Mobile Antenna Design & Performance Trade-Offs

Antenna Engineering Vancouver

White Paper: Mobile antenna design and performance trade-offs

Aaron B. Logan Field Applications Engineer June 16, 2009

White Paper: Mobile Antenna Design Report Author: Aaron B. Logan

Date Start: 6/16/2009

Abstract Quarter-wave monopoles, half-wave dipoles, and axial collinear arrays are the three primary types of antennas used for mobile applications. These antennas have distinct electrical and mechanical differences and costs associated with each design. Exploring a few fundamental concepts is critical to understanding the differences and trade-offs involved with each type of antenna. Consumer-specific considerations such as height, performance and cost trade-offs are also explored to enables a consumer to select the right antenna for their application.

Antenna Fundamentals Before focusing on the differences between these types of antennas, a few antenna fundamentals must be reviewed. Mobile radios require the antenna to be designed for the same electrical frequency as the radio itself in order to operate properly. The frequency (f) of a radio wave is the number of cycles the wave transverses a full cycle in one second. To visualize this concept, Figure 1 shows an image of a 10 Hertz (Hz) cycle displayed over the course of two seconds.

Figure 1: 10Hz cycle over 2 seconds 1



Knowing the frequency is critical for defining what the wavelength of an antenna is. The wavelength is the distance the radio frequency travels during one cyclic period. Radio waves travel at the speed of light which is approximately 300,000 kilometers per second. The relationship of wavelength versus frequency is inversely proportional (Schmitt, 2002). This means that as the frequency goes up, the wavelength gets shorter and when the frequency goes down, the wavelength gets longer. Wavelength is commonly symbolized with the Greek letter lambda (λ). To visualize this relationship, refer to Equation 1(Carr, 2002)..

Equation 1: Relationship of frequency vs. wavelength

It is important to state that the electrical wavelength is always shorter in coaxial cable than in free-space (air) due to the physical properties of the cable such as the dielectric constant between the conductors (Hall, 1991). Electrical length can be defined as the physical length divided by the electrical length (Schmitt, 2002). With respect to mobile antenna design, the electrical and physical lengths are used interchangeably as the difference between the two lengths is small.

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The fundamental differences between a quarter-wavelength monopole, a half-wavelength dipole, and a collinear array become apparent when analyzing the voltage and current distribution differences between the three. Voltages and currents exist on an antenna’s radiator 90° out of phase with each other. Figure 2 shows the phase relationship differences between voltage and current. The feed point of a mobile antenna is the antenna mount itself, and as such, most mobile antennas are considered to be “end-fed” structures (Hall, 1991). All end-fed antennas are designed to have the peak voltage potential at the tip of the antennas where the current is forced to a zero potential. It is this relationship that ties our monopoles, dipoles, and axial collinear arrays together.

Voltage & Current Relationship 90 45 °)( s e e rg e D

0 0

0.25

0.5

0.75

1

-45

-90 Wavelength (?) I

V

Figure 2: Voltage and current phase relationship

The physical length corresponds with the electrical wave-type. Monopoles are designed to be a quarter wavelength tall, dipoles are designed to be a half wavelength tall, and an axial collinear array is designed to be the tallest combining different radiator lengths into a single phased antenna structure. Figure 3 shows an example of these three types of antennas over a ground plane while Figure 4 shows the current distribution. Each antenna has the same size 3



ground plane. A ground plane is a reflective surface, such as the roof of a vehicle, which is electrically connected to the antenna. The critical difference between these antennas is the effective antenna height. The voltage and current distribution on these structures are used to determine the open-circuit voltages and ultimately the gain of the antenna (Johnson, 1993).

Figure 3: Physical differences between a monopole, dipole, and collinear array

Figure 4: Current distribution on a monopole, dipole, and collinear array

The first antenna in Figure 4 shows the current flows from a zero potential at the tip of the antenna to approximately 90° of a full cycle. This equates to a quarter wavelength of current. The second image shows current starting from a zero potential and terminating close to 180° with minimal potential at a half wavelength. The third image shows the current distribution of a 4



collinear array consisting of a 5/8ths over 5/8ths over a 1/2 wave. This structure phases the currents by employing a phasing coil to enable a full cycle of current to transverse and setups up the phasing for the subsequent sections. This configuration allows for a nearly symmetric omnidirectional pattern (Polivka, 2007). Figure 5 shows the collinear array and the currents combine to complete the phasing. By definition, a collinear array is a “broadside radiator composed of multiple elements operating in phase” (Hall, 1991).

Figure 5: Current phasing on a collinear array

The gain of the antenna helps define the distance an antenna can radiate as well as describing the directionality of the antenna. As the height of the antenna increases, along with proper phasing of the currents (Scholz, nd), the further on the horizon an antenna can radiate thus 5



increasing the effective radio coverage area. It must be clarified that gain in an antenna is not like gain measured in an amplifier. Gain, commonly referred to as an “S21” measurement, is described as the “forward transmission coefficient” which could result in an increase in forward power (Agilent, 2002). An increase in antenna gain does not increase the total available power. Rather, gain in an antenna increases as the directionality of the antenna increases.

To visualize this concept, imagine a flashlight and a light bulb. Both bulbs have the same amount of power applied and put off the same amount of radiated energy. However, the light on a flashlight travels further than a light bulb because of the increased directionality induced by the reflector portion of the flashlight which focuses the energy in a specific direction. “When we make a map of the gains in all directions, we have the radiation pattern of the antenna,” (Carr, 2002).

The potential gain of a monopole and dipole are similar in performance. Both are considered low gain antennas that radiates out on the horizon. Using a Numerical Electromagnetic Code (NEC) program to calculate the radiation patterns, Figure 6 shows the differences in the gains. Both the monopole and dipole have similar gains on the same ground plane while the collinear array pushes the energy further onto the horizon.

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Figure 6: Elevation patterns of a monopole, dipole, and collinear array

Consumer Specific Considerations Height, performance, and cost are primary considerations a consumer must ponder before purchasing a roof-mounted antenna for their radio. The most common types of antennas used today are quarter-wave monopoles, half-wave dipoles, and collinear arrays. All three types exist for the same frequency, but each has its own unique advantage.

Antenna height should always be considered before purchasing. Garages and carports can be problematic for taller antennas. Repetitive impacts from entering and exiting a garage can degrade performance or permanently damage the antenna. If height may be an issue, the best option would be a monopole. Monopoles are the shortest antenna available for mobile radios. As such, it is ideally suited for users with garages or people who park in covered parking areas. End7



fed dipoles are twice the height of monopoles and are better suited for consumers where height is a minimal consideration. Collinear arrays are the tallest antenna option.

Antenna height is an indicator of performance. Taller antennas have a greater size aperture than shorter antennas. A larger aperture can be translated as a “structure capable of having more gain,” (Johnson, 1993). Recall that higher gain means the signal travels farther out on the horizon resulting in an increased coverage area. Low-gain antennas push the energy higher in elevation. Low gain antennas are ideal for use in cities with tall buildings.

A monopole is a low gain antenna whose performance is dependent on not only the size of the vehicle, but also the placement on the vehicle due to the dependency of a ground plane (Nordic Semi, 2005). Dipole antennas have the same low gain performance as monopoles; however, dipoles are not dependent on the vehicle’s ground plane size (Ott, 2002). Dipoles can be mounted on vehicles with fiberglass roofs where monopoles cannot. This is the primary tradeoff between the monopole and dipole; ground plane dependency.

Collinear arrays are high-gain antennas. Similar to monopoles, most collinear arrays are dependent on the vehicle size and placement on the ground plane. For rural areas or sparsely populated areas, collinear arrays are a better choice where maximum coverage area is needed. Placed into perspective, collinear arrays can double the coverage area that a monopole covers.

Cost seems to be the most important factor when considering an antenna. Cost is driven by the complexity of the antenna design, the number of components required to match the 8



antenna to the radio system, and the total antenna performance as a whole. Monopoles are the most economical antenna solution. The design is simple and may consist of only four or five components. By design, monopoles do not require any real matching components at the antenna mount. When proper materials are used and the design correct, minimal mismatch loss occurs through the antenna mount and is efficiently radiated by the antenna (Logan, 2006). If theft or repetitive damage is a problem, this is a great choice as replacements are cheap.

Dipoles are a higher-cost antenna solution; however, the buyer is paying for assured performance over installation location. Dipoles require a matching network at the base of the antenna to electrically match the antenna to the antenna mount and radio system. The necessity of the matching network is a direct result of the end-fed method used to feed the antenna. This antenna is moderately priced because of the additional components and the assured performance over the smaller monopole.

A collinear array typically costs the most as the design is the most complex and may require additional matching components. Depending on the phasing mechanism, the primary radiator may be a turned coil or some other arrangement employed to facilitate current phasing. This adds cost to the bill of materials. Additionally, depending on the design, a matching network may be required similar to a dipole. As collinear arrays provide the widest coverage area, buyers are paying for the technology used and the improved performance more than the added cost of components.

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Height, performance, and cost are just a few of the primary considerations a consumer must ponder before purchasing a roof-mounted antenna for their mobile radio. The shortest, cheapest, low-gain antenna is the monopole. For a modest cost and assured low-gain performance, a dipole is an excellent choice. For those who need maximum coverage and can accept a tall antenna, the collinear array should be chosen. By understanding the trade-offs in antenna selection and basic antenna concepts, a consumer can make the best choice based on financial needs, performance requirements, and application limitations.

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References Agilent Technologies, Inc. (2002). Agilent 2002 back to basics seminar: Learn the fundamentals of RF measurements from experienced Agilent engineers. Santa Clara, CA: Agilent Technologies, Inc. Carr, J.J. (2002). Practical radio frequency test & measurement: A technician's handbook. Burlington, MA: Newnes Publishing. Hall, Gerald (Ed.). (1991). The ARRL antenna handbook. Newington, CT: The American Radio Relay League. Johnson, R. (1993). Antenna engineering handbook (3rd ed.), New York, NY: McGraw-Hill, Inc. Logan, A.B. (2006, June 12). White paper: NMOHF antenna mounts. Retrieved May 29, 2009, from Larsen-Antennas Web site: http://www.larsenantennas.com/docfiles/NMOHFWhitePaper.pdf Nordic Semiconductor ASA, (2005, January 21). White paper: 1/4 printed monopole antenna for 2.45 GHz. Retrieved May 29, 2009, from Nordic Semiconductor Web site: http://www.nordicsemi.com/files/Product/white_paper/PCB-quarterwave-2_4GHzmonopole-jan05.pdf Ott, H (2004, October 6). Seminar, Dipoles for dummies, part 1 - Basic . Retrieved May 29, 2009, from Hott Consultants Web site: http://www.hottconsultants.com/pdf_files/dipoles1.pdf Polivka,M & Holub A. (2007). Collinear and coparallel principles in antenna design. Progress In Electromagnetics Research Symposium 2007, (August 27-30), pages 337-341. Retrieved May 29th, 2009, from http://piers.mit.edu/piersproceedings/download.php?file=cGllcnMyMDA3cHJhZ3VlfDN BMmJfMDMzNy5wZGZ8MDcwMjIwMTgzNzQ4 Schmitt, Ron (2002). Electromagnetics explained: A handbook for wireless/RF,EMC, and highspeed electronics. Burlington, MA: Newnes. Scholz, P. (Date unknown). Basic antenna principles for mobile communications. Retrieved May 29, 2009, from Kathrein Web site: http://www.kathrein.de/en/mcs/techninfos/download/basicantenna.pdf

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Mobile Antenna Design & Performance Trade-Offs

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