The avionics shop should be able to test COM receive sensitivity through the
plane's antenna with the unit installed in the plane, very simply using a
suitable signal generator (that every shop should have.) The signal
generator is set up to transmit a very low level signal, modulated with a
tone, through a whip antenna located some distance (maybe 100 feet or so)
from the plane, in line-of-sight to its COM antenna. Signal level is
adjusted to the point that the tone is just heard through the COM receiver.
Calculating free-space loss for the distance (a standard calculation), one
can determine within a few dB the signal strength incident to the COM
antenna. This is not as precise as doing it on the bench with the signal
generator connected directly to the receiver, but it is sufficient to find a
gross problem and to troubleshoot it. This method CAN accurately measure
the difference in sensitivity between a plane's two COM receivers.

If your avionics guy doesn't understand this test you need a new avionics
guy.


-Elliott Drucker