transponder interfere audio reception

[Paul Michel Fenouil]

Good morning flyers,
Have you experienced this problem:
On my single Comanche 180 I installed a new TRIG Transponder TT31 and an intercom PM1000 II.
When the TRIG is on, I hear little crackles in the headset. These crackles appears each time the TRIG sent a signal (i.e. when blinks the antenna icon on the screen). When I switch off COM/NAV and PM1000 intercom, I always hear the crackling in headphones.
Of course crackling stops when I switch off the TRIG.
These cracks are not very loud but very annoying when I use the intercom. Indeed the intercom interprets these crackles as an ATC reception and it automatically reduces any conversation between pilot and copilot.
I think this problem is due to the radioelectric field sent by the TRIG, which cross all the wiring.
I suppose I should have to shield most part of the wires (?).

Thank you for your attention. Have nice flights.

Paul

[Francois Marquis]

Paul,

Pretty hard to shoot a precise diagnostic on something that I have not seen, but this sounds like a ground loop coupling to me.

The coupling you hear can be RF induced, but it can also be DC induced. The former is done through capacitive/inductive coupling with the transponder RF signal output via the antenna, the cable to it, etc. The later is done through current spikes that goes through the airframe as the transponder pulses its energy demand.

The simplest and very first thing I would look for is whether the microphone/headphone jacks are insulated from ground. Secondly, I would make absolutely certain that all audio and your com equipments are grounded at (or very near) the same location. In fact, you must make absolutely sure the audio side has no ground loops. Audio shields must be connected only on one end (equipment side is preferred), etc.

Also, check that the transponder is not grounding itself through its antenna coax.

That being cleared away, and still hearing the transponder replies, then... time to chase RF couplings.

Good luck!

Francois Marquis

[Paul Michel Fenouil]

Thank you very much Francois, I continue my investigations and will provide the solution (if I can find)....

Paul