Some great information, thanks! My EMI test plan will be much the better for it! It's nice to get it right, but even nicer to know why you're getting it right! BTW, this started out as a very simple EMI test, for cabin installed survey equipment. It's kind of snowballed from there. It is not proposed as a highly qualified examination. This would be done, if I observed any EMI during the basic test. My test is only eyes and ears, no special equipment.
This will be my 13th time doing this test on the aircraft. Interestingly, last September I was witnessing this test in flight, while riding jump seat. I could hear a faint transponder signal over the DC noise cancelling headset I was wearing. When I unplugged the headset, the noise continued! I sat, looking at the two plugs in my hand, and hearing the signal. When I moved away, it stopped. I can only presume that some form of induction was affecting the headset, while in close proximity of the avionics compartment.