Well, it seems like I have the RF issue cured - maybe. It’s testing good in the hangar. I’ll be more confident when I fly tomorrow if the problem doesn’t return. I tried a different PTT switch - no help, new 7.5a fuse - no help, I put a new BNC connector on the radio end of the antenna cable - no help, tried different grounding locations for the PTT switch - no help, I re-routed power wires away from signal wires where that existed in a couple places - no help, I put a ferrite clip on the serial wire running from the EIS to the EFIS- no help, then I put a ferrite clip on the RG400 COM antenna cable, and like the parting of the Red Sea, it was gone! I moved the ferrite clip to different locations along the cable, and it didn’t seem to make a difference, it still worked. I removed the ferrite, and the condition returned and certain indications went wild. Put it back on, and it’s gone.
So this ferrite (whatever that is) is correcting something that I’ve never dealt with before, and it’s correcting something that is wrong with my electrical system as installed. The original avionics on this airplane, that I didn’t build, worked fine with this same antenna, but it was a bare bones steam panel, except for the early generation GRT EIS system, and that worked fine. Then I installed digital EFIS based avionics and things went wild if I dared to transmit on my only (new) radio. There is a lot of very low power signal data moving around through my new, very organized (at least it was) wiring bundles, and if there is any RF looming around looking for a low energy host it can invade - like a COVID-19 hairball - it’s going to grab it. This is the sort of nightmare I’ve been having lately.
My take at this point is, my antenna, with it’s unprotected center core connection is allowing this RF electronic pathogen to cause a pandemic in my airplane. It seems like a new more modern, BNC connected com antenna is my vaccine cure.
So what do you experts say? As a senior citizen with a rapidly shrinking retirement nest egg, I don’t want to waste money on an antenna of that does no good - although a DP antenna ($125) is a pretty cheap fix in airplane terms, if it is a fix....... and again, the proof will be a flight test tomorrow, not a no noise check in my hangar with the engine not running.
Thanks for all the help. It has been a mostly fun learning experience. And I might not be done yet, but hope springs eternal.