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

On Saturday, December 28, 2019 at 12:10:29 PM UTC-8, Darryl Ramm wrote:
On Saturday, December 28, 2019 at 10:44:46 AM UTC-8, George Haeh wrote:
"You are bathing all the other avionics with high-energy RF pulses from the transponder... 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."

The avionics, except for the Oudie, are all in metal enclosures.

"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?"

The PF Flarm and ADS-B antennas are mounted on the sides of the canopy quite some distance from the xpdr antenna which itself is about a foot forward of the instrument panel. The PF range is better than the four other gliders I've checked. No PF problem with own glider xpdr.


"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."

Will be doing another transponder check in the spring.

"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."

Most of the unshielded wiring is in the bottom of the instrument enclosure directly below the L2 antenna where the radiated energy may be lower. But so far, everything seems to be working.

Yes with wires running into them, and power cables, and metal construction is not perfectly RF tight, all of which will leak some RF power into the enclosures. And none of which are designed to be close to high power transponder signals.

Well, they BETTER be! Any aircraft can be expected to have a transponder with an antenna at a unknown location, so it must be assumed that the antenna is close to the particular avionics. The transponder response is only 25 microsec long, so you are being exposed to an RF pulse that averages 1 to 2 mW, depending upon the code and altitude being transmitted (1202 has only 3 ONEs out of a possible 12, and each ONE bit has power on for one third of the bit time). By way of comparison, a cell phone transmits at 600 mW, and as much as 3 W - and you are holding that thing right next to your head and think nothing of it.

Tom