Transponder interference

[Pilotlight11]

Lets start with the antenna. It is not the antenna. How do I know you ask. I have run tests. And so has the avionics shop where I had the new transponder certified. 1) We disconnected the monopole antenna and the popping stops. 2) we connected a different monopole antenna and it came back. 3) I bought a dipole antenna and connected it to a 25 foot cable and walked away from the plane - popping stops. 4) walk back to the plane with the antenna and the popping starts again. 5) same results with a monopole on a long cable. Sit in the plane where the monopole is about 2 feet above your head and block the path with your hand - popping attenuates. I have a spare RG400 rigged in my plane, same results - so while it is possible they are both internally grounded - not likely. As I said, the only thing that makes a difference is distance between the antenna and the headset. The dipole, mounted back inside the hull near the tail has made a difference, but not eliminated it since I can not get 25 feet away. The previous transponder was a Garmin GTX330, they are both 250 watt transponders. The old transponder was stack mounted. The Dynon is a remotely mounted 261 about 12 inches in front of and to the side of the old location. NO firewall involved. This is a pylon mounted pusher prop with the engine behind (Searey). The transponder sits next to the comm radio, but so did the stack version on top of a Garmin SL40. All the harnesses were made by Aircraft Spruce for the specific purpose of connecting all the Dynon components were a harness form Dynon was not available. The ELT is a brand new ACK E-04. In addition to the ELT Led lighting up, when I key the mic the RF interference overrides a radio tuned to a different frequency with static. That was a hand held 20 feet away - engine off, no non-avionics active. The avionics shop looked at the harness to the headsets and confirmed the grounding was correct. And just a note to RayinGA: the link you posted goes directly to the antenna I used as the replacement for the monopole. I don't know what else I can add. I'm going to the plane tomorrow and will go ahead and check that the cable is not internally shorted. However, just to get it all on the record, I also conducted all these tests with RG174 with identical results.

 

[jakej]

Bob, as much as I hate the use of ferrites why don't you just buy 2 X ferrites (clamp on type) for the coax & add 1 to each antenna cable as close to the antenna as you can get & report the results ? IMO it's worth a try & a cheap easy solution if it works.
Some years ago a kit maker found that the same Elt type as yours was activating when a customer started Transmitting. They suggested we replace the ELT with a different brand guess what, the $4 Ferrite fixed the problem.

The symptoms you are having can be more prevalent in composite, wood & rag/tube type aircraft & a whole different set of factors come into play that you don't see in aluminium airframes, just saying.......

Please bear with us in trying to assist you, I believe there always is a solution, sometimes it can take a while to resolve.

 

[Pilotlight11]

jakej: thanks you so much for the suggestion. I will indeed try it. My "testiness" is really triggered when it is apparent that people leaving comments have either not read what is right in front of them or believe something has to be said twice before it is true. I truly appreciate new directions to try. The issues I've been having have caused me to "deconstruct" the front end of my plane (about 75 rivets) to be able to satisfy all the "are you sure about this" checks and advice I've been getting.

 

[jakej]

Bob, you can pm me if you wish. It could save a lot of to/froing but I ask that when the 'problem' is resolved that you publish it here so that others do not have to go throughn the same frustration.