Ok here's a new puzzle for you all. Maybe someone will have an idea I haven't yet thought of.

I have a Garmin GTX 320A transponder. I was installed and working fine with an internal antenna like this:



However it had bad range and since it was installed on one side of the fuselage it only worked when the airport was on the right side

So I replaced it with one of these:



Mounted under the seat on the metal hard point that the series 7 has. The little stub is grounded to the airframe with it's mounting.

Now for the problem. After the upgrade every time the transponder fires I get some scratching audio sound in my headsets. Also the transponder "light" on the front of the unit seems to flicker when it gets activated by the sweep. If I turn it to standby all is quiet. If I unhook the antenna all is quiet. Same antenna cable with both setup's just I shortened about 5 feet and installed a new crimped end. It's RG400 cable. I've tested with a meter and there is no connection from the center of the antenna to ground. There is good grounding from the outside of the bnc to the airframe via the antenna mounting. Thinking it's using the airframe as the ground plane right?

Headsets are Lightspeed Zulu and are powered by the plane's electrical system.

That's all I can think of off hand. Any thoughts about what to test next to resolve?