heinrich Welker. However, as with the early Texas Instruments units (and others) only prototypes were ever built; it was never put into commercial production. RCA had demonstrated a prototype transistor radio as early as 1952 and it is likely that they and the other radio makers were planning transistor radios of their own, but Texas Instruments and Regency were the first to offer a production model.

Sanyo 8S-P3 transistor radio, which received AM and shortwave bands.

The use of transistors instead of vacuum tubes as the amplifier elements meant that the device was much smaller, required far less power to operate than a tube radio, and was more shock-resistant. Transistors are current amplifiers, while tubes are voltage amplifiers. Since the transistor base draws current, its impedance is low in contrast to the high impedance of the vacuum tubes.[4] It also allowed "instant-on" operation, since there were no filaments to heat up. The typical portable tube radio of the fifties was about the size and weight of a lunchbox, and contained several heavy, non-rechargeable batteries — one or more so-called "A" batteries to heat the tube filaments and a large 45- to 90-volt "B" battery to power the signal

Transistor radio circuits. By comparison, the "transistor" could fit in a pocket and weighed half a pound, or less, and was powered by standard flashlight batteries or a single compact 9-volt battery. (The now-familiar 9-volt battery was introduced for powering transistor radios.) Listeners sometimes held an entire transistor radio directly against the side of the head, with the speaker against the ear, to minimize the "tinny" sound caused by the high resonant frequency of its small speaker enclosure. Most radios included earphone jacks and came with single earphones that provided only mediocre-quality sound reproduction. To consumers familiar with the earphone-listening experience of the transistor radio, the first Sony Walkman cassette player, with a pair of high-fidelity stereo earphones, would provide a greatly contrasting display of audio fidelity. The transistor radio remains the single most popular communications device in existence. Some estimates suggest that there are at least seven billion of them in existence, almost all tunable to the common AM band, and an increasingly high percentage of those also tunable to the FM band. Some receive shortwave broadcasts as well. Most operate on battery power. They have become small and cheap due to improved electronics which has the ability to pack millions of transistors on one integrated circuit or chip. To the general public, the prefix "transistor" means a pocket radio; it can be used to refer to any small radio, but the term itself is now obsolete, since virtually all commercial broadcast receivers, pocket-sized or not, are now transistor-based.

Regency TR-1 — the first transistor radio Two companies working together, Texas Instruments of Dallas, Texas and Industrial Development Engineering Associates (I.D.E.A.) of Indianapolis, Indiana, were behind the unveiling of the Regency TR-1, the world's first commercially produced transistor radio. Previously, Texas Instruments was producing instrumentation for the oil industry and locating devices for the U.S. Navy, and I.D.E.A. built home television antenna boosters, but the two companies worked together on the TR-1, looking to grow revenues for their respective companies by breaking into this new product area.[3] In May 1954, Texas Instruments had designed and built a prototype and was looking for an established radio manufacturer to develop and market a radio using their transistors. None of the major radio makers including RCA, Philco, and Emerson were interested. The President of I.D.E.A. at the time, Ed Tudor, jumped at the opportunity to manufacture the TR-1, predicting sales of the transistor radios at "20 million radios in three years".[5] The Regency TR-1 was announced on October 18, 1954 by the Regency Division of I.D.E.A., was put on sale in November 1954, and was the Regency TR-1. first practical transistor radio made in any significant numbers. One year after the release of the TR-1 sales approached the 100,000 mark. The look and size of the TR-1 was well received, but the reviews of the TR-1's performance were typically adverse.[5] The Regency TR-1 is patented by Richard C. Koch, US 2892931 [6], former Project Engineer of I.D.E.A.

Raytheon 8-TP-1 — the second transistor radio In February 1955 the second transistor radio, the 8-TP-1, was introduced by Raytheon. It was a larger portable transistor radio, including an expansive four-inch speaker and four additional transistors (the TR-1 used only four). As a result the sound quality was much better than the TR-1. An additional benefit of the 8-TP-1 was its efficient battery consumption. In July 1955, the first positive review of a transistor radio appeared in the Consumer Reports that said, "The transistors in this set have not been used in an effort to build the smallest radio on the market, and

115

Transistor radio good performance has not been sacrificed." Following the success of the 8-TP-1, Zenith, RCA, DeWald, and Crosley began flooding the market with additional transistor radio models.[5]

Pricing Prior to the Regency TR-1, transistors were difficult to produce. Only one in five transistors that were produced worked as expected (only a 20% yield) and as a result the price remained extremely high.[5] When it was released in 1954, the Regency TR-1 cost $49.95 (roughly $364 in 2006 U.S. dollars) and sold about 150,000 units. Raytheon and Zenith Electronics transistor radios soon followed and were priced even higher. In 1955, Raytheon's 8-TR-1 was priced at $80 (approximately $425 in 1994 U.S. dollars).[5] Sony's TR-63, released in December 1957 cost $39.95.[5] Following the success of the TR-63 the Japanese companies continued to make their transistor radios smaller. Coupled with the extremely low labor costs in Japan, the Japanese transistor radios began selling for as low as $25. In 1962 American manufacturers dropped prices of transistor radios to as low as $15.[5]

Japanese history in the market While on a trip to the United States in 1952, Masura Ibuka, founder of Tokyo Telecommunications, discovered that AT&T was about to make licensing available for the transistor. Ibuka and physicist Akio Morita convinced the Ministry of International Trade and Industry (MITI) in Japan to finance the $25,000 licensing fee. For several months Ibuka traveled around the United States borrowing ideas from the American transistor manufacturers. Improving upon the ideas, Tokyo Telecommunications made its first functional transistor radio in 1954.[5] Within five years, Tokyo Telecommunications grew from seven employees to approximately five hundred. Other Japanese companies soon followed their entry into the American market and the grand total of electronic products exported from Japan in 1958 increased 2.5 times in comparison to 1957.[7]

TR-55 and TR-7 In August 1955, still a small company named Tokyo Tsushin Kogyo, Ltd. (Tokyo Telecommunications Engineering Corporation), Ibuka and Morita introduced their own five-transistor radio into the U.S. market, the TR-55, under the new brand name Sony.[8] [9] With its release, Sony became the first company to manufacture a radio from the transistors on up, and to utilize all miniature components. It is estimated that only 5,000 to 10,000 units were produced. Coupled by a lack of advertising the result was the demise of this initial attempt. In 1955, in addition to the TR-55, the TR-7 was introduced in the United States by Sony through trade magazines, but was as equally unsuccessful.[5]

TR-63 The TR-63 was introduced by Sony to the United States in December 1957. The TR-63 was 1/4" narrower and 1/2" shorter than the original Regency TR-1. Like the TR-1 it was offered in four colors: lemon, green, red, and black. In addition to its smaller size, the TR-63 had a small tuning capacitor and required a new nine-volt battery which would become the standard. Approximately 100,000 units of the TR-63 were imported in 1957.[5] This "pocketable" (The term "pocketable" was a matter of some interpretation, as Sony allegedly had special shirts made with oversized pockets for their salesmen) model proved highly successful in the market.[10] With the visible success of the TR-63 Japanese competitors such as Toshiba and Sharp joined the market. By 1959, in the United States market, there were more than six million transistor radio sets produced by Japanese companies that represented $62 million in revenue.[5]

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117

In popular culture Transistor radios were extremely successful because of four social forces — a large number of young people, a post-World War II baby boom, a public with a disposable income amidst a period of prosperity, and the growing popularity of rock 'n' roll music. The transistor radio appeared in many popular films such as Lolita and the term "transistor radio" can be heard in the lyrics of Van Morrison's "Brown-Eyed Girl", a famous top 10 Billboard hit in the late 1960s. Starting in 1954, transistor radios took on more elaborate designs as a result of heated competition. Eventually, transistor radios doubled as novelty items. The small components of transistor radios that became smaller over time were used to make anything from "Jimmy Carter Peanut-shaped" radios to "Gun-shaped" radios to "Mork from Ork Eggship-shaped" radios. Corporations used transistor radios to advertise their business. "Charlie the Tuna-shaped" radios could be purchased from Star-Kist for an insignificant amount of money giving their company visibility amongst the public. These novelty radios are now bought and sold as collector's items amongst modern day collectors.[11]

Transistor radio decline The emergence of Hong Kong in the transistor radio market resulted in the decline of Japanese participation in the late-1960's. Japan continued to dominate the electronics and semiconductor markets, but now shied away from radio manufacturing leaving production to not only Hong Kong, but also Korea, Taiwan, and other Pacific Rim countries who picked up where Japan left off. Currently, China is the foremost producer of transistor radios. In 1970 the last assembly line producing transistor radios in America shut down. The Zenith Trans-Oceanic 7000 was the last American-made transistor radio.[5]

Rise of digital audio players Use of air signal only radios (AM/FM) have declined in popularity with the rise of portable digital audio players, which allow users to carry and listen to the music of their choosing and may also include a radio tuner. This is a popular choice with listeners who are dissatisfied with terrestrial music radio because of a limited selection of music or other criticisms. However, transistor radios are still popular for news, talk radio, weather, live sporting events and emergency alert applications.

References [1] The Invention of the Transistor (http:/ / www. juliantrubin. com/ bigten/ transistorexperiments. html) [2] Handy, Erbe, Blackham, Antonier (1993). Made In Japan : Transistor Radios of the 1950s and 1960s. Chronicle Books. ISBN 0-8118-0271-X. page 13 [3] "The Revolution in Your Pocket" (http:/ / www. americanheritage. com/ articles/ magazine/ it/ 2004/ 2/ 2004_2_12. shtml). . Retrieved 2010-04-20. [4] Donald L. Stoner and L.A. Earnshaw (1963). The Transistor Radio Handbook: Theory, Circuitry, and Equipment. Editors and Engineers, Ltd.. page 32

A modern transistor radio (Sony Walkman SRF-S84 transistor radio, released 2001, shown without earphones)

[5] David Lane and Robert Lane (1994). Transistor Radios: A Collector's Encyclopedia and Price Guide. Wallace-Homestead Book Company. ISBN 0-87069-712-9. pages 2-7 [6] http:/ / v3. espacenet. com/ textdoc?DB=EPODOC& IDX=US2892931 [7] Handy, Erbe, Blackham, Antonier (1993). Made In Japan : Transistor Radios of the 1950s and 1960s. Chronicle Books. ISBN 0-8118-0271-X. pages 23-29 [8] John Nathan (1999). SONY : the private life. Houghton Mifflin Company. ISBN 0-395-89327-5. page 35 [9] "Transistor Radios" (http:/ / www. pbs. org/ transistor/ background1/ events/ tradio. html). ScienCentral. 1999. . Retrieved 2010-01-19. [10] "Sony Global - Sony History" (http:/ / www. sony. net/ Fun/ SH/ 1-6/ h2. html). . Retrieved 2008-09-01.

Transistor radio [11] Handy, Erbe, Blackham, Antonier (1993). Made In Japan : Transistor Radios of the 1950s and 1960s. Chronicle Books. ISBN 0-8118-0271-X. pages 46-51

Further reading • Michael F. Wolff: "The secret six-month project. Why Texas Instruments decided to put the first transistor radio on the market by Christmas 1954 and how it was accomplished." IEEE Spectrum, December 1985, pages 64–69 • Transistor Radios: 1954-1968 (Schiffer Book for Collectors) by Norman R. Smith • Made in Japan: Transistor Radios of the 1950s and 1960s by Handy, Erbe, Blackham, Antonier (1993) (ISBN 0-8118-0271-X) • Unique books on Transistor Radios (http://www.ericwrobbel.com/) by Eric Wrobbel • The Portable Radio in American Life by University of Arizona Professor Michael Brian Schiffer, Ph.D. (The University of Arizona Press, 1991). • Restoring Pocket Radios (DVD) by Ron Mansfield and Eric Wrobbel. (ChildhoodRadios.com, 2002). • The Regency TR-1 story, based on an interview with Regency co-founder, John Pies (partner with Joe Weaver) www.regencytr1.com/Regency_Early_Years.html (http://www.regencytr1.com/Regency_Early_Years.html/)

External links • http://www.jamesbutters.com/Focusing on the history and design elements of early pocket transistor radios. • http://www.pbs.org/transistor/materials/how-europe-missed-transistor.pdfPDF • TI Information Bulletin (http://www.ti.com/corp/docs/company/history/timeline/semicon/1950/docs/ 54regency.htm) First Commercial Transistor Radio October 18, 1954 • Web site about the first transistor radio (http://people.msoe.edu/~reyer/regency/) by Dr. Steven Reyer, a Professor in the Electrical Engineering and Computer Science Department at the Milwaukee School of Engineering. • M31 Galaxy of Transistor Radios (http://www.abetterpage.com/transistors/trans/1trans.html) Transistor radio site with photos and infoormation on classic transistor radios from the U.S., Japan, and Europe. • Radio Wallah (http://tabiwallah.com/radiowallah/) Historical data accompanied by hundreds of images covering early transistor radios. • Sarah's Transistor Radios (http://www.transistor.org/) Web site displaying over 1500 transistor radios and other information. • Regency TR-1 Transistor Radio History: (http://www.regencytr1.com) Web site with many historical references on the web and in published literature • 1954 to 2004, the TR-1's Golden Anniversary (http://people.msoe.edu/~reyer/regency/). In depth coverage of the Regency radio. • The Transistor Radio Directory (http://sites.google.com/site/aldoandr/home/the-transistor-radio-directory). • Beautiful vintage sound equipment (http://www.flickr.com/photos/nfsa/sets/72157623995696900/) from the National Film and Sound Archive, Australia (http://www.nfsa.gov.au)

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Walkie-talkie A walkie-talkie (more formally known as a handheld transceiver) is a hand-held, portable, two-way radio transceiver. Its development during the Second World War has been variously credited to Donald L. Hings, radio engineer Alfred J. Gross, and engineering teams at Motorola. Similar designs were created for other armed forces, and after the war, walkie-talkies spread to public safety and eventually commercial and jobsite work. Major characteristics include a half-duplex channel (only one radio transmits at a time, though any number can listen) and a "push-to-talk" (PTT) switch that starts transmission. Typical walkie-talkies resemble a telephone handset, possibly slightly larger but still a single unit, with an antenna sticking out of the top. Where a phone's earpiece is only loud enough to be heard by the user, a walkie-talkie's built-in speaker can be heard by the user and those in the user's immediate vicinity. Hand-held transceivers may be used to communicate between each other, or to vehicle-mounted or base stations.

Recreational, toy and amateur radio walkie talkies.

A picture of two consumer-grade walkie-talkies (PMR446-type).

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History The first radio receiver/transmitter to be widely nicknamed "Walkie-Talkie" was the backpacked Motorola SCR-300, created by an engineering team in 1940 at the Galvin Manufacturing Company (fore-runner of Motorola). The team consisted of Dan Noble, who conceived of the design using frequency modulation, Raymond Yoder, Henryk Magnuski who was the principal RF engineer, Marion Bond, Lloyd Morris, and Bill Vogel.

SCR-300-A "walkie talkie"

Motorola also produced the hand-held AM SCR-536 radio during World War II, and it was called the "Handie-Talkie" (HT). The terms are often confused today, but the original walkie talkie referred to the back mounted model, while the handie talkie was the device which could be held entirely in the hand (but had vastly reduced performance). Both devices ran on vacuum tubes and used high voltage dry cell batteries. (Handie-Talkie became a trademark of Motorola, Inc. on May 22, 1951. The application was filed with the U.S. Patent and Trademark Office, and the trademark registration number is 71560123.)

SCR-536 "handie talkie".

Radio engineer Alfred J. Gross also worked on the early technology behind the walkie-talkie between 1934 and 1941, and is sometimes credited with inventing it. Gross had developed and tested a small portable high-frequency radio with two-way communications features which he dubbed a "walkie-talkie". The device caught the attention of the U.S. Office of Strategic Services (now the Central Intelligence Agency), which recruited him to develop the Joan-Eleanor system, a two-way, air-to-ground radio system for covert use by troops behind enemy lines during World War II.[1]

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Also credited with the invention of the walkie talkie is Canadian inventor Donald Hings who created a portable radio signaling system for his employer CM&S in 1937 which he called a "packset", but which later became known as the "walkie talkie". Hings was formally decorated for its significance to the war effort.[2] [3] Hing's model C-58 "Handy-Talkie" was in military service by 1942, the result of a secret R&D effort that began in 1940. Following World War II, Raytheon developed the SCR-536's military replacement, the AN/PRC-6. The AN/PRC-6 circuit uses 13 vacuum tubes (receiver and transmitter); a second set of 13 tubes is supplied with the unit as running spares. The unit is factory set with one crystal and may be changed to a different frequency in the field by replacing the crystal and re-tuning the unit. It uses a 24 inch whip antenna. There is an optional handset H-33C/PT that can be connected to the AN/PRC-6 by a 5 foot cable. A web sling is provided.

Noemfoor, Dutch New Guinea, July 1944. A US soldier (foreground) uses a walkie-talkie during the Battle of Noemfoor. (Photographer: Allan F. Anderson.)

In the mid-1970s the Marine Corps initiated an effort to develop a squad radio to replace the unsatisfactory helmet-mounted AN/PRR-9 receiver and receiver/transmitter hand-held AN/PRT-4 (both developed by the Army). The AN/PRC-68 was first produced in 1976 by Magnavox, was issued to the Marines in the 1980s, and was adopted by the US Army as well. The abbreviation HT, derived from Motorola's "Handie Talkie" trademark, is commonly used to refer to portable handheld ham radios, with "walkie-talkie" often used as a layman's term or specifically to refer to a toy. Public safety or commercial users generally refer to their handhelds simply as "radios". Surplus Motorola Handie Talkies found their way into the hands of ham radio operators immediately following World War II. Motorola's public safety radios of the 1950s and 1960s, were loaned or donated to ham groups as part of the Civil Defense program. To avoid trademark infringement, other manufacturers use designations such as "Handheld Transceiver" or "Handie Transceiver" for their products.

Developments Some cellular telephone networks offer a push-to-talk handset that allows walkie-talkie-like operation over the cellular network, without dialling a call each time. Walkie-talkies for public safety, commercial and industrial uses may be part of trunked radio systems, which dynamically allocate radio channels for more efficient use of limited radio spectrum. Such systems always work with a base station that acts as a repeater and controller, although individual handsets and mobiles may have a mode that bypasses the base station.

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Contemporary use Walkie-talkies are widely used in any setting where portable radio communications are necessary, including business, public safety, military, outdoor recreation, and the like, and devices are available at numerous price points from inexpensive analog units sold as toys up to ruggedized (i.e. waterproof or intrinsically safe) analog and digital units for use on boats or in heavy industry. Most countries, at the very least, will allow the sale of walkie-talkies for business, marine communications, and some personal uses such as CB radio, as well as amateur radio designs. Walkie-talkies, thanks to increasing use of miniaturized electronics, can be made very small, with some personal two-way UHF radio models being smaller than a deck of cards (though VHF and HF units can be substantially larger due to the need for larger antennas and battery packs). In addition, as costs come down, it is possible to add advanced squelch capabilities such as CTCSS (analog squelch) and DCS (digital squelch) (often marketed as "privacy codes") to inexpensive radios, as well as voice scrambling and trunking capabilities. Some units (especially amateur HTs) also include DTMF keypads for remote operation of various devices such as repeaters. Some models include VOX capability for hands-free operation, as well as the ability to attach external microphones and speakers.

A modern Project 25-capable professional walkie talkie.

Consumer and commercial equipment differ in a number of ways; commercial gear is generally ruggedized, with metal cases, and often has only a few specific frequencies programmed into it (often, though not always, with a computer or other outside programming device; older units can simply swap crystals), since a given business or public safety agent must often abide by a specific frequency allocation. Consumer gear, on the other hand, is generally made to be small, lightweight, and capable of accessing any channel within the specified band, not just a subset of assigned channels.

Motorola HT1000 two-way radio

Military

Walkie-talkie Military organizations use handheld radios for a variety of purposes. Modern units such as the AN/PRC-148 Multiband Inter/Intra Team Radio (MBITR) can communicate on a variety of bands and modulation schemes and include encryption capabilities.

Amateur radio Walkie-talkies (also known as HTs or "handheld transceivers" ) are widely used among amateur radio operators. While converted commercial gear by companies such as Motorola are not uncommon, many companies such as Yaesu, Icom, and Kenwood design models specifically for amateur use. While superficially similar to commercial and personal units (including such things as CTCSS and DCS squelch functions, used primarily to activate amateur radio repeaters), amateur gear usually has a number of features that are not common to other gear, including: • Wide-band receivers, often including radio scanner functionality, for listening to non-amateur radio bands. • Multiple bands; while some operate only on specific bands such as 2 meters or 70 cm, others support several UHF and VHF amateur allocations available to the user. • Since amateur allocations usually are not channelized, the user can dial in any frequency desired in the authorized band. • Multiple modulation schemes: a few amateur HTs may allow modulation modes other than FM, including AM, SSB, and CW,[4] [5] and digital modes such as radioteletype or PSK31. Some may have TNCs built in to support packet radio data transmission without additional hardware. A newer addition to the Amateur Radio service is Digital Smart Technology for Amateur Radio or D-STAR. Handheld radios with this technology have several advanced features, including narrower bandwidth, simultaneous voice and messaging, GPS position reporting, and callsign routed radio calls over a wide ranging international network. As mentioned, commercial walkie-talkies can sometimes be reprogrammed to operate on amateur frequencies. Amateur radio operators may do this for cost reasons or due to a perception that commercial gear is more solidly constructed or better designed than purpose-built amateur gear.

Personal use The personal walkie-talkie has become popular also because of the U.S. Family Radio Service (FRS) and similar unlicensed services (such as Europe's PMR446 and Australia's UHF CB) in other countries. While FRS walkie-talkies are also sometimes used as toys because mass-production makes them low cost, they have proper superheterodyne receivers and are a useful communication tool for both business and personal use. The boom in unlicensed transceivers has, however, been a source of frustration to users of licensed services that are sometimes interfered with. For example, FRS and GMRS overlap in the United States, resulting in substantial pirate use of the GMRS frequencies. Use of the GMRS frequencies (USA) requires a license; however most users either disregard this requirement or are unaware. Canada reallocated frequencies for unlicensed use due to heavy interference from US GMRS users. The European PMR446 channels fall in the middle of a United States UHF amateur allocation, and the US FRS channels interfere with public safety communications in the United Kingdom. Designs for personal walkie-talkies are in any case tightly regulated, generally requiring non-removable antennas (with a few exceptions such as CB radio and the United States MURS allocation) and forbidding modified radios.

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Most personal walkie-talkies sold are designed to operate in UHF allocations, and are designed to be very compact, with buttons for changing channels and other settings on the face of the radio and a short, fixed antenna. Most such units are made of heavy, often brightly colored plastic, though some more expensive units have ruggedized metal or plastic cases. Commercial-grade radios are often designed to be used on allocations such as GMRS or MURS (the latter of which has had very little readily available purpose-built equipment). In addition, CB walkie-talkies are available, but less popular due to the propagation characteristics of the 27 MHz band and the general bulkiness of the gear involved.

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A Motorola FRS radio with labeled parts

Personal walkie-talkies are generally designed to give easy access to all available channels (and, if supplied, squelch codes) within the device's specified allocation. Personal two-way radios are also sometimes combined with other electronic devices; Garmin's Rino series combine a GPS receiver in the same package as an FRS/GMRS walkie-talkie (allowing Rino users to transmit digital location data to each other) Some personal radios also include receivers for AM and FM broadcast radio and, where applicable, NOAA Weather Radio and similar systems broadcasting on the same frequencies. Some designs also allow the sending of text messages and pictures between similarly equipped units. While jobsite and government radios are often rated in power output, consumer radios are frequently and controversially rated in mile or kilometer ratings. Because of the line of sight propagation of UHF signals, experienced users consider such ratings to be wildly exaggerated, and some manufacturers have begun printing range ratings on the package based on terrain as opposed to simple power output. While the bulk of personal walkie-talkie traffic is in the 27 MHz area and in the 400-500 MHz area of the UHF spectrum, there are some units that use the 49 MHz band (shared with cordless phones, baby monitors, and similar devices) as well as the 900 MHz band; in the US at least, units in these bands do not require licenses as long as they adhere to FCC power output rules. A company called TriSquare [6] is, as of July 2007, marketing a series of walkie-talkies in the United States based on frequency-hopping spread spectrum technology operating in this frequency range under the name eXRS (eXtreme Radio Service—despite the name, a proprietary design, not an official allocation of the US FCC). The spread-spectrum scheme used in eXRS radios allows up to 10 billion virtual "channels" and ensures private communications between two or more units.

Recreation Low-power versions, exempt from licence requirements, are also popular children's toys such as the Fischer Price Walkie-Talkie for children illustrated in the top image on the right. Prior to the change of CB radio from licensed to "permitted by part" (FCC rules Part 95) status, the typical toy walkie-talkie available in North America was limited to 100 milliwatts of power on transmit and using one or two crystal-controlled channels in the 27 MHz citizens' band using amplitude modulation (AM) only. Later toy walkie-talkies operated in the 49 MHz band, some with frequency modulation (FM), shared with cordless phones and baby monitors. The lowest cost devices are very simple electronically (single-frequency, crystal-controlled, generally based on a simple discrete transistor circuit where "grownup" walkie-talkies use chips), may employ superregenerative receivers, and may lack even a volume control, but they may nevertheless be elaborately decorated, often superficially resembling more "grown-up" radios such as FRS or public safety gear. Unlike more costly units, low-cost toy walkie-talkies may not have separate microphones

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and speakers; the receiver's speaker sometimes doubles as a microphone while in transmit mode. An unusual feature, common on children's walkie-talkies but seldom available otherwise even on amateur models, is a "code key", that is, a button allowing the operator to transmit Morse code or similar tones to another walkie-talkie operating on the same frequency. Generally the operator depresses the PTT button and taps out a message using a Morse Code crib sheet attached as a sticker to the radio; however, as Morse Code has fallen out of wide use outside amateur radio circles, some such units either have a grossly simplified code label or no longer provide a sticker at all. An inexpensive children's walkie-talkie.

In addition, personal UHF radios will sometimes be bought and used as toys, though they are not generally explicitly marketed as such (but see Hasbro's ChatNow line, which transmits both voice and digital data on the FRS band).

Specialized uses In addition to land mobile use, walkie-talkie designs are also used for marine VHF and aviation communications, especially on smaller boats and aircraft where mounting a fixed radio might be impractical or expensive. Often such units will have switches to provide quick access to emergency and information channels. Intrinsically safe walkie-talkies are often required in heavy industrial settings where the radio may be used around flammable vapors. This designation means that the knobs and switches in the radio are engineered to avoid producing sparks as they are operated.

Accessories There are various accessories available for walkie talkies such as rechargeable batteries, drop in rechargers, multi-unit rechargers for charging as many as six units at a time, and an audio accessory jack that can be used for headsets or speaker microphones.[7] When headsets are used with voice activation (VOX) capability the user can talk with hands free operation. Several types of audio accessories are available such as speaker microphones that clip near the ear, security type earpieces with a pendant push-to-talk switch and a built-in microphone, a headset that has a push-to-talk switch earbud that looks more like what you would find on a music player, or a single-ear lightweight behind-the-head headset with boom microphone and pendant push-to-talk switch similar to that worn by a telephone call center agent.

Walkie-talkie

References Footnotes [1] "Al Gross" (http:/ / web. mit. edu/ invent/ a-winners/ a-gross. html). Lemelson-MIT Program. . Retrieved 2008-12-16. [2] http:/ / www. telecomhall. ca/ tour/ inventors/ 2006/ donald_l_hings/ WalkieTalkie. pdf?sourceid=navclient& ie=UTF-8& rls=GGLJ,GGLJ:2006-10,GGLJ:en& q=Donald+ L. + Hings+ . THE VANCOUVER SUN, Friday August 17, 2001 Walkie-Talkie Inventor Receives Order of Canada [3] "CBC.ca - The Greatest Canadian Invention" (http:/ / www. cbc. ca/ inventions/ inventions. html). CBC News. . [4] http:/ / www. rigpix. com/ tokyohypower/ ht750. htm Tokyo HyPower HT750 [5] http:/ / www. rigpix. com/ mizuho/ mizuho_mx2. htm Mizuho MX2 [6] http:/ / www. trisquare. us [7] "Two Way Radios" (https:/ / www. intercomsonline. com/ Articles. asp?ID=181) page of IntercomsOnline.com (http:/ / www. intercomsonline. com/ ).

Notations • Onslow, David. "Two-Way Radio Success: How to Choose Two-Way Radios, Commercial Intercoms, and Other Wireless Communication Devices for Your Business" (https://www.intercomsonline.com/Articles. asp?ID=181). IntercomsOnline.com. Retrieved 2008-10-24.

Further reading • Dunlap, Orrin E., Jr. Marconi: The man and his wireless. (Arno Press., New York: 1971) • Harlow, Alvin F., Old Waves and New Wires: The History of the Telegraph, Telephone, and Wireless. (Appleton-Century Co., New York: 1936) • Herrick, Clyde N., Radio: Theory and Servicing. (Reston Publishing Company, Inc., Virginia 1975) • Martin, James. Future Developments in Telecommunications 2nd Ed., (Prentice Hall Inc., New Jersey: 1977) • Martin, James. The Wired Society. (Prentice Hall Inc., New Jersey: 1978) • Silver, H. Ward. Two-Way Radios and Scanners for Dummies. (Wiley Publishing, Hoboken, NH, 2005, ISBN 978-0-7645-9582-0)

External links • SCR-300-A Technical Manual (http://www.scr300.org) • U.S. Army Signal Corp Museum - exhibits and collections (http://gordon.army.mil/ocos/Museum/exhibits. asp) • Al Gross, 2000 Lemelson-MIT Lifetime Achievement Award Winner, developed the "walkie-talkie". (http://web. mit.edu/invent/a-winners/a-gross.html) • RetroCom, a collection of vintage radio gear images and articles. (http://www.retrocom.com) • Simple VHF walkie-talkie circuit (http://www.freeinfosociety.com/electronics/schemview.php?id=1870) (note: requires an experienced builder, and may be illegal to operate without a proper license from your local communications agency)

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Extra Knowledge Noise (electronics) Electronic noise [1] is a random fluctuation in an electrical signal, a characteristic of all electronic circuits. Noise generated by electronic devices varies greatly, as it can be produced by several different effects. Thermal noise is unavoidable at non-zero temperature (see Fluctuation-dissipation theorem), while other types depend mostly on device type (such as shot noise [1] [2] , which needs steep potential barrier) or manufacturing quality and semiconductor defects (such as conductance fluctuations, including 1/f noise). In communication systems, the noise is an error or undesired random disturbance of a useful information signal, introduced before or after the detector and decoder. The noise is a summation of unwanted or disturbing energy from natural and sometimes man-made sources. Noise is, however, typically distinguished from interference, (e.g. cross-talk, deliberate jamming or other unwanted electromagnetic interference from specific transmitters), for example in the signal-to-noise ratio (SNR), signal-to-interference ratio (SIR) and signal-to-noise plus interference ratio (SNIR) measures. Noise is also typically distinguished from distortion, which is an unwanted alteration of the signal waveform, for example in the signal-to-noise and distortion ratio (SINAD). In a carrier-modulated passband analog communication system, a certain carrier-to-noise ratio (CNR) at the radio receiver input would result in a certain signal-to-noise ratio in the detected message signal. In a digital communications system, a certain Eb/N0 (normalized signal-to-noise ratio) would result in a certain bit error rate (BER). While noise is generally unwanted, it can serve a useful purpose in some applications, such as random number generation or dithering.

Types [1] Thermal noise Johnson–Nyquist noise (sometimes thermal, Johnson or Nyquist noise) is unavoidable, and generated by the random thermal motion of charge carriers (usually electrons), inside an electrical conductor, which happens regardless of any applied voltage. Thermal noise is approximately white, meaning that its power spectral density is nearly equal throughout the frequency spectrum. The amplitude of the signal has very nearly a Gaussian probability density function. A communication system affected by thermal noise is often modeled as an additive white Gaussian noise (AWGN) channel. The root mean square (RMS) voltage due to thermal noise Δf (hertz), is given by

, generated in a resistance R (ohms) over bandwidth

where kB is Boltzmann's constant (joules per kelvin) and T is the resistor's absolute temperature (kelvin). As the amount of thermal noise generated depends upon the temperature of the circuit, very sensitive circuits such as preamplifiers in radio telescopes are sometimes cooled in liquid nitrogen to reduce the noise level.

Noise (electronics)

Shot noise Shot noise in electronic devices consists of unavoidable random statistical fluctuations of the electric current in an electrical conductor. Random fluctuations are inherent when current flows, as the current is a flow of discrete charges (electrons).

Flicker noise Flicker noise, also known as 1/f noise, is a signal or process with a frequency spectrum that falls off steadily into the higher frequencies, with a pink spectrum. It occurs in almost all electronic devices, and results from a variety of effects, though always related to a direct current.

Burst noise Burst noise consists of sudden step-like transitions between two or more levels (non-Gaussian), as high as several hundred millivolts, at random and unpredictable times. Each shift in offset voltage or current lasts for several milliseconds, and the intervals between pulses tend to be in the audio range (less than 100 Hz), leading to the term popcorn noise for the popping or crackling sounds it produces in audio circuits.

Avalanche noise Avalanche noise is the noise produced when a junction diode is operated at the onset of avalanche breakdown, a semiconductor junction phenomenon in which carriers in a high voltage gradient develop sufficient energy to dislodge additional carriers through physical impact, creating ragged current flows.

Quantification The noise level in an electronic system is typically measured as an electrical power N in watts or dBm, a root mean square (RMS) voltage (identical to the noise standard deviation) in volts, dBμV or a mean squared error (MSE) in volts squared. Noise may also be characterized by its probability distribution and noise spectral density N0(f) in watts per hertz. A noise signal is typically considered as a linear addition to a useful information signal. Typical signal quality measures involving noise are signal-to-noise ratio (SNR or S/N), signal-to-quantization noise ratio (SQNR) in analog-to-digital coversion and compression, peak signal-to-noise ratio (PSNR) in image and video coding, Eb/N0 in digital transmission, carrier to noise ratio (CNR) before the detector in carrier-modulated systems, and noise figure in cascaded amplifiers. Noise is a random process, characterized by stochastic properties such as its variance, distribution, and spectral density. The spectral distribution of noise can vary with frequency, so its power density is measured in watts per hertz (W/Hz). Since the power in a resistive element is proportional to the square of the voltage across it, noise voltage (density) can be described by taking the square root of the noise power density, resulting in volts per root hertz ( ). Integrated circuit devices, such as operational amplifiers commonly quote equivalent input noise level in these terms (at room temperature). Noise power is measured in Watts or decibels (dB) relative to a standard power, usually indicated by adding a suffix after dB. Examples of electrical noise-level measurement units are dBu, dBm0, dBrn, dBrnC, and dBrn(f1 − f2), dBrn(144-line). Noise levels are usually viewed in opposition to signal levels and so are often seen as part of a signal-to-noise ratio (SNR). Telecommunication systems strive to increase the ratio of signal level to noise level in order to effectively transmit data. In practice, if the transmitted signal falls below the level of the noise (often designated as the noise floor) in the system, data can no longer be decoded at the receiver. Noise in telecommunication systems is a product of both internal and external sources to the system.

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Dither If the noise source is correlated with the signal, such as in the case of quantisation error, the intentional introduction of additional noise, called dither, can reduce overall noise in the bandwidth of interest. This technique allows retrieval of signals below the nominal detection threshold of an instrument. This is an example of stochastic resonance.

References [1] C.D. Motchenbacher, J.A. Connelly (1993). Low-noise electronic system design. Wiley Interscience. [2] L.B. Kish, C.G. Granqvist (2000). "Noise in nanotechnology (invite paper)". Microelectronics-Reliability 40: 1833.

• White noise calculator, thermal noise - Voltage in microvolts, conversion to noise level in dBu and dBV and vice versa (http://www.sengpielaudio.com/calculator-noise.htm)

External links • Active Filter (Sallen & Key) Noise Study (http://www.c-c-i.com/node/77)

Further reading • Sh. Kogan (1996). Electronic Noise and Fluctuations in Solids. Cambridge University Press. ISBN 0521460344.  This article incorporates public domain material from websites or documents of the General Services Administration (in support of MIL-STD-188).

Induction plasma technology The 1960s were the incipient period of Thermal Plasma Technology, driven by the necessity of aerospace programs. Among the various methods of thermal plasma generation, induction plasma (or inductively coupled plasma) takes up an important role. The early effort to maintain inductively coupled plasma on a stream of gas could retrospect to Babat[1] in 1947, and Reed[2] in 1961. The earlier stage of the investigations was concentrated in the fundamental studies of the energy coupling mechanism and the characteristics of the flow, temperature and concentration fields in the plasma discharge. In 1980’s, with the increasing demand for high performance materials and other scientific issues, people demonstrated high interest in applications of induction plasma technology in industrial scale production and other projects, for example, the waste treatment. Numerous research and development were devoted to bridge the gap between the laboratory gadget and the industry integration. After decades’ effort, induction plasma technology has got a firm foothold in modern advanced industry.

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The generation of induction plasma Induction heating is a very mature technology of hundred years’ history. A conductive metallic piece, inside a coil of high frequency, will be “induced”, and heated to the red-hot state. There is no difference in cardinal principle for either induction heating or “inductively coupled plasma”, only that the medium to induce, in the later case, is replaced by the flowing gas, and the temperature obtained is extremely high, as it arrives the "fourth state of the matter”—plasma. As shown in picture, inductively coupled plasma (ICP) torch is essentially a copper coil of several turns, through which cooling water is running in order to dissipate the heat produced in operation. The coil wraps a confinement tube, inside which the induction plasma is generated. One end of the confinement tube is open; the plasma is actually maintained on a continuum gas flow. During induction plasma operation, the generator supplies an alternating current (ac) of radio frequency (r.f.) to the torch coil; this ac induces an alternating magnetic field inside the coil, after Ampère’s law (for a solenoid coil): 2

ΦB=(μ0Icn)(πr0 ) (1)

(left) Induction heating; (right) Inductively coupled plasma.

where, ΦB is the flux of magnetic field, µ0 is permeability constant (4π x 10−7 Wb/A.m), Ic is the coil current, n is the number of coil turns per unit length, and r0 is the mean radius of the coil turns. According to Faraday’s Law, a variation in magnetic field flux will induce a voltage, or electromagnetic force: E=-N(ΔΦB/Δt) (2) where, N is the number of coil turns, and the item in parenthesis is the rate at which the flux is changing. The plasma is conductive (assuming a plasma already exists in the torch). This electromagnetic force, E, will in turn drive a current of density j in closed loops. The situation is much similar to heating a metal rod in the induction coil: energy transferred to the plasma is dissipated via Joule heating, j2R , from Ohm’s Law, where R is the resistance of plasma. Since the plasma has a relatively high electrical conductivity, it is difficult for the alternating magnetic field to penetrate it, especially at very high frequencies. This phenomenon is usually described as the “skin effect”. The intuitive scenario is that the induced currents surrounding each magnetic line counteract each other, so that a net induced current is concentrated only near the periphery of plasma. It means the hottest part of plasma is off-axis. Therefore, the induction plasma is something like an “annular shell”. Observing on the axis of plasma, it looks like a bright “bagel”. In practice, the ignition of plasma under low pressure condition(<300 torr), is almost “spontaneous”, once the r.f. power imposed on the coil achieves a certain threshold value (depending on the torch configuration, gas flow rate etc.) the state of plasma gas (usually Argon) will swiftly transit from glow-discharge to arc-break and create a stable induction plasma. It goes without saying, the system matching should have been well-tuned beforehand. For the case of atmospheric ambient pressure conditions, the ignition is often fulfilled with the aid of so-called TESLA, one device producing high-frequency; high-voltage electric sparks, which induces local arc-break inside the torch and stimulates a cascade through ionization of plasma gas, ultimately resulting in an established stable plasma.

Induction plasma, observed from side and from the end

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Induction plasma torch Induction plasma torch is the core of the induction plasma technology. Despite the existence of hundreds of different designs, an induction plasma torch consists of essentially three components:

Induction plasma torch for industrial applications

• coil The induction coil consists of several spiral turns, depending on the r.f. power source characteristics. Coil parameters including the coil diameter, number of coil turns, and radius of each turn, are specified in such a way to create an electrical "tank circuit" with proper electrical impedance. Coils are typically hollow along their cylindrical axis, filled with internal liquid cooling (e.g., de-ionized water) to mitigate high operating temperatures of the coils that result from the high electrical currents required during operation.

• confinement tube This tube serves to confine the plasma. Quartz tube is the common implementation. The tube is often cooled either by compressed air (<10 kW) or cooling water. While the transparency of quartz tube is demanded in many laboratory applications (such as spectrum diagnostic), its relatively poor mechanical and thermal properties pose a risk to other parts (e.g., o-ring seals) that may be damaged under the intense radiation of high-temperature plasma. These constraints limit the use of quartz tubes to low power torches only (<30 kW). For industrial, high power plasma applications (30~250 kW), tubes made of ceramic materials are typically used.[3] The ideal candidate material will possess good thermal conductivity and excellent thermal shock resistance. For the time being, silicon nitride (Si3N4) is the first choice. Torches of even greater power employ a metal wall cage for the plasma confinement tube, with engineering tradeoffs of lower power coupling efficiencies and increased risk of chemical interactions with the plasma gases. • gas distributor Often called a torch head, this part is responsible for the introduction of different gas streams into the discharge zone. Generally, there are three gas lines passing to the torch head. According to their distance to the center of circle, these three gas streams are also arbitrarily named as Q1, Q2, and Q3. Q1 is the carrier gas that is usually introduced into the plasma torch through an injector at the center of the torch head. As the name indicates it, the function of Q1 is to convey the precursor (powders or liquid) into plasma. Argon is the usual carrier gas, however, many other reactive gases (i.e., oxygen, NH3, CH4, etc.) are often involved in the carrier gas, depending on the processing requirement. Q2 is the plasma forming gas, commonly called as the “Central Gas”. In today’s induction plasma torch design, it is almost unexceptional that the central gas is introduced into the torch chamber by tangentially swirling. The swirling gas stream is maintained by an internal tube that hoops the swirl till to the level of the first turn of induction coil. All these engineering concepts are aiming to create the proper flow pattern necessary to insure the stability of the gas discharge in the center of the coil region. Q3 is commonly refereed as “Sheath Gas” that is introduced outside the internal tube mentioned above. The flow pattern of Q3 can be either vortex or straight. The function of sheath gas is twofold. It helps to stabilize the plasma discharge; most importantly, it protects the confinement tube, as a cooling medium. • Plasma gases and plasma performance The minimum power to sustain an induction plasma depends on pressure, frequency and gas composition. The lower sustaining power setting is achieved with high r.f. frequency, low pressure, and monatomic gas, such as argon. Once diatomic gas is introduced into the plasma, the sustaining power would be drastically increased, because extra dissociation energy is required to break gaseous molecular bonds first, so then further excitation to plasma state is possible. The major reasons to use diatomic gases in plasma processing are (1) to get a plasma of high energy content and good thermal conductivity (see Table below), and (2) to conform the processing chemistry.

Induction plasma technology

Gas

Specific [4] gravity

Thermal dissociation energy (eV)

132

Ionization energy (eV)

[5]

[5] Enthalpy (MJ/mol)

Thermal conductivity (W/m.K)

Ar

1.380

not applicable

15.76

0.644

0.24

He

0.138

not applicable

24.28

2.453

0.21

H2

0.069

4.59

13.69

3.736

0.91

N2

0.967

9.76

14.53

1.675

1.49

O2

1.105

5.17

13.62

1.370

0.99

Air

1.000

n.a.

n.a.

1.709

1.39

In practice, the selection of plasma gases in an induction plasma processing is first determined by the processing chemistry, i.e., if the processing requiring a reductive or oxidative, or other environment. Then suitable second gas may be selected and added to argon, so as to get a better heat transfer between plasma and the materials to treat. Ar-He, Ar-H2, Ar-N2, Ar-O2, Air, etc. mixture are very commonly used induction plasmas. Since the energy dissipation in the discharge takes places essentially in the outer annular shell of plasma, the second gas is usually introduced along with the sheath gas line, rather than the central gas line.

The industrial application of induction plasma technology Following the evolution of the induction plasma technology in laboratory, the major advantages of the induction plasma have been distinguished: • Without the erosion and contamination concern of electrode, due to the different plasma generation mechanism compared with other plasma method, for example, direct current non-transfer arc (dc) plasma. • The possibility of the axial feeding of precursors, being solid powders, or suspensions, liquids. This feature overcomes the difficulty of exposing materials to the high temperature of plasma, from the high viscosity of high temperature of plasma. • Because of non electrode problem, a wide versatile chemistry selection is possible, i.e., the torch could work in either reductive, or, oxidative, even corrosive conditions. With this capability, induction plasma torch often works as not only a high temperature, high enthalpy heat source, but also chemical reaction vessels. • Relatively long residence time of precursor in the plasma plume (several milliseconds up to hundreds milliseconds), compared with dc plasma. • Relatively large plasma volume. These features of induction plasma technology, has found niche applications in industrial scale operation in the last decade. The successful industrial application of induction plasma process depends largely on many fundamental engineering supports. For example, the industrial plasma torch design, which allows high power level (50 to 600 kW) and long duration (three shifts of 8 hours/day) of plasma processing. Another example is the powder feeders that convey large quantity of solid precursor (1 to 30 kg/h) with reliable and precise delivery performance. Nowadays, we have been in a position to be able to numerate many examples of the industrial applications of induction plasma technology, such as, powder spheroidisation, nanosized powders synthesis, induction plasma spraying, waste treatments, etc.,[6] [7] However, the most impressive success of induction plasma technology is doubtless in the fields of spheroidisation and nano-materials synthesis.

Induction plasma technology

Powder spheroidisation[8] The requirement of powders spheroidisation (as well as densification) comes from very different industrial fields, from powder metallurgy to the electronic packaging. Generally speaking, the pressing need for an industrial process to turn to spherical powders is to seek at least one of the following benefits which result from the spheroidisation process: 1. 2. 3. 4. 5.

Improve the powders flow-ability. Increase the powders packing density. Eliminate powder internal cavities and fractures. Change the surface morphology of the particles. Other unique motive, such as optical reflection, chemical purity etc.

Spheroidisation is a process of in-flight melting. The powder precursor of angular shape is introduced into induction plasma, and melted immediately in the high temperatures of plasma. The melted powder The dense microstructure of the spheroidised cast particles are assuming the spherical shape under the action of surface tungsten carbide powders tension of liquid state. These droplets will be drastically cooled down when fly out of the plasma plume, because of the big temperature gradient exciting in the plasma. The condensed spheres are thus collected as the spheroidisation products. A great variety of ceramics, metals and metal alloys have been successfully spheroidized/densified using induction plasma spheroidisation. Following are some typical materials spheroidized on commercial scale. • • • •

Oxide ceramics: SiO2, ZrO2, YSZ, Al2TiO5, glass Non-oxides: WC, WC-Co, CaF2, TiN Metals: Re, Ta , Mo, W Alloys: Cr-Fe-C, Re-Mo, Re-W

Nano-materials synthesis It is the increased demand for nanopowders that promotes the extensive research and development of various techniques for nanometric powders. The challenges for an industrial application technology are productivity, quality controllability, and affordability. Induction plasma technology implements in-flight evaporation of precursor, even those raw materials of the highest boiling-point; operating under various atmospheres, permitting synthesis of a great variety of nanopowders, and thus become much more reliable and efficient technology for synthesis of nanopowders in both laboratory and industrial scales. Induction plasma used for nanopowder synthesis has many advantages over the alternative techniques, such as high purity, high flexibility, easy to scale-up, easy to operate and process control. In the nano-synthesis process, material is first heated up to evaporation in induction plasma, and the vapours are subsequently subjected to a very rapid quenching in the quench/reaction zone, The quench gas can be inert gases such as Ar and N2 or reactive gases such as CH4 and NH3, depending on the type of nanopowders to be synthesized. The nanometric powders produced are usually collected by porous filters, which are installed away from the plasma reactor section. Because of the high reactivity of metal powders, special attention should be given to powder pacification prior to the removal of the collected powder from the filtration section of the process. The induction plasma system has been successfully used in the synthesis nanopowders. The typical size range of the nano-particles produced is from 20 to 100 nm, depending on the quench conditions employed. The productivity varies from few hundreds g/h to 3~4 kg/h, according to the different materials' physical properties. A typical induction plasma nano-synthsize system for industrial application is shown below. The photos of some nano-product from the same equipment are included.

133

Induction plasma technology

134

Gallery

The flaky interlocking rhenium powders become dense separate spheres after the induction plasma spheroidisation processing

The SiO2 powder spheroidised by induction plasma (air plasma), output 15~20 kg/h

The induction plasma installation for nanopowders synthesis

Some samples of the nanoparticles prepared by induction plasma processing

Summary Induction plasma technology aims mainly those “adding-high-value” processes. Besides the “spheroidisation” and “nanomaterial synthesis”, the high risk waste treatment, refractory materials deposit, noble material synthesis etc. may be the next industrial fields for induction plasma technology.

Notes [1] [2] [3] [4] [5] [6] [7] [8]

G. I. Babat, Inst. Elec. Eng., London, England, 94, 27(1947) T. B. Reed, J. Appl. Phys., 32, 821(1961) United States Patent 5200595 at standard temperature and pressure at 10000 K M.I. Boulos, Radio frequency plasma developments, scale-up and industrial applications, High Temp. Chem. Processes, 1(1992)401-411 M.I. Boulos, The Inductively Coupled Radio Frequency Plasma, High Temp. Material Processes, 1(1997)17-39 M.I. Boulos, Plasma power can make better powders, Metalpowder Report, No. 5, (2004)16-21

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Article Sources and Contributors Ultra high frequency  Source: http://en.wikipedia.org/w/index.php?oldid=435569548  Contributors: AJenbo, Acs4b, Alexwcovington, Anaxial, AndreyMavlyanov, Angilbas, Angstorm, Ansbaradigeidfran, AnthonyQBachler, Arteitle, AxG, BAxelrod, Balaji7, BaronLarf, Bergsten, BevanFindlay, Bigbob, BilCat, Bloodshedder, Bogsat, Bovineone, Bridesmill, Bryan Derksen, Bucketsofg, Bumm13, Caerwine, CamdenTommy, Camilo Sanchez, Canberra photographer, Carlb, Cfrjlr, Chris 1127, Chris the speller, ChrisPUT, CieloEstrellado, Ciphers, Clawson, Cleosimble, Cntras, Co149, Collabi, Colonies Chris, Coloursinmyhead, Commander Keane, Coneslayer, Cootiequits, CosineKitty, Courcelles, Crmadsen, D119, DSRH, DV8 2XL, Dale Arnett, Dcandeto, Denni, Dethme0w, DisambiguationGuy, Dismas, DocWatson42, Docu, Ds9kicks, ERobson, Edman274, Eetvartti, Ejay, Emricha, Enigmaman, EoGuy, Ericd, Evice, Feydey, Firsfron, Fluteboy, Frankieroberto, Franl, Funandtrvl, Fursday, GSK, Gavinatkinson, GerbilSoft, Giraffedata, Glenn, Gpvos, Graymornings, Hadal, Haikupoet, Ianblair23, Idontthinkso, Iluvcapra, JFG, JWM83, Jakes18, Jdaloner, Jeffq, Jerzy, Jhapney, Jim.henderson, Joel7687, Jonverve, Jrdioko, Kascatu, Kelisi, Keraunos, Krash, Kreline, Krisorey, Laurence Gilcrest, Leafyplant, Lee M, Liftarn, MBread, Ma3nocum, Mahanga, Martian, Martin451, Martnym, Mav, Mdebets, Meano.Culpa, Mellery, Metroccfd, Michael Hardy, Misternuvistor, Misto, Mrschimpf, Nascarkylebuschj12, Nedim Ardoğa, Nedlowe, Nkshanl, Noisy, NorthernThunder, Nufy8, Nuggetboy, OlEnglish, Oli Filth, Omegatron, Optikos, PSzalapski, Patrick, Pauladin, Pavel Kolotilov, Picapica, PigFlu Oink, Pjvpjv, Platinumfawkes, Prefect, Preslethe, Prince wiki thai, Puckly, Qui1che, Qutezuce, RAMChYLD, RTC, RTG, Radiojon, Reaper Eternal, Reddi, RingtailedFox, Rjhanson54, Rjwilmsi, RoyBoy, RussBlau, Scchipli, Semiwiki, Sineui, SiobhanHansa, Slawojarek, Smack, Ssd, Starionwolf, Stephan Leeds, Stereorock, Swid, Tabletop, Tagishsimon, Template namespace initialisation script, The Anome, The PIPE, TheGerm, Thingg, ThinkBlue, ThomasPusch, Thunderbird2, Timc, TomCat4680, Tompagenet, Tonsofpcs, Towel401, TrbleClef, Troyoda1990, Umerfarooqawan, ValRon, Verkhovensky, Viking880, Voidxor, Wfeidt, Wireless friend, Wordie, Wordsmith, XL2D, Y control, Ynhockey, Zaphraud, Zariane, Zeno333, 374 anonymous edits Super high frequency  Source: http://en.wikipedia.org/w/index.php?oldid=423642606  Contributors: Adamantios, Black-Velvet, Bobblewik, Caerwine, D.bennett08, Eetvartti, Esradekan, Forajump, Gene Nygaard, Gurch, Jonah Saltzman, Jonverve, Mlaffs, Nnh, Raz1el, Rgrg, ThomasPusch, TubularWorld, Voidxor, Whaa?, Whitepaw, Wireless friend, 32 anonymous edits Extremely high frequency  Source: http://en.wikipedia.org/w/index.php?oldid=430130010  Contributors: 4kinnel, Adamantios, Antonrojo, Ashishbhatnagar72, Ayecee, Baellen, Beland, Black Pullet, Bluemoose, Bobblewik, Bpack1, Chowbok, Cobalttempest, DV8 2XL, Dalillama, Debresser, Deville, Eetvartti, EmreDuran, Firsfron, Galoubet, Goobergunch, Ground Zero, Imoeng, InsufficientData, JLaTondre, Jaraalbe, Jerzy, Jitse Niesen, Jonverve, Joolsr, Jordan Brown, Keenan Pepper, Leonard G., Lloyd Wood, Lmdlmd, Lotje, Lt coolbud, Maximus Rex, Meneth, Mjmarcus, Mmwaveguru, Nandhp, Neutrality, Patrick, Pdklein, Phelonius Friar, Pjvpjv, Polsok, Radiojon, Rchandra, Rgrg, Rjwilmsi, Skapur, Small black sun, Stevertigo, Template namespace initialisation script, Teohhanhui, The Anome, ThomasPusch, Thunderbird2, Veinor, Walld, Wdfarmer, Wmt, Woohookitty, Ygtai, 102 anonymous edits Modulation  Source: http://en.wikipedia.org/w/index.php?oldid=434174346  Contributors: 62.163.16.xxx, 63.192.137.xxx, Abrech, Alex.muller, Alinja, Altenmann, Argilo, Ashwink911, Atropos235, Aua, AxelBoldt, Bdesham, Berean Hunter, Bernard François, Berserkerus, Bfallik, Binksternet, Bjarkef, Bluemouse2306, Canaima, Cbradiomagazine, Chetvorno, Cihan, Conversion script, Coralmizu, Cuddlyable3, DV8 2XL, Dandv, Danh, Daniel.Cardenas, DavidCary, Denelson83, Dicklyon, Doktor Who, Dreadstar, Dtwitkowski, EEPROM Eagle, EugeneZelenko, Favonian, Frehley, Giftlite, Glenn, Grafen, Gsmcoupe, HappyCamper, Hatch68, Hmo, HowardMorland, Inkling, Ixhotl, JClark2906, Jhbdel, JohnTechnologist, Johnuniq, Joy, Jpat34721, Jpmonroe, Karlhendrikse, Krishnavedala, Lee Carre, Leszek Jańczuk, Light current, MC10, Mahamahamaha, Mahanga, Mange01, MaratL, Matt Yeager, Maziaar83, McNeight, Meodou, Michael Hardy, MichaelStanford, MuthuKutty, Mysid, Oli Filth, One-dimensional Tangent, Philippe23, Plamka, Playclever, Pranav v, Pyr0technician, Qartis, Rabarberski, Ralmin, Read-write-services, Rebmertaumer, RobertYu, Rw4ni, Sam Blacketer, Simon South, Siwiak, Smack, Smshaner, Splash, Ssd, Starx, Stradivariusis, Stw, Suffusion of Yellow, Superborsuk, Svenjissom, Technicolorcavalry, Tedder, The Anome, The Thing That Should Not Be, Towel401, Tprentice, Tuxa, Unknownx123, Urfriendshailesh, Vanwhistler, West London Dweller, Wikiborg, Wireless friend, Wispanow, Wksalar, Yaco, Yerpo, Yeryry, Z3r0kw3l, Zhouyuanxin, 270 anonymous edits Transmitter  Source: http://en.wikipedia.org/w/index.php?oldid=424720902  Contributors: 16@r, Alansohn, Aldie, Anna Lincoln, Atlant, Axeman89, BAxelrod, Biblbroks, Bidgee, Bobo192, BorgQueen, Broadcasttransmitter, CALR, COMPATT, Callidior, Chairboy, Chetvorno, Chillysnow, Cmdrjameson, Cutler, D, Daniel C, Danny, Deor, Deville, Dfrg.msc, Dpv, Dtgriscom, E-s-B, Edison, Engineer Bob, Erzahler, Everyking, Ewlyahoocom, Flowerpotman, Footwarrior, Fredbauder, GRAHAMUK, GarnetRChaney, Geac, Gene Nygaard, Glenn, Goudzovski, Gpvos, Ground Zero, HamburgerRadio, Heron, Hlucho, Hooperbloob, Hornet35, Hydrogen Iodide, Ixfd64, Jaho, Jc3s5h, Jeff3000, JeremyA, JustinSmith, K1Bond007, Karen Johnson, Karl-Henner, Kku, KrakatoaKatie, Levineps, Lightmouse, Lssg124, LuckyLouie, Ma3nocum, ManN, Marnanel, Matthew Yeager, Mets501, MichaelMaggs, Mikael Häggström, Mikeblas, Mintleaf, Mrschimpf, Nedim Ardoğa, Njiro, Nneonneo, Obersachse, Oldphella, Palfrey, Perfectajay, Pjvpjv, Pontificalibus, Prefect, R'n'B, RadioBroadcast, Radiojon, RainbowOfLight, RickReinckens, SCEhardt, Samir, Shalom Yechiel, Spliced, Stoneygirl45, Sv1xv, Tawglobal, The Thing That Should Not Be, TheM62Manchester, Thingg, Timo Honkasalo, Tresckow, Tubenutdave, Tycho, UsagiM, Vegaswikian, Wikijens, Wtshymanski, Zidonuke, °, 149 anonymous edits Antenna (radio)  Source: http://en.wikipedia.org/w/index.php?oldid=435109972  Contributors: A. B., Aarchiba, Abhivyakti s, Aceman2000, Acimatti, Adamantios, Addshore, Adrian, Adziura, Ai4ijoel, Alanacheng, Algocu, AltairPayne, Altenmann, Andre Engels, Andrewjuren, Angr, AngryParsley, Anonym1ty, Anthony Appleyard, Archanamiya, Arnero, Arteitle, Asrghasrhiojadrhr, AugPi, Auntof6, BD2412, Bernd in Japan, Betacommand, Bidgee, Blackjack3, Blainster, Bobo192, Bonzo, Borgx, Brandon, Brandon.irwin, Broadcasttransmitter, Bryan Derksen, Bsskchaitanya, Catslash, Chetvorno, ChrisHodgesUK, Christopher Parham, Cimon Avaro, Clarkefreak, Clubjuggle, Coolvariant, CosineKitty, Crisis, Cybercobra, Cybernetic, Cyrius, DV8 2XL, DWaterson, Dan Parnell, Daniel Christensen, Daniel.Cardenas, Dante Alighieri, Darguz Parsilvan, David Jordan, David R. Ingham, Dawnseeker2000, Dcljr, [email protected], Deor, Digitat, Dleather, Dogcow, Doradus, Dougher, Dratman, Drkirkby, Drys, Ekconklin, Emoboy2, Enochlau, EscapeVelocity, Euchiasmus, Ewlyahoocom, Ezeu, Fallschirmjäger, Francis E Williams, Fresheneesz, GLPeterson, GRAHAMUK, GaeusOctavius, Gaius Cornelius, Gary King, Gassaver, Gene Nygaard, George100, GetsEclectic, Glenn, GoingBatty, Graeme Bartlett, Greenrd, Haham hanuka, HereToHelp, Heron, Hertzian, Hickorybark, Hooperbloob, Horsten, IPSOS, Interferometrist, Intgr, Iridescent, It1224, JA.Davidson, Jamesqjf, Jc3s5h, Jcbarr, Jddriessen, Jdiyef, JesseW, Jim.henderson, Jjensen347, Jmrowland, John of Reading, JohnOwens, JohnTechnologist, Johnnymartins, Johnor, Joy, Jpgordon, Jswd, Jugni, Juhachi, Julesd, JurgenG, JustinSmith, Jyril, KD5TVI, Kaimbridge, Kamenlitchev, Karl gregory jones, Kcordina, Keesiewonder, KeithH, KelleyCook, Kevin Rector, Kevmitch, Kgrr, Kharker, Kingturtle, Kostisl, Kotoviski, Krish Dulal, Kristian Ovaska, Kymacpherson, LMB, LPFR, LQ, Lethe, Light current, Liquidcable, Loren.wilton, LorenzoB, LouI, Lukeseed, Mac, Mako098765, Mandarax, Matthias Holger, Mauls, Mazarin07, Meisam, Michael Hardy, Mifter, Migo, Mikeblas, Moggie2002, Mondebleu, Mouchoir le Souris, MrRK, N0YKG, N5iln, NameThatWorks, Nedim Ardoğa, Networkingguy, NeutralLang, Nicholasrs, Nobodyinpart, Norm mit, Nuggetboy, Ohnoitsjamie, Ominae, Onco p53, Optimist on the run, Pete463251, Peter Harriman, Pezant, PhantomS, Pixel ;-), Pol098, Quantumobserver, R'n'B, RDT2, RTG, RadioFan, Ransu, Razimantv, Read-write-services, Red Winged Duck, Reddi, Rememberway, Requestion, Rheostatik, RickK, Rmrfstar, Rob.desbois, RobertG, Rogerbrent, Sadads, Sauermfj, Scrabbler, Searchme, Sergioledesma, Shadowjams, ShelfSkewed, SiobhanHansa, Smndalila, Snafflekid, Spamhog, SparhawkWiki, SpeedyGonsales, SpuriousQ, Sreeram bh, Srleffler, Ssd, Stannered, Stephan Leeds, Stepp-Wulf, Steve Quinn, Steve carlson, Stevenj, SunCreator, Sv1xv, The Original Wildbear, TheGerm, Thunderchild, Tim Starling, Timo Honkasalo, Timothy Truckle, Tizio, Tlusťa, TomCat4680, Tony Sidaway, Txinviolet, Un chien andalou, Unyoyega, Vanessaezekowitz, Vegaswikian, Vegaswikian1, Voyajer, W.F.Galway, WO2, Warut, Waveguy, Wavelength, Whpq, Wikieditoroftoday, Wjbeaty, Wmahan, Wolfkeeper, Wolfmankurd, Woohookitty, Wskish, Ww, Yaf, Yoenit, Zoicon5, Zoohouse, しまでん, 大西洋鲑, 448 anonymous edits Receiver (radio)  Source: http://en.wikipedia.org/w/index.php?oldid=431432809  Contributors: A little insignificant, Aaagmnr, Abortz, AeroPsico, Altermike, Alynna Kasmira, Andrevan, Appraiser, Arch dude, Bobo192, CDN99, ChrisRuvolo, Clive Buxton, Cmacd123, Colonies Chris, CvetanPetrov1940, DV8 2XL, Eddie.willers, Elcairo, Elvisdang1, Engineer Bob, Eskimospy, Flowerpotman, Fosnez, Fuby, Gfoley9999, Giftlite, GillesAuriault, Glenn, Gutta Percha, Haikupoet, Homerjay, Hooperbloob, Joy, Keraunoscopia, Khaledelmansoury, Kku, LadyofHats, Lawrencekhoo, Lixuesong, LuckyLouie, Lumos3, M jurrens, MSTCrow, Mac, Marknew, Markus Kuhn, Marshawnlynch23, Martial Law, Maximaximax, McTavidge, Melos Antropon, Mikeblas, Mitchan, Mlewis000, Momo san, Mr squelch, Neelix, Nixeagle, Nol888, Nopetro, Octahedron80, OlavN, Ozguy89, PFHLai, Pak21, Pastorius, Petri Krohn, Potatoswatter, Quota, Raise-the-Sail, Rees11, Rich Farmbrough, Ritabest, Rjstott, Rjwilmsi, Rui Silva, SeventyThree, Shawnc, SimonP, Stoneygirl45, Sv1xv, Tan90deg, Tangopaso, Tkynerd, User2004, Wikipelli, William Greene, Wizzy, Woohookitty, Wtshymanski, Zidonuke, しまでん, 90 anonymous edits Tuned radio frequency receiver  Source: http://en.wikipedia.org/w/index.php?oldid=430407296  Contributors: Armstrong1113149, Charles Matthews, CosineKitty, DV8 2XL, Firsfron, Glenn, Glrx, Hooperbloob, Interferometrist, Jamelan, KD5TVI, Liampr, Mercyjhansi, Misza13, Nkendrick, OlavN, Pol098, Rchandra, Redheylin, Sweidman, Wtshymanski, 10 anonymous edits Radar  Source: http://en.wikipedia.org/w/index.php?oldid=435296979  Contributors: 119, 128.59.58.xxx, 209.239.196.xxx, 2over0, A. Carty, A.R., ADude, Aarchiba, AdjustShift, AeroPsico, Ahbushnell, Ahoerstemeier, Ahunt, Al Silonov, Alai, Alanbrowne, Alansohn, Alex.tan, Alexander.stohr, Alga, Allstarecho, Alphasinus, Altenmann, Altmany, Alvaro, Amalas, American Eagle, American2, Amoruso, Amorymeltzer, Andre Engels, Andrea105, Andux, Andyjsmith, Angusmclellan, Animum, Anna Lincoln, Annetna, Anttin, Apoc2400, Arch dude, Armstrong1113149, ArnoldReinhold, Arodin66, Arpingstone, Arz1969, Ashley Y, Astairefred, Astromahitis, Atif.t2, Atollinchi, Aude, Ausir, Avaragado, Average Earthman, Axlq, B. 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Article Sources and Contributors LilHelpa, Little Mountain 5, Lnbogoda, Lowellian, Lperez2029, Luna Santin, Lupin, M7, MASFROG, Maheshkale, Malcolm Farmer, Mangoe, Mani1, Mark.murphy, Mark83, MarkSutton, Marquez, Marshall46, Martin Hogbin, Martinf550, Materialscientist, Matt Britt, Maury Markowitz, Mav, Maxí, Meffertf, Mens Sana, Mexeno1, MiG, Michael Devore, Michael Hardy, MichaelBillington, Mike Rosoft, Mike65535, MikeF9, Mikeo, Minesweeper, Miquonranger03, Miterdale, Mklo, Mlaffs, Moglucy, Mooncow, Mosslabel, Mugaliens, Muhends, Mulad, Mushin, MuthuKutty, Mvannier, Mxn, Myanw, Mygerardromance, Mynameinc, Naddy, Nageh, Nakon, Nathan, NatusRoma, Neckro, Neightdogwiki, Neilc, Nigelj, Nimur, Nmajdan, Noah Salzman, Noctibus, Noctuidae, Norm, OUGryphon, Oberiko, Oblivious, Oda Mari, Okkervil River King, Ombudswiki, Onorem, Ortolan88, Osurak, Ouishoebean, OwenBlacker, Oxymoron83, PEHowland, Page blanking expert, Palica, Pascal.Tesson, Patrick, Paul Barlow, Paul Siebert, Paxsimius, Pb30, Pcr, Penkala, Peruvianllama, Peter, Petr.adamek, Phantomsteve, Pharaoh of the Wizards, Philbert2.71828, Philip Baird Shearer, Philippep, Pierre cb, Pill, PimRijkee, Pingu Is Sumerian, Pjedicke, Plugwash, Pmetzger, Pol098, Pot, Powerboat-charter, Prari, PrestonH, Prmacn, Ptdecker, Quantumobserver, QuiteUnusual, Qxz, R'n'B, RadarCzar, Radarguy, Rama, RamenFueled, Raryel, Rasmus Faber, Raul654, Ravensgrace, Rawling, Raymond C. Watson, Jr., Rboatright, Read-write-services, Reaper Eternal, Reconsider the static, RedWolf, Reddi, Reisio, Rejnal, Remotelysensed, Rense, ResearchRave, RetiredUser2, RexNL, Rich Farmbrough, Richard Weil, Richj, Rjakew, Rjwilmsi, Rls, Rmhermen, Roperbw, Rpalmerarrc, Rpfraser, Rtdrury, Runmiler429, Ryanmarks0321, Sabranan, Sachin mahapurush, Sam Spade, Sander123, Sanders muc, SandyGeorgia, SchickEe2346812, ScottAlanHill, ScottDavis, SeaValeYen, Searchme, Sfmammamia, Shaddack, Shadowjams, Shattered, Shirik, Sicooke, SimonD, Slawojarek, Smack, Smalljim, Softce, Someguy1221, Sonett72, Sosacrl, SpNeo, Spalding, Special-T, SpeedyGonsales, SperryTS, Spinningspark, SpookyMulder, Srleffler, Stalwart111, Starnestommy, SteinbDJ, Stevenj, Suisui, SunCreator, Susurrus, Sylent, Symane, TJC, TaintedMustard, Tannin, Tanthalas39, Taroaldo, Taxman, Tedernst, Tedness, The Epopt, The High Fin Sperm Whale, TheGerm, TheGrimReaper NS, Themfromspace, Thenthornthing, Thick as a Planck, Thumperward, Tide rolls, Timc, Timo Kouwenhoven, Timothy 1623, Tivedshambo, Tlmurray, TomCerul, Tomwalden, Tracyvznh, Trafford09, Tree Biting Conspiracy, Treesmill, Trelvis, Tridy, Twentysomkaos, Ultraexactzz, Uncle Dick, Utcursch, Uunter, V79benno, Vanessaezekowitz, Vanzandtj, Vary, Vbarwick, Vgy7ujm, VladimirKorablin, Vorodin, Vsmith, Vykischandra, Walkerma, Waveguy, Wayward, Wazzup1223, Wernher, Wicked1948, Wik, Wiki libs, WikiSans, Wikiborg, Wikinaut, Wknight94, Wolfkeeper, Wootonius, Wtshymanski, Wwoods, Wyllium, XieChengnuo, Xmnemonic, Xorx, Yeokaiwei, Zanaq, Zharradan.angelfire, Zigger, Zntrip, Zoney, Ztobor, Скампецкий, Чръный человек, ‫ملاع بوبحم‬, 974 anonymous edits Transistor radio  Source: http://en.wikipedia.org/w/index.php?oldid=435213765  Contributors: A930913, Adashiel, Ahkitj, Alai, Alsandro, Altermike, Arpingstone, Ashmoo, Asperal, Awhit003, Betacommand, Bethaso, BillFlis, Boguslinks, Can't sleep, clown will eat me, Chetvorno, Ck lostsword, DV8 2XL, Daltonls, Dbpies, DeadEyeArrow, DerHexer, Dpbsmith, Edg2103, Everyking, Ewrobbel, FPAtl, Favonian, Form follows function, Frap, Frosted14, Furry, Gene Nygaard, Glenn, Glyn.nelson, Gmaxwell, Haikupoet, HeikoEvermann, Hooperbloob, Hydrargyrum, Iridescent, Issanotomo, Itactics, J.delanoy, Jasonuhl, Jeffutz, Jimfbleak, KD5TVI, Kanags, Keilana, Kilo-Lima, Kvng, Lightmouse, Livonia Mall, LokiClock, LorenzoB, Lots42, Magyaradio, Maury Markowitz, Mdf, Megan.rw1, Mets501, Michael Shields, Michaeljpro, Nekokaitou, Nicholas Tan, Nil Einne, Opus33, Piano non troppo, Piledhigheranddeeper, Pinkadelica, PipOC, PoccilScript, Qdr, QuantumEleven, Rich Farmbrough, Rjwilmsi, Robert31, Roger sandwich, Silvonen, Skarebo, Someone else, Sub40Hz, Tabletop, Ted Wilkes, Thadius856, Transisto, Transistorradioguy, Turnbull1, Twas Now, Vancouver Outlaw, Wavelength, Wikinaut, Wtshymanski, Ykerzner, Zaxem, 157 anonymous edits Walkie-talkie  Source: http://en.wikipedia.org/w/index.php?oldid=428921977  Contributors: 2D, 462712, Abi79, Addihockey10, Alansohn, Altenmann, Amikake3, Arakunem, ArnoldReinhold, Arny, Aruton, Azumanga1, Babbage, Binksternet, BlueStraggler, Bluerasberry, Borgx, Bradder555, Brenont, Brian Crawford, Brian in denver, Cacophony, Calmer Waters, Caltas, Can't sleep, clown will eat me, Capricorn42, Cleared as filed, Cmacd123, Courcelles, Crzrussian, Csl77, DVD R W, David Jordan, Dawn Bard, Dawnseeker2000, Dbfirs, Dejudicibus, Dfrg.msc, Dougmc, EagleOne, Edward, Eleassar777, Esb, Etimbo, Exert, FayssalF, Franamax, Free Citizen, Frzl, Furrykef, GRBerry, GoingBatty, Grant65, Greensburger, Gwernol, Hadley, Hadlock, Haikupoet, Halibutt, Hank Magnuski, Heterodyne, Houshfan84, Ht1848, Hydrargyrum, Illinoisavonlady, Illumynite, Ingolfson, Iridescent, JForget, Jhsounds, John yazzie1963, John254, Julienlecomte, Jwilkers, Kencaesi, Kevin Rector, Kgutwin, Kharker, Kindall, Kingpin13, Kookyunii, Kozuch, Krash, Krashlandon, LFaraone, Linusthefish, LuckyLouie, Mark7211, MarkGallagher, Marktreut, Mattwic, May Cause Dizziness, MementoVivere, Mercy, Michal Nebyla, Mike Rosoft, Mjponso, Mmajka, Monoet, Mrath, MuZemike, Mulad, Naturespace, Nikuda, Nivix, NorthernThunder, OS2Warp, OhanaUnited, Omegatron, Pengyanan, Pfaff9, Pgr94, Pingualot, Pip2andahalf, Prolog, RadicalBender, Radiojon, Requestion, Richardpitt, Rifleman 82, RyanCross, Sbharris, Shentosara, SimoneIcough, Spider63, Squidfryerchef, Surv1v4l1st, Sv1xv, Syphxxx, Tadpole9, Tanvir Ahmmed, Tellyaddict, Terrypin, Theo hamer, Thick Peter, Thingg, Thunderbird2, Timc, Tinton5, Tkaasbell, TubularWorld, Utcursch, Varitek, Vegaswikian, Vikiniho84, Violetriga, WRK, Wernher, WikHead, Wiki Wikardo, Wjejskenewr, Wtshymanski, Xlaran, Yamamoto Ichiro, 229 anonymous edits Noise (electronics)  Source: http://en.wikipedia.org/w/index.php?oldid=432665873  Contributors: Alan Liefting, Chem-awb, Deville, Dicklyon, Dreadstar, Fyrael, Gene Nygaard, Giftlite, GyroMagician, Htavroh, Il palazzo, Improve, Jamelan, Jeepday, Keenan Pepper, Kle0012, Light current, Lindosland, Macintosh User, Mange01, Martin Hedegaard, Materialscientist, Michael93555, Miyagawa, Mytomi, Nageh, Nasa-verve, Nedim Ardoğa, Nightkey, Ok2ptp, Oli Filth, Omegatron, Paolo.dL, Penwhale, Pitoutom, Pol098, RSRScrooge, Rich257, Riichrd, Rjwilmsi, Rod57, Rogerbrent, Rror, SchfiftyThree, Smither, Splintercellguy, Ssbohio, Tbone2001, Tevildo, TheArmadillo, Toffile, TristanJ, Valueyou, Vcaeken, Waggers, William Avery, Yngvarr, 49 anonymous edits Induction plasma technology  Source: http://en.wikipedia.org/w/index.php?oldid=413105365  Contributors: Aditya.m4, Alexius08, Carlog3, Chzz, HughRandolph, IVAN3MAN, Kkmurray, MSGJ, Vegaswikian, Xfanplasma, 15 anonymous edits

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Image Sources, Licenses and Contributors

Image Sources, Licenses and Contributors File:CNAM-IMG 0564.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:CNAM-IMG_0564.jpg  License: Creative Commons Attribution-Sharealike 2.0  Contributors: User:Rama File:OptischerTelegraf.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:OptischerTelegraf.jpg  License: Creative Commons Attribution-ShareAlike 3.0 Unported  Contributors: Lokilech File:Fibreoptic.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Fibreoptic.jpg  License: GNU Free Documentation License  Contributors: BigRiz File:Digital broadcast standards.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Digital_broadcast_standards.svg  License: Public Domain  Contributors: EnEdC File:Osi-model.png  Source: http://en.wikipedia.org/w/index.php?title=File:Osi-model.png  License: GNU Free Documentation License  Contributors: Dino.korah Image:Visible EM modes.png  Source: http://en.wikipedia.org/w/index.php?title=File:Visible_EM_modes.png  License: Creative Commons Attribution-ShareAlike 3.0 Unported  Contributors: Gerbrant, Maxhurtz, TimVickers, WillowW Image:Onde electromagnetique.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Onde_electromagnetique.svg  License: Creative Commons Attribution-ShareAlike 3.0 Unported  Contributors: SuperManu Image:EM spectrum.svg  Source: http://en.wikipedia.org/w/index.php?title=File:EM_spectrum.svg  License: Creative Commons Attribution-Sharealike 2.5  Contributors: User:Sakurambo Image:Light spectrum.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Light_spectrum.svg  License: GNU Free Documentation License  Contributors: Light_spectrum.png: Original uploader was Denelson83 at en.wikipedia derivative work: B. Jankuloski (talk) Image:Freq synth.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Freq_synth.svg  License: Creative Commons Attribution-Share Alike  Contributors: w:User:Stuuf Image:IdealMixer.JPG  Source: http://en.wikipedia.org/w/index.php?title=File:IdealMixer.JPG  License: Public Domain  Contributors: AdamDoehling, Kvng, 2 anonymous edits File:Diode DBM.png  Source: http://en.wikipedia.org/w/index.php?title=File:Diode_DBM.png  License: Creative Commons Attribution-Sharealike 3.0,2.5,2.0,1.0  Contributors: しまでん Image:PD-icon.svg  Source: http://en.wikipedia.org/w/index.php?title=File:PD-icon.svg  License: Public Domain  Contributors: Various. See log. (Original SVG was based on File:PD-icon.png by Duesentrieb, which was based on Image:Red copyright.png by Rfl.) Image:VHF Usage.svg  Source: http://en.wikipedia.org/w/index.php?title=File:VHF_Usage.svg  License: Creative Commons Attribution 3.0  Contributors: ZanderSchubert (talk) File:Primary Network Affiliates May 1954.png  Source: http://en.wikipedia.org/w/index.php?title=File:Primary_Network_Affiliates_May_1954.png  License: GNU Free Documentation License  Contributors: user:Firsfron File:Amfm3-en-de.gif  Source: http://en.wikipedia.org/w/index.php?title=File:Amfm3-en-de.gif  License: Creative Commons Attribution-Sharealike 2.5  Contributors: Berserkerus File:baud.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Baud.svg  License: Creative Commons Attribution-Sharealike 3.0  Contributors: Jhbdel (talk) Image:CrystalPalaceMast(large).jpg  Source: http://en.wikipedia.org/w/index.php?title=File:CrystalPalaceMast(large).jpg  License: Creative Commons Attribution-ShareAlike 1.0 Generic  Contributors: Original uploader was Secretlondon at en.wikipedia Image:WDET-FM transmitter.png  Source: http://en.wikipedia.org/w/index.php?title=File:WDET-FM_transmitter.png  License: Creative Commons Attribution-Sharealike 3.0  Contributors: w:en:LuckyLouieLuckyLouie Image:Radiotower.png  Source: http://en.wikipedia.org/w/index.php?title=File:Radiotower.png  License: GNU Free Documentation License  Contributors: User:LuckyLouie File:Car radio antenna extended portrait.jpeg  Source: http://en.wikipedia.org/w/index.php?title=File:Car_radio_antenna_extended_portrait.jpeg  License: Creative Commons Attribution-Sharealike 3.0  Contributors: Zuzu File:Half – Wave Dipole.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Half_–_Wave_Dipole.jpg  License: Creative Commons Attribution-ShareAlike 3.0 Unported  Contributors: Schwarzbeck Mess-Elektronik File:Canberra Deep Dish Communications Complex - GPN-2000-000502.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Canberra_Deep_Dish_Communications_Complex_-_GPN-2000-000502.jpg  License: Public Domain  Contributors: NASA File:Antenna.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Antenna.jpg  License: Creative Commons Attribution-ShareAlike 3.0 Unported  Contributors: Yonatan Horan File:Rabbit-ears dipole antenna with UHF loop 20090204.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Rabbit-ears_dipole_antenna_with_UHF_loop_20090204.jpg  License: Creative Commons Attribution 2.5  Contributors: Mark Wagner (User:Carnildo) File:6 sector site in CDMA.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:6_sector_site_in_CDMA.jpg  License: Public Domain  Contributors: hardikvasa File:TV antenna.JPG  Source: http://en.wikipedia.org/w/index.php?title=File:TV_antenna.JPG  License: Creative Commons Attribution 3.0  Contributors: Krish Dulal File:Bundesarchiv Bild 183-29802-0001, MTS Strehla, Bezirk Dresden, Ukw-Sprechfunk.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Bundesarchiv_Bild_183-29802-0001,_MTS_Strehla,_Bezirk_Dresden,_Ukw-Sprechfunk.jpg  License: Creative Commons Attribution-Share Alike 3.0 Germany  Contributors: Braun File:Superturnstile Tx Muehlacker.JPG  Source: http://en.wikipedia.org/w/index.php?title=File:Superturnstile_Tx_Muehlacker.JPG  License: Creative Commons Attribution-Sharealike 2.5  Contributors: Hans-Peter Scholz, Birkenfeld (Enzkreis), Germany File:Folded dipole.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Folded_dipole.jpg  License: Creative Commons Attribution 3.0  Contributors: Bidgee File:Antenna visalia california.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Antenna_visalia_california.jpg  License: Creative Commons Attribution-ShareAlike 3.0 Unported  Contributors: Henryk KotowskiKotoviski File:2008-07-28 Mast radiator.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:2008-07-28_Mast_radiator.jpg  License: Creative Commons Attribution-Share Alike  Contributors: Ildar Sagdejev (Specious) Image:Sidelobes en.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Sidelobes_en.svg  License: Creative Commons Attribution-Share Alike  Contributors: Timothy Truckle Image:Atmosphericnoise.PNG  Source: http://en.wikipedia.org/w/index.php?title=File:Atmosphericnoise.PNG  License: Public Domain  Contributors: http://en.wikipedia.org/wiki/User:Threeme3 File:TVAerial.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:TVAerial.jpg  License: Public Domain  Contributors: Daniel Christensen (talk) File:Old rabbit ears.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Old_rabbit_ears.jpg  License: Creative Commons Attribution 3.0  Contributors: Daniel Christensen (talk) File:A6-1EN.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:A6-1EN.jpg  License: Creative Commons Attribution-ShareAlike 3.0 Unported  Contributors: LP, Santosga File:A6-2.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:A6-2.jpg  License: Creative Commons Attribution-ShareAlike 3.0 Unported  Contributors: Anonymous Dissident, Inductiveload, LP, Santosga, 1 anonymous edits File:A6-4.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:A6-4.jpg  License: Creative Commons Attribution-ShareAlike 3.0 Unported  Contributors: LP, Santosga, 1 anonymous edits File:Zij-en.png  Source: http://en.wikipedia.org/w/index.php?title=File:Zij-en.png  License: Creative Commons Attribution-ShareAlike 3.0 Unported  Contributors: Alejo2083, LP, 1 anonymous edits Image:Montreal-tower-top.thumb2.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Montreal-tower-top.thumb2.jpg  License: Public Domain  Contributors: Original uploader was Aarchiba at en.wikipedia Image:Antenna d44ac.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Antenna_d44ac.jpg  License: Creative Commons Attribution-ShareAlike 3.0 Unported  Contributors: Adamantios, ChrisRuvolo, Croquant, HenkvD, Kotoviski, Liftarn, Siebrand, Stepa, Waldir Image:Television Antenna.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Television_Antenna.jpg  License: Creative Commons Attribution-ShareAlike 3.0 Unported  Contributors: Andreas -horn- Hornig, Conti, Daniel Christensen, Enochlau, Jerome Charles Potts Image:Space diversity.gif  Source: http://en.wikipedia.org/w/index.php?title=File:Space_diversity.gif  License: GNU Free Documentation License  Contributors: David Jordan, Jim.henderson File:136 to 174 MHz base station antennas.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:136_to_174_MHz_base_station_antennas.jpg  License: Creative Commons Attribution-Sharealike 3.0  Contributors: w:en:David JordanDavid Jordan on English Wikipedia Image:Low cost DCF77 receiver.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Low_cost_DCF77_receiver.jpg  License: Creative Commons Attribution-Sharealike 3.0  Contributors: User:Jaho Image:VHF UHF LP-antenna.JPG  Source: http://en.wikipedia.org/w/index.php?title=File:VHF_UHF_LP-antenna.JPG  License: Attribution  Contributors: Copyright ©2008 K. Krallis, SV1XV Image:Delano VOA.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Delano_VOA.jpg  License: Public Domain  Contributors: Akitoki7 (talk)

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Image Sources, Licenses and Contributors Image:OldTVAntenna.JPG  Source: http://en.wikipedia.org/w/index.php?title=File:OldTVAntenna.JPG  License: Creative Commons Attribution-Sharealike 3.0  Contributors: Sreerambh Image:T2FD_Antenna.png  Source: http://en.wikipedia.org/w/index.php?title=File:T2FD_Antenna.png  License: Public Domain  Contributors: Spamhog File:Aerial antenna.JPG  Source: http://en.wikipedia.org/w/index.php?title=File:Aerial_antenna.JPG  License: Creative Commons Attribution 3.0  Contributors: Daniel Christensen (talk) File:Philco am loop.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Philco_am_loop.jpg  License: Creative Commons Attribution 3.0  Contributors: Daniel Christensen (talk) Image:Doncastertower.JPG  Source: http://en.wikipedia.org/w/index.php?title=File:Doncastertower.JPG  License: GNU Free Documentation License  Contributors: Original uploader was Skyscraper297 at en.wikipedia File:Palmerston-water-tank.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Palmerston-water-tank.jpg  License: Creative Commons Attribution 3.0  Contributors: Bidgee Image:base_station_mexico-city.JPG  Source: http://en.wikipedia.org/w/index.php?title=File:Base_station_mexico-city.JPG  License: Public Domain  Contributors: Pptudela Image:PalmCellTower.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:PalmCellTower.jpg  License: Public Domain  Contributors: Gary Minnaert (Minnaert) Image:Trunked 5ch central control.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Trunked_5ch_central_control.svg  License: GNU Free Documentation License  Contributors: Traced by User:Stannered from an original by David Jordan Image:Base station antenna network.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Base_station_antenna_network.svg  License: Public Domain  Contributors: Original uploader was David Jordan at en.wikipedia. Later version(s) were uploaded by MashrurR at en.wikipedia. Image:Truetone-Radio.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Truetone-Radio.jpg  License: GNU Free Documentation License  Contributors: User:Raul654 Image:BC-224-D.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:BC-224-D.jpg  License: Creative Commons Attribution-Sharealike 3.0  Contributors: Copyright ©2006 K. Krallis, SV1XV Image:1920s TRF radio manufactured by Signal.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:1920s_TRF_radio_manufactured_by_Signal.jpg  License: Public Domain  Contributors: Armstrong1113149 Image:TFR Tube Layout.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:TFR_Tube_Layout.jpg  License: Public Domain  Contributors: Sweidman Image:TRF Dials.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:TRF_Dials.jpg  License: Public Domain  Contributors: Sweidman Image:TRF Component Layout.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:TRF_Component_Layout.jpg  License: Public Domain  Contributors: Sweidman Image:TRF_Schematic.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:TRF_Schematic.jpg  License: Public Domain  Contributors: Sweidman Image:Zn414-basic-circuit.png  Source: http://en.wikipedia.org/w/index.php?title=File:Zn414-basic-circuit.png  License: Public domain  Contributors: Nkendrick (talk) Image:Radar antenna.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Radar_antenna.jpg  License: Public Domain  Contributors: Angeloleithold, Dual Freq, Get It, Huntster, Saperaud, Tony Wills, 1 anonymous edits File:Radar-hatzerim-1-1.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Radar-hatzerim-1-1.jpg  License: Creative Commons Attribution-Sharealike 3.0  Contributors: Bukvoed File:GBH_Primary_Secondory_Radar.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:GBH_Primary_Secondory_Radar.jpg  License: Creative Commons Attribution-Sharealike 3.0  Contributors: Lnbogoda File:Chain home.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Chain_home.jpg  License: Public Domain  Contributors: Original uploader was Stuart166axe at en.wikipedia Image:Radar antennas on USS Theodore Roosevelt SPS-64.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Radar_antennas_on_USS_Theodore_Roosevelt_SPS-64.jpg  License: Public Domain  Contributors: U.S. Navy Image:weather radar.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Weather_radar.jpg  License: unknown  Contributors: NOAA's National Weather Service File:Radar-height.PNG  Source: http://en.wikipedia.org/w/index.php?title=File:Radar-height.PNG  License: GNU Free Documentation License  Contributors: User:Pierre_cb File:Multipath propagation diagram en.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Multipath_propagation_diagram_en.svg  License: Public Domain  Contributors: Original image: Lithium57 English translation: MichaelBillington (talk) Image:Radaroperation.gif  Source: http://en.wikipedia.org/w/index.php?title=File:Radaroperation.gif  License: Creative Commons Attribution-Sharealike 2.0  Contributors: Original uploader was Averse at de.wikipedia Image:Sonar Principle EN.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Sonar_Principle_EN.svg  License: Creative Commons Attribution-ShareAlike 3.0 Unported  Contributors: Georg Wiora (Dr. Schorsch) Image:Radar composantes.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Radar_composantes.svg  License: Creative Commons Attribution-Sharealike 3.0  Contributors: Pierre cb, Vanessaezekowitz, WikipediaMaster, 1 anonymous edits File:SPS-10 radar antenna on a Knox class frigate.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:SPS-10_radar_antenna_on_a_Knox_class_frigate.jpg  License: unknown  Contributors: DON S. MONTGOMERY File:Radar antennas on USS Theodore Roosevelt SPS-64.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Radar_antennas_on_USS_Theodore_Roosevelt_SPS-64.jpg  License: Public Domain  Contributors: U.S. Navy Image:PAVE PAWS Radar Clear AFS Alaska.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:PAVE_PAWS_Radar_Clear_AFS_Alaska.jpg  License: Public Domain  Contributors: AirBa, Avron, Chetvorno, Tony Wills Image:Pocket radio open english.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Pocket_radio_open_english.jpg  License: Public Domain  Contributors: Pocket_radio_open.jpg: Ulfbastel derivative work: Chetvorno (talk) Image:Sanyo Transistor.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Sanyo_Transistor.jpg  License: Public Domain  Contributors: Jorge Barrios Image:Regency transistor radio.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Regency_transistor_radio.jpg  License: GNU Free Documentation License  Contributors: Gregory F. Maxwell < [email protected]> Image:Sony-walkman-srfs84s 0001.JPG  Source: http://en.wikipedia.org/w/index.php?title=File:Sony-walkman-srfs84s_0001.JPG  License: GNU Free Documentation License  Contributors: Original uploader was Ahkitj at en.wikipedia Image:Recreational Walkie Talkies.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Recreational_Walkie_Talkies.jpg  License: Public Domain  Contributors: BesigedB, Burgundavia, Cmdrjameson, Haikupoet, John254, Qero, S9c31r1jo, Wtshymanski, Xlaran, YUL89YYZ, 6 anonymous edits Image:Wikipedia images 011.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Wikipedia_images_011.jpg  License: Public Domain  Contributors: Ahoerstemeier, Haikupoet, Tellyaddict, 2 anonymous edits Image:Scr300.png  Source: http://en.wikipedia.org/w/index.php?title=File:Scr300.png  License: Public Domain  Contributors: LuckyLouie (talk) 12:23, 18 July 2008 (UTC) File:Portable radio SCR536.png  Source: http://en.wikipedia.org/w/index.php?title=File:Portable_radio_SCR536.png  License: Public Domain  Contributors: Mattes, Sv1xv Image:AWM 017402 Noemfoor radio.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:AWM_017402_Noemfoor_radio.jpg  License: Public Domain  Contributors: Allan F. Anderson Image:Motorola hand-held.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Motorola_hand-held.jpg  License: Public Domain  Contributors: David Jordan, Monkeybait File:Motorola HT1000.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Motorola_HT1000.jpg  License: Public Domain  Contributors: w:en:User:Heterodyne. Image:Mot t5400.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Mot_t5400.jpg  License: Public Domain  Contributors: Image:Toywt.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Toywt.jpg  License: Public Domain  Contributors: Haikupoet File:Induction heating on solid and gas.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Induction_heating_on_solid_and_gas.jpg  License: Creative Commons Attribution-Sharealike 3.0  Contributors: Xfanplasma File:Structure of induction plasma.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Structure_of_induction_plasma.jpg  License: Creative Commons Attribution-Sharealike 3.0  Contributors: Xfanplasma File:Tekna PL50 torch.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Tekna_PL50_torch.jpg  License: Creative Commons Attribution-Sharealike 3.0  Contributors: Xfanplasma File:Dense section of spherical tungsten carbide.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Dense_section_of_spherical_tungsten_carbide.jpg  License: Creative Commons Attribution-Sharealike 3.0  Contributors: Xfanplasma File:Flaky interlocking rhenium powder.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Flaky_interlocking_rhenium_powder.jpg  License: Creative Commons Attribution-Sharealike 3.0  Contributors: Xfanplasma File:Spheroidised quartz powders.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Spheroidised_quartz_powders.jpg  License: Creative Commons Attribution-Sharealike 3.0  Contributors: Xfanplasma

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License Creative Commons Attribution-Share Alike 3.0 Unported http:/ / creativecommons. org/ licenses/ by-sa/ 3. 0/

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