Eureka!!! (maybe)
Once upon a time, when Design Engineers had permanent jobs and worked in Research Laboratories, our Research Director used to say we had all our best ideas when shaving. And he was right, it was after all the hard work of the previous day and a good night's sleep that a new idea would surface and we would bound into work shouting 'Eureka' and applying for patents.
Nowadays I take advantage of retirement to let my beard grow, so I don't have that shaving experience, but on Sunday mornings I soak in a piping hot bath and that has about the same effect. Today, for example, I was having that long hot soak and dreaming of exquisitely beautiful and sublimely aerobatic Fuji 200s painting elegant patterns in the sky.
One of the Fujis had an ADF aerial from cabin to vert stab and had its coms antennas fuselage topside beneath it. And I saw the radio waves approach that area and vanish.
At this point I have to explain that electromagnetic radiation consists of two perpendicular components which must travel together. One is electric and in this case is vertical, and the other is magnetic and is horizontal.
When the field approaches the tail of the aircraft, the loop consisting of metal fuselage and metal ADF aerial constitutes a shorted turn and prevents any magnetic field from existing in a direction perpendicular to the loop. This kills the loop as a path for the signal so there is no electric field for the coms aerial to pick up. The effect is only strong enough to kill the signal when it somes from directly ahead or astern.
The shorted turn does not have to be a continuous dc path, it only has to be a path at vhf frequencies, so a very small amount of capacitive coupling at each end will be sufficient to make it a short.
My suggestion is to fly (or taxi) with ADF aerial physically removed and check the coms using the remaining topside coms antenna. It shouldn't cost a lot to try and hf can either mail me a pickled wallaby or else have me consigned to the funny farm for demented Engineers.
When our Director told us his the ideas-when-shaving theory, we asked to be paid a shaving allowance, whereupon he went very quiet and then changed the subject.
Last edited by mike-wsm; 27th February 2011 at 15:28.
Reason: typos and clarity