I work for the FAA and we are trying to find an antenna that will reduce multipath from VHF and UHF signals. Currently we are testing some antenna's that are designed for receiving circularly polarized waves even though the transmissions from the aircraft are linearly polarized. These antennas are performing better than a regular omnidirectional antenna. Any suggestions would be appreciated. Thank you.
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VHF/UHF Anennas with multipath canceling abilities
- Thread starter FAA
- Start date
Given that the aircraft antennas are linear (and not likely to change to circular - at least not in our lifetimes), I wonder if your CP ground antenna is working better simply because it has some better rejection towards the ground reflections ?
The only simple approach that I can think of is to provide an antenna that rejects any signals from below the horizon. The problem is that you would want to accept the signals from just above the horizon from distant aircraft. You have a very fine line between where you want maximum sensitivity and rejection. Probably not practical.
Ref. "Airborne Measurements of Electromagnetic Wave Reflections from Land and Sea Water", Allen L. Johnson, AGARD Confernece Proceedings No. 269 (AGARD-CP-269), paper 11. Might be of some interests... there's probably better references (I just happen to know this one).
Another, more expensive approach, would be to use multiple antennas on the assumption that if the direct and reflected waves are cancelling at one spot, they might be combining constructively at another spot. This would require an active receiving system and wouldn't help in the reverse direction.
The only simple approach that I can think of is to provide an antenna that rejects any signals from below the horizon. The problem is that you would want to accept the signals from just above the horizon from distant aircraft. You have a very fine line between where you want maximum sensitivity and rejection. Probably not practical.
Ref. "Airborne Measurements of Electromagnetic Wave Reflections from Land and Sea Water", Allen L. Johnson, AGARD Confernece Proceedings No. 269 (AGARD-CP-269), paper 11. Might be of some interests... there's probably better references (I just happen to know this one).
Another, more expensive approach, would be to use multiple antennas on the assumption that if the direct and reflected waves are cancelling at one spot, they might be combining constructively at another spot. This would require an active receiving system and wouldn't help in the reverse direction.
There could be several reasons that CP antenna works better than linearly polarized antenna.
1. Your CP antenna may have a better Front/Back ratio that allows to better mitigate reflected signal from the ground.
2. Your CP antenna may have a sharper radiation pattern roll-off near and below horizon, again allowing to better mitigate interfering signal.
3. The signal radiated from the aircraft may not be purely linearly polarized but eliptical with let say right-hand oriantion in polarization. Let say your CP antenna is right-hand polarized too, therefore it will allow to mitigate the reflected signals from the ground that are now left-hand oriented.
Hope this help
Waldemar
1. Your CP antenna may have a better Front/Back ratio that allows to better mitigate reflected signal from the ground.
2. Your CP antenna may have a sharper radiation pattern roll-off near and below horizon, again allowing to better mitigate interfering signal.
3. The signal radiated from the aircraft may not be purely linearly polarized but eliptical with let say right-hand oriantion in polarization. Let say your CP antenna is right-hand polarized too, therefore it will allow to mitigate the reflected signals from the ground that are now left-hand oriented.
Hope this help
Waldemar
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