Engineering often comes down to deciding between the best set of compromises. Unfortunately, the “perfect” antenna does not exist (for example, something the size of a pin head that covers every commonly used band, and has 100% efficiency). Optimizing one parameter invariably effects other parameters.
For instance, if an antenna must be small, efficiency at low frequencies will drop and the bandwidth will suffer. But perhaps a loss of efficiency at low frequencies is acceptable; perhaps it is more important that the antenna is small. Therefore, what needs to be done in this scenario is to determine how small the antenna needs to be and what the minimum antenna efficiency should be in order for the antenna to perform in its application.
Below we will cover common criteria and how they relate to design.
Size
One of the most important requirements invariably ends up being allowable size. Any application has some form of size limit; therefore it is important to know ahead of time how much size is available for the antenna to work.
Sometimes the available size is more than sufficient to build a high-performance antenna. However, most of the time size ends up being a major limiting factor in antenna design and performance. Therefore, the allowable size for an antenna really needs to be specified.
Bandwidth
Bandwidth is quite size dependent. As the size of the antenna goes down, the bandwidth suffers. There are ways to bring back the bandwidth, but efficiency may suffer in turn, resulting in less antenna gain.
Of course, some applications really don’t need a broad bandwidth. In other cases, the antenna only needs to pick up specific bands. It is important to specify the desired bandwidth, because gain will sometimes need to be sacrificed to achieve the bandwidth or visa versa.
Efficiency and Gain
Both efficiency and gain relate to the type of coverage that may be expected.
Gain is also related to directivity, which is a measure of how directional an antenna is. Sometimes directivity is useful; sometimes it is not. It is important to decide in what directions the antenna will need to radiate and receive, and what the gain should be in these regions.
Be aware, however, that a truly omnidirectional antenna cannot have a gain of more than 1. This is because an increase of gain in one direction comes at the expense of gain in other directions.
Efficiency describes how much of the signal going to an antenna will actually be used. Broadly speaking, efficiency drops as the size of the antenna shrinks below a certain minimum dimension determined by the frequency of operation. The efficiency of an antenna can be brought back to some degree for a small antenna, but at the expense of bandwidth.
Efficiency and gain go hand in hand; antenna gain is a function of both efficiency and directivity. Therefore, a low efficiency antenna will have a low gain. However, it is possible to increase gain by making the antenna more directive when acceptable.
Most mobile devices will need more omnidirectional antennas to ensure complete coverage.
VSWR
VSWR is a measurement of the amount power reflected back to the device powering the antenna, and is related to impedance. The greater the mismatch between an antenna and the device it is intended for, the worse the VSWR.
Unless otherwise specified in the design specs, most devices are assumed to have a 50 ohm impedance. Impedance mismatch can occur for reasons other than an overall mismatch between the antenna and the source. The impendence of an antenna tends to vary across the frequency spectrum when its bandwidth is not sufficient. This can lead to unacceptable VSWRs at certain frequencies.
Matching networks are the most efficient way to fix frequency-related VSWR problems. A worst case VSWR should be specified for a new antenna design as some devices are extremely sensitive to VSWR problems. Sometimes optimizing VSWR will result in less gain; it depends on how the VSWR is improved.
It is worth mentioning here that a good VSWR does not mean good antenna “sensitivity,” i.e. gain. A 50 ohm load has a nearly perfect VSWR and yet doesn’t radiate power at all. This is why the antenna gain is important, too.
Putting It All Together
Antenna design attempts to achieve the best solution that will meet the requirements of the application. Often, the results can exceed the specifications; other times some of the specifications may prove mutually exclusive.
Also, some otherwise good designs may prove failures due to cost. As an example, high frequency PC board antennas need high quality PC board material to ensure performance and repeatability. High quality PC boards are quite expensive, and yet low quality PC boards tend to be lossy and have variable characteristics that can cause widely varying antenna performance from board to board.
Time is sometimes another constraint. Finding a good design takes time that may or may not be available. As a rule, the more difficult the criteria, the more time and effort that will be needed to meet the design objectives.
Nevertheless, a good antenna design can often be realized by understanding how each property and criteria of an antenna affect the other characteristics of the design. The key is to determine what exactly the antenna will need to do for it to be useful in its application. This determined, an antenna type that will best match the application can be chosen, and then adapted to the application at hand.
ARSI 