Ampcamp said
Prop mod or wavelength is interesting but does not explain, as you say, why it just appeared out of the blue or intermittant.
It is engine related 2200 rpm (was 1900 but went up with mag rebuild if remember correctly)
It was made worse with high gain antenna so it is going down the stick
It is intermittant.
It is only off the nose (noise source between antennas and transmitting station).
Non airframe grounded handheld receiver unaffected.
Lots of clues there.
One othe big clue is that it happens at -every- distance to base station so almost has to be some modulation effect as opposed to an independant (fixed level) rfi source.
Both the intemitant and RPM sensitivity can be explained by prop vibration, which if I understood a previouse post correctly, is basically a "in the direction of spin" vibration (torque load/unload) that occurs above a certain speed. This onset speed was raised slightly when the ignition system was improved.
My theory is that this vibration is intermitantly grounding/ungrounding one (or both) of the blades, or less likely some mechanically related structure.
Only when ungrounded does the prop blade create a reflection/multipath/interfernce situation.
The signal does not have to go directly "through" the prop for this to happen, think tall buildings and ghosty (analog) TV reception. It does have to be mostly in line, hence the 20 degree range of the effect.
The handheld not seeing the noise can be explained by the effect being larger at "magic" ( ~ 1/4 wavelength multiples) distances from the prop.
Worse with new antenna could be that a slight difference in (electrical/effective) distance to prop.
One interesting test would be to take the handeld outside and slowly move toward the plane when noise is happening.
Not sure one could (safely!!!) get close enough to get the effect or not though. Results of handheld connected to antenna would also be usefull.