Mark, I don't know too much about transponders, but I guess the little red light comes off the back of the receiver amplifier, and that it just responds to RF and does not care too much about the frequency.
Co-ax cable is very, very good at shielding signal, nothing gets out of those suckers, running the cables close together should not be a problem. So I think it is possibly back to your grounding. Your antentas presumably have ground plains. The antenna shield should be connected to the ground plain, but this ground plain must NOT be connected to the aircraft ground, and it must NOT be connected to your transponder antenna ground plain. If it is, then you have effectively created a single turn transformer, signal on the radio cable inner will induce a signal in the circular path made up of the two outer shields. Quite a heavy current will flow around that loop, and that will induce a signal in the transponder cable inner wire. As these two cables are very well matched, a very large part of your radio signal will end up driving into your transponder. I am not certain, but my guess is that inside the radio, the coax outer does not connected to the low voltage ground, or that would cause a similar problem on metal aircraft. Mark Jones wrote: >Yesterday, I checked all wiring to the transponder and transceiver. All hook >ups are correct. I then disconnected the transceiver antenna at the radio >and the symptom went away. Hooked it back up and when I transmitted it lit >up the ident transmit button. Both the antenna coax cables are run together >down the fuselage side to behind the seat where they split up again. I think >I may need to run one antenna down one side of the fuselage and the other >down the opposite side. The way they are, I feel the transponder coax is >picking up the transmit signal of the radio tricking it into lighting up the >ident button. . > > > >
