Selecting the Proper Antenna

Antenna Gain

The ability of the antenna to shape the signal and focus it in a particular direction is called "antenna gain" and is expressed in terms of how much stronger the signal in the desired direction is, compared to the worst possible antenna, which distributes the signal evenly in all directions (an "isotropic radiator"). To express the relationship to the isotropic reference, this is abbreviated dBi. The typical omni-directional "stick" antenna is rated at 6-8 dBi, indicating that by redirecting the signal that would have gone straight up or down to the horizontal level, 4 times as much signal is available horizontally. A parabolic reflector design can easily achieve 24 dBi.

The antenna gain factor applies to the received signal as well as to the transmitted signal. By focusing the incoming signal from a particular direction onto the radiating element, the antenna also shields the receiver from interference from noise sources outside of the amplified angle.

Point-to-Point Applications

For point-to-point applications, you generally want to use high-gain directional antennas. The tight beam gives you better signal strength, and it also helps lock out potential sources of noise and interference in the environment.
Remember to adjust your transmit power to comply with FCC regulations in the 2.4GHz band: With a 24 dBi antenna, the maximum transmit power in the USA is 24 dBm. (In the 900 MHz band, the limit is 36 dBm EIRP, so with a 24 dBi antenna, max output power is 12 dBm.) A 24 dBi parabolic grid antenna has a beam width of about 10 degrees both horizontally and vertically. Align the beam carefully, and make sure that the mast does not sway more than 4-5 degrees under maximum wind load.

Multi-Point Applications

Multipoint systems have a hub node, and a number of subscriber nodes. Each of the subscriber nodes communicates directly only with its hub, so select directional antennas as for a point-to-point application (see above). For the hub of a multipoint application, the picture is much more complicated. The hub must have a beam open enough to encompass all the subscriber nodes. In most cases, this means an omnidirectional or a sector antenna, which needs to be mounted at an elevated point. It is tempting to select the highest possible gain antenna you can find, but if you are in hilly terrain, that may not be the best solution.
Omnidirectional antennas achieve a high gain by shaping their beam to a flat disk. The higher the gain, the flatter the disk. A 6 dBi omni antenna may have a vertical beamwidth of 16 degrees, but a 10 dBi omni is typically only about 8 degrees, and a 12 dBi omni is only 4 degrees. If the antenna is mounted on a tower in a valley, subscribers on a hillside looking down on the tower may be outside the beam. (This is exacerbated by the fact that the antenna designer typically expects the antenna to be on a tower above the subscribers and therefore may have tilted the beam down, shaping it like a flat cone.)

In a mountainous area, the best location for the hub is often on a mountaintop to one side of the coverage area, with a low-gain directional antenna such as a 12 dBi Yagi which is likely to have an beamwidth of about 45 degrees both vertically and horizontally. Panel antennas of similar beam shape and gain are also readily available.

Inexperienced system installers often design for the nodes farthest out, and assume that subscriber nodes at shorter distances will work. "They may be outside the core of the beam, but they have less loss due to distance, and that will make up for it." This is not true. Outside the main beam, signal strength does not decrease evenly towards the backside. Rather, the edge of the beam consists of a complex pattern of side lobes with nulls (areas of no signal whatsoever) in between. Usually, the first null is at an angle twice as far from the center of the beam as the 3 dB dropoff point normally counted as the edge of the beam.

How to Read a Specification Sheet

Because there are many tradeoffs between different performance parameters, it is useful to review the manufacturer's specification sheet before committing to an antenna. The following are some of the data you will find:

Frequency Range - The frequency range that the manufacturer specifies for the antenna is typically larger than the band you intend to operate in. Make sure that the stated specifications are valid for the entire listed range, or at least in the part of it that you will be using. Watch out for a footnote that the stated values are "typical mid band values" unless the stated range is MUCH wider than the band you will be using.

Beamwidth - Two times the angle of deviation from the center of the beam where the signal strength drops 3 dB below the peak value. The higher the gain the narrower the angle.

Gain - The signal level measured in the direction at which it is strongest.
Front/Back ratio - How well does the antenna suppress signal from sidelobes on the back of the antenna. A high front/back ratio is important for sites with multiple antennas.

Cross polarization discrimination - How well can you separate signals at the same frequency with opposite polarizations?
Rated wind velocity/Horizontal thrust at rated wind - Make sure your mounting hardware will handle the load!