I wish I'd spotted this thread from the get-go. Fascinating.
Is it still an ongoing mystery, or is the fitting of the lower aerial considered a final fix/workaround?
I used to commute from the east coast to Luton, often at night in a 4 seat metal aircraft. It was fairly new, but the radio used to get worse as I passed near to Stansted, then improve with distance. I did exactly what I'd want to do with this problem, and measure the AGC voltages. However, in those days, I was able to open the box without breaking hidden seals.
In this case, after a few pages I have to not spend any more time today, but after so much testing and swapping - I was getting a gut feeling (after a C&G and being a telecom tekki) in my yoof, that something exotic was happening. Thinking laterally, I wondered if the
basic design of the comms boxes radiated a spurious signal, which was being reflected by by a prop disc that only qualified as a dish at a certain RPM. I'd want to know what happened to that AGC line as the signal was swamped/diminished.
I know it was easy in my day. AGC feedback was a simple voltage that could be measured with an AVO. Now I suppose it's a digital 'statement.'
