Not RFI
A: The problem occurs over a wide range of recevied signal strengths. This rules out reduction of signal strength by shadowing.
B: Everything but the battery and one radio have been disabled and noise was still there.
C: No hint of it on the handheld.
D: Most telling: The noise is only there when receiving, not in background with squelch disabled.
My theory:
The prop is modulating the Rx signal. This would account for the extreme directional sensititivity, only happpens whe prop is between antenna and source.
I have no idea of the normal grounding(/bonding) path between prop blade
and airplane but suspect as the prop vibration sets in at ~2100 rpm one or more of the blades are intermitantly disconnected.
Given prop length and radio wavelength this could cause a a reflection/phase shift that would modulate the signal.
The impact would also be critically dependant on distance from prop to antenna, which would explain lack of noise on handheld.
One test, not conclusive, would be to measure resistance from each blade to chassis while applying force in every direction to each blade of prop (engine secured!!).
Few more things to try: Is there any hint of the noise when the base station is directly behind you?
Turn to the "edge" of the noise (~15degrees?) and see if problem goes away with airplane pitch. (Another way of moving prop in/out of line of sight of antenna.).
(Added)
Note the theory behind this is that the rapdidly fluctuating Rx signal strength is causing the receivers AGC (automatic gain control) to wig out resulting in incorrect levels at the detector.
More on why not RFI:
Simple shadowing would slowly change the Rx level, much less than flying from .5 miles to 20 miles.
If the inteference was external generated RFI it would be at a (relatively) constant level so a few Db change in Rx level would only matter at some fairly critical distance from base station, farther away it would be pressent all the time, close in would not be there at all.
Last edited by MurphyWasRight; 22nd Feb 2011 at 19:22.
Reason: Typos, clarity. added bit more explanation