Locating Transponder Antenna on top of the fuselage instead of under it.

On Friday, December 27, 2019 at 6:05:20 PM UTC-8, George Haeh wrote:
I had a standard length PowerFLARM and ADS-B antenna through a hole in the glareshield of my ASW-27. After upgrading to dual long PF antennas that I placed on opposite sides of the canopy with Velcro, I placed an internal L2 transponder antenna in the glareshield hole reaching to just below the canopy favoring visibility to TCAS above and satellites. Being a long way from any big airports, I haven't verified with ATC, but suspect I'm visible to ground installations unless close in above or pointed directly away when my body and carbon fiber will be in the way.

Uh this seems pretty bad advice. And there are great reasons why glider manufacturers do not have anything like this as a recommended installation.

You are bathing all the other avionics with high-energy RF pulses from the transponder. An external quarter wave antenna with a ground plane helps get that energy away from the other avionics and pilot. A L2 type wave antenna for mounted inside a non-conductive fuselage should normally be mounted back in the fuselage away from the pilot and avionics. And things like PowerFLARM 1090ES In are designed to work by seeing somewhat reduced power leakage of your transponder signal, not being blasted by the direct high-energy signal.

You may be operating the FLARM antennas closer to a transponder antenna than a good idea (~30cm minimum recommended by FLARM... but that's when both externally mounted, this may be worse mounted inside). I'm not even sure something as close as 30cm is a good idea externally. Does ownship PowerFLARM transponder identification work OK? Or do you have to disable PCAS etc.? FLARM range checks all look good? How do you know if the Transponder is working OK until you check/ask?. You also might be flying in a remote enough area that you get few transponder interrogations, so things seem to work OK. A close encounter with a TCAS equipped aircraft might result in a chirp of ~1kHz transponder interrogations. Even if things are are working OK now who knows what that will do to your electronics, PowerFLARM, traffic display, etc. when you might need to most rely on it.

You likely have lots of conductive objects within the antenna RF near field near field (including the Schleicher panel support brass rod, all the usual wiring and conductive instrument cases). That can significantly affect the radiation pattern in ways unlikely to be obvious.

And you are possibly exposing yourself to excessive RF radiation.

In the USA I would expoect that an A&P doing a transponder RF check would not sign off on such an install.