"What if this is caused by presence or absence of front seat pax, or caused by another factor in the flight profile that is concomittant to the RPM and not actually the RPM at all? AOA or trim at that weight/speed condition?" It appears that all the logical steps have been taken (for a radio problem) already. Good luck if this starts a new chain of thought.
Thanks for looking in and thinking outside of the box for a while. We need that, we certainly do. One of the problem here - if I read HF's notes correctly is that the problem - when chooses to be evident - can be reproduced on the ground (post #92 applies).
The positive outcome from relocating one antenna to the bottom of the future is a ray of hope, but the over-riding frustrations remain.
So what do we know ?
1) The problem suddenly appeared without any apparent reason.
2) Even so, it is intermittent in nature without any identified reason(s) thus far for that factor.
3) Both radios are affected even though they are (I think)of different types.
4) Substitution of one of the radios with a known "good" one still produced the problem as previously.
5) Changing the loads on the DC circuit /bus seems to have no effect on the behaviour of the problem.
6) Improving the engine's running (sorry - I'm sure there's a better way of saying that) by changing spark plugs etc changed the onset of the problem to a point some 300 erpm higher than previously.
7) A different alternator (and presumably) regulator was tried - no change.
8) The problem is directional and appears within the predicted rev range over a wide range of engine operating parameters - but always at the known RPM.
9) Problem cannot be reproduced on a handheld in the cockpit and independent of the aircraft power or antenna circuits.
10) Problem is evident over a very wide range of distances (and therefore signal strengths) - could hardly be a RF signal then could it ...?
I'm still without explanation or answer. It the defies every understanding of radio behaviour that I have ever read about or encountered. The only consolotion is that I'm not alone.
So is the radio system essentially OK - something else altogether is at fault?
Can we establish that this has been observed with a sample of other transmission sources - ie just not the tower or base - other aircraft?
I know nothing about ADF operation. Is there something which "cuts in" within the ADF system within the observed heading range? (but why only at that RPM range also
??)
This one really defies logical analysis. I hope I'm still around when it's finally solved once and for all. It's a real worry.
FOR