Thanks HF for the additional information. Some other contributions have been very useful, one or two not quite so IMHO (that's all I want to say on that
)
One says that the problem must be shadowing of the antenna by the fuselage in front which would cause a weaker signal and hence the directional nature of the problem.
The other says that the entire aircraft acts as an antenna and when the aircraft is aligned with the station you get the strongest signal and hence the directional nature of the problem.
Does anyone actually know which one of them is correct?
There is a small amount of truth in each of them but neither would get a satisfactory pass in the real world with my old radio theory tutor.
The first statement might be a consideration if we were dealing in micro-watts over a considerable distance. Unless we have a seriously desensitised reciever (two on them in fact) I suggest that the shielding effect of the fuselage would be minimal given the transmit power of the radio equipment used in the tower ground installations which I would expect to be 'significant'. This theory is further eroded by the fact (if I understand correctly) that even with the aircraft on the ground in close proximity to the tower such that line of sight conditions between the transmitting and receiving antennas, the problem is still evident.
The second statement has some credibility in that if the shield of coax cable to the antenna is bonded to the skin of the aircraft (metal is assumed here) and thus to the airframe (grounded to use the more common terminology)
at the antenna feedpoint the entire aircraft becomes the ground (some would say counterpoise) against which the whip of the antenna either radiates or receives a signal. This is the sort of system I would expect in this aircraft (the turtle deck
is metal (aluminium) isn't it ?????) .
There is usually some directivity in that the radiation patten of the antenna would normally be the strongest in the direction of the largest area of ground plane. In this instance I think the directivity effect would probably favour signals in front of the aircraft if at all. Others might argue for signals from the rear of the aircraft being stronger in both transmit and receive. A beer or several are needed here to arrive at the correct answer by concensus. But I would be astonished beyond measure if either possibility were serious enough to produce the effect we are seeing here.
The prop modulation possibility as presented and argued by others earlier today is very interesting and seemingly credible. However I am troubled by the intermittent nature of the problem, and that the onset of the problem has been lifted by quite a few hundred RPM as changes were made to engine equipment which improved the performance. Prop modulation is possible perhaps, but surely there would have to be other contributing factors.
Finally, one especially puzzling aspect is that, as I understand it, the noise is stronger on the radio which has the newer, higher gain antenna. That again really suggests some sort of RF interference which is being received by that antenna at a higher received sig strength than the other one. (No it doesn't help a lot - at least I don't think it does - but it's just another question without logical answer)
What is the answer then? I don't know. I'm baffled like almost everyone else but somehow, someway, the cause will be identified and the solution found.
FOR
(all written in hast - with apologies to grammatical and/or technical purists - it's been a long hard day, this might be better attempted after a good sleep)
