Electromagnetic waves, often referred to as radio waves, are waves of energy that are similar to light waves and travel through the air at the speed of light. To understand how an antenna works you must first have a basic idea of the make-up of a radio wave. A radio wave can be visualized as a sine wave. The distance a wave travels to complete one cycle is known as the wavelength of the signal. A 2.4GHz signal (such as Bluetooth™, WiFi, Zigbee or WiMedia) completes a cycle as it travels through the air every 12.5 cm. The wavelength of visible light is less than 5 um. The formula for wavelength is: where is the wavelength, c is the speed of light, and f the frequency (cycles per second). In vacuum and air, c is equal to the speed of light (299 793 077 m/s), but as you will soon see, radio waves are slower when passing through other materials and hence the wavelength will be shorter. This is of great importance when designing antennas. See “Material influence on wavelength”. Electromagnetic spectrum
Basically, a transmitting antenna transmits by exciting it at the base (or at a pair of antinodes), while in a receiving antenna, the applied electromagnetic field is distributed throughout the
entire length of the antenna to receive the signal. The magnetic field that the transmitting antenna radiates will produce an electric current on any metal surface that it strikes. However, if the metal that the signal strikes has a certain length relation to the wavelength the induced current will be much stronger on the object. We stated before that as a signal at 2.4GHz travels through the air, it completes a cycle in approximately 12 cm. If the signal strikes a 12 cm antenna or fractions of it (1/2 or 1/4 wavelength = 6 or 3 cm), then the induced current will be much higher than if the signal struck a metal object that was not some appreciable fraction of the wavelength. This is known as antenna resonance. Every antenna has at least one exact resonance point. Note that an antenna also transmits a stronger signal if it is resonant on the frequency used. Antennas have a number of important parameters, those of most interest include the gain, radiation pattern, bandwidth and polarization.
Without digging too deep into a complex domain, let’s look at a few facts that have great impact on antenna design. The dielectric constant is the relative permittivity of a material. It is dimensionless and always greater than 1. A dielectric constant of "1" is equivalent to the permittivity of a vacuum, which is a fundamental constant (associated with the speed of light). In other words, vacuum has the lowest possible permittivity.
The higher figures a material shows for permittivity, the slower the radio waves will pass and thus making a radio a signal of e.g. 2.4 GHz present a wavelength shorter than 12 cm. This means that if an antenna is covered with a material with high permittivity it will, for the same frequency, find its resonant point with a shorter (smaller) antenna than would have been the case if it was an open wire. A value will result in half the wavelength. This sounds good when you want to build antennas for small devices, but what’s the catch? The catch is that the higher the permittivity, the more the energy will be reflected inside the antenna before leaving it, and the more inferior and narrow-banded the antenna will get. Also, a very small antenna has less surface to absorb the incoming wave. Ceramic antennas can be built very small .
VSWR is a measure of impedance mismatch between the transmission line and its load. The higher the VSWR, the greater the mismatch. The minimum VSWR, i.e., that which corresponds to a perfect impedance match, is unity. To understand the definition above we must understand what impedance is. Impedance in antenna terms refers to the ratio of the voltage to current (both are present on an antenna) at any particular point of the antenna. This ratio of voltage to current varies on different parts of the antenna, which means that the impedance is different on different spots on the antenna if you could pick any spot and measure it. As stated before, the impedance for the entire chain from the radio to the antenna must be the same, and almost all radio equipment is built for
an impedance of 50 ohm. If any part of this chain fails to show a 50 ohm impedance due to e.g. bad connections, incorrect antenna length, etc., the maximum power will not be radiated from the antenna. Instead part (or all) of the wave is reflected back down the line. The amount of the wave reflected back depends on how bad the mismatch is. The combination of the original wave traveling down the coaxial cable (towards the antenna or opposite during receive) and the reflecting wave is called a standing wave. The ratio of the two above described waves is known as the Standing Wave Ratio. The result is presented as a figure describing the power absorption of the antenna. A value of 2.0:1 VSWR, which is equal to 90 % power absorption, is considered very good for a small antenna: 3.0:1 is considered acceptable (-6dB) which is equal to 75 % power absorption.
Smith Chart One common way of visualizing the VSWR is a polar plot called Smith chart. From this plot the VSWR value, the return loss and the impedance for the different frequencies can be derived. Therefore it is an important instrument for
understanding antennas. To learn more about the SMITH chart, see e.g. http://sssmag.com/smith.html
This is basically the same thing as VSWR. If 50 % of the signal is absorbed by the antenna and 50 % is reflected back, we say that the Return Loss is -3dB. A very good antenna might have a value of -10dB (90 % absorbed & 10 % reflected). When studying a graph showing Return Loss/VSWR, a deep and wide dip of the curve is good since this shows an antenna with good bandwidth (spreadband). Consequently, the narrower the dip is, the bigger risk that also desired channels will be reflected away (narrow band). Return Loss Chart
Note: To be able to compare figures from different manufacturers, you must be aware of the conditions under which the measurement was made. Was impedance matching used or not?
Conversion table VSWR / Return Loss Performance VSWR Return Loss (dB) Better
Worse
1.01 1.05 1.1 1.2 1.3 1.4 1.5 1.75 2.0 2.5 3.01 5.85 8.72 17.4
-46.1 -32.3 -26.4 -20.8 -17.7 -15.6 14.0 -11.3 -9.5 -7.4 -6.0 -3.0 -2.0 -1.0
Normally a radio needs to work on multiple frequencies. For example, the 2.4 GHz ISM band used by Bluetooth/Wi-Fi/Zigbee/WiMedia devices has a range from 2400-2483 MHz. In this band WPAN communication uses 78 channels for its frequency hopping technique, 1 MHz between each channel. This means that the antenna must perform well over a range of frequencies. So, the goal must be to make it resonant in the middle of that band. The term that is important here is bandwidth or how much band your antenna works well over. One method of judging how well (efficiently) your antenna is working is by measuring VSWR. Typically, bandwidth is measured by looking at SWR, i.e., by finding the frequency range over
which the SWR is less than 2.
Efficiency is a figure showing the ratio of the total radiated power to the total input power . Efficiency has no unit and the ideal figure is 1.
It is essential to know how the measurement was performed before comparing figures from different manufacturers: was a matching network used? Was the measuring point as close to the antenna as possible or was the transmission line included? Often, the figure for efficiency will dramatically decrease when the antenna is built into a device. Note: This is a good figure of merit, especially for small antennas. Efficiency
Antenna gain is a measure of directivity. In order to explain this better, we must first have a look at the different antenna types and their radiation patterns. Basically there are only two types of antennas: The dipole antenna (Hertzian) and the vertical antenna (Marconi). All antennas can be broken down to one of these types (although some say that there is only one - the dipole). In addition to this we have a theoretical perfect antenna (non-existent) that radiates equally in all directions with 100% efficiency. This antenna is called an isotropic radiator. Basic Antenna types
Antenna Radiation Patterns
This is similar to gain but the heat losses (i.e. the efficiency) are disregarded. We will then get a pattern as the dotted line shown in the figure. Point "d" refers to directivity, point "a" to gain and point "b" to the isotropic reference.
Gain presented as 3D gain
The gain can also be presented as a 3D gain. The radius of the spheriod is proportional to the antenna gain. Gain in theory Since all real antennas will radiate
more in some directions than in others, you can say that gain is the amount of power you can reach in one direction at the expense of the power lost in the others. When talking about gain it is always the main lobe that is discussed. Gain may be expressed as dBi or dBd. The first is gain compared to the isotropic radiator and the second gain is compared to a half-wave dipole in free space (0 dBd=2.15 dBi). It may be worthwhile considering the fact that instead of doubling your amplifier output, you could alternatively use an antenna that has 3db more gain than your current antenna and achieve exactly the same effect. Note: Small antennas usually have low gain, often between 0 and 2dBi. Note: Regarding efficiency and radiation patterns what is true for transmission is generall also true for reception.
This is similar to gain but the heat losses (i.e. the efficiency) are disregarded. We will then get a pattern as the dotted line shown in the figure. Point "c" refers to directivity, point "a" to gain and point "b" to the isotropic reference.
Radio waves are built by two fields, one electric and one magnetic. These two field are perpendicular to each other. The sum of the fields is the electromagnetic field. Energy flows back and forth from one field to the other - This is what is known as "oscillation". The position and direction of the electric field with reference to the earth’s surface (the ground) determines wave polarization. In general, the electric field is the same plane as the antenna's radiator. Horizontal polarization —— the electric field is parallel to the ground. Vertical polarization -- the electric field is perpendicular to the ground. There is one special polarization known as Circular polarization. As the wave travels it spins, covering every possible angle. It can either be righthanded or lefthanded circular polarization depending on which way its spinning.
Note: Small antennas have no clear polarization. Polarization chart
An ideal antenna solution has an impedance of 50 ohm all the way from the transceiver to the antenna, to get the best possible impedance match between transceiver, transmission line and antenna. Since ideal conditions do not exist in reality, the impedance in the antenna interface often must be compensated by means of a matching network, i.e. a net built with inductive and/or capacitive components. The VSWR result is optimized by choosing the proper layout and component values for the matching net and the maximum potential of the antenna is shown.
Decibel (dB) is a mathematical expression showing the relationship between two values. The RF power level at either transmitter output or
receiver input is expressed in Watts, but it can also be expressed in dBm. The relation between dBm and Watts can be expressed as follows: P dBm = 10 x Log P mW For example: 1 Watt = 1000 mW; P dBm = 10 x Log 1000 = 30 dBm 100 mW; P dBm = 10 x Log 100 = 20 dBm Conversion table dBm / Watt dBm Watt 0
0,001
10
0,01
20
0,1
30
1
40
10
The following definitions are taken from IEEE Standard Definitions of Terms for Antennas, IEEE Std 145-1983. Adaptive (smart) antenna: An antenna system having circuit elements associated with its radiating elements such that one or more of the antenna properties are controlled by the received signal. Antenna polarization: In a specified direction from an antenna and at a point in its far field, is the polarization of the (locally) plane wave which is used to represent the radiated wave at that point.
Antenna: That part of a transmitting or receiving system which is designed to radiate or to receive electromagnetic waves. Coaxial antenna: An antenna comprised of a extension to the inner conductor of a coaxial line and a radiating sleeve which in effect is formed by folding back the outer conductor of the coaxial line. Collinear array antenna: A linear array of radiating elements, usually dipoles, with their axes lying in a straight line. Co-polarization: That polarization which the antenna is intended to radiate Cross-polarization: In a specified plane containing the reference polarization ellipse, the polarization orthogonal to a specified reference polarization. Directional antenna: An antenna having the property of radiating or receiving electromagnetic waves more effectively in some directions than others. Effective radiated power (ERP): In a given direction, the relative gain of a transmitting antenna with respect to the maximum directivity of a half-wave dipole multiplied by the net power accepted by the antenna from the connected transmitter. E-plane: For a linearly polarized antenna, the plane containing the electric field vector and the direction of maximum radiation. Far-field region: That region of the field of an antenna where the angular field distribution is essentially independent of the distance from a specified point in the antenna region. Frequency bandwidth: The range of frequencies
within which the performance of the antenna, with respect to some characteristics, conforms to a specified standard. Front-to-back ratio: The ratio of the maximum directivity of an antenna to its directivity in a specified rearward direction. Half-power beamwidth: In a radiation pattern cut containing the direction of the maximum of a lobe, the angle between the two directions in which the radiation intensity is one-half the maximum value. Half-wave dipole: A wire antenna consisting of two straight collinear conductors of equal length, separated by a small feeding gap, with each conductor approximately a quarter-wave length long. H-plane: For a linearly polarized antenna, the plane containing the magnetic field vector and the direction of maximum radiation. Input impedance: The impedance presented by an antenna at its terminals. Isolation: A measure of power transfer from one antenna to another. Isotropic radiator: A hypothetical, loss less antenna having equal radiation intensity in all directions. Log-periodic antenna: Any one of a class of antennas having a structural geometry such that its impedance and radiation characteristics repeat periodically as the logarithm of frequency. Major/main lobe: The radiation lobe containing the direction of maximum radiation. Microstrip antenna: An antenna which consists of a thin metallic conductor bonded to a thin
grounded dielectric substrate. Omnidirectional antenna: An antenna having an essentially non-directional pattern in a given plane of the antenna and a directional pattern in any orthogonal plane. Radiation efficiency: The ratio of the total power radiated by an antenna to the net power accepted by the antenna from the connected transmitter. Side lobe suppression: Any process, action or adjustment to reduce the level of the side lobes or to reduce the degradation of the intended antenna system performance resulting from the presence of side lobes.
