Antenna Aperture

VHF Marine Band radios, protocol, radio communication theory, practical advice; AIS; DSC; MMSI; EPIRB.
jimh
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Antenna Aperture

Postby jimh » Sat Feb 10, 2018 12:03 pm

One way to understand how a passive device like a radio antenna can possess the ability to have gain is to consider the behavior of the antenna when receiving. When a signal arrives at a receiving antenna, it has lost a great deal of its power due to propagation path loss. The power-density of the signal is very low. By power density I mean the amount of signal per area. If we have two antennas, and one antenna produces a stronger signal, we then say that antenna must have a larger effective aperture (area), because it gathered or collected more signal power from the radio wave coming from the remote transmitter.

If we compare an antenna with a gain of 2 (3 dB) with an antenna with a gain of 4 (6 dB), the antenna with more gain must have a greater effective aperture. The increased gain is a very positive factor, but it comes with a liability: because the antenna's effective aperture is greater, the antenna will be affected more by any other conductors close to it that come within the effective aperture. Simply stated, the antenna with 6 dB gain must have a larger aperture than an antenna with 3 dB gain, and the 6 dB gain antenna requires a greater separation from all other conductors near it to avoid those conductors causing interference. Or, even simpler, higher gain antennas must have a clearer mounting to work properly. An antenna with twice the gain of another needs twice as much clearance around it to work as intended compared to the antenna with half the gain.

If we mount a 6 dB-gain antenna on a small boat and locate it amid all sorts of other metal conductors, its performance will be more likely to be degraded in that environment than a 3 dB-gain antenna. The more gain an antenna has, the more isolated it should be from other metal conductors--particularly metal conductors that might be resonant at the operating frequency.

The most obvious other metal conductor that might be resonant at the same frequency as an antenna on a small boat is another antenna for that same frequency. If you don't want two antennas to affect each other, you have to keep them apart. The more gain each co-resonant antenna has, the farther apart they need to be from each other.