Transponder question

RogerT

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RogerT
I'm in the process of putting a Searey back together. My hangar partner bought it a few months ago in pieces in a trailer. Hopefully the pic came across.
The transponder was mounted up behind the pilot. Who knows why. I've moved it to the panel. The antenna is mounted above the cockpit on the underside of the wing. That means RG58 about 8 or 9 ft long. I'm thinking about moving it to the front deck in front of the windshield for a shorter run. I don't know that the current location is that good. I know I'd have to make a ground plane behind the fiberglass deck panel. Ideas? Thoughts? Suggestions? I guess changing to RG400 is an option too.

Thanks.

RT
 

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Arent transponder antennae supposed to be on the underside? I've never paid attention to flying boats and txp antenna locations.
 
A couple of pieces of foil tape stuck to the inside of the hull will make a reasonable counterpoise.
 
Another thought is to put it inside the fiberglass nose. Thoughts?
 
Sorry, no good answer to your question.
I just wanted to urge you to finish up the plane and go fly it. I took a ride in it with a friend and it was a hoot and half. We went to the lake, did some nice landings, got a lot of thumbs up from boaters and jet-skiers and then headed home. That Rotax purrs like a kitten too.
And awesome convertible boat plane toy!! :)
 
Another thought is to put it inside the fiberglass nose. Thoughts?
The Van's crowd puts them beneath the fiberglass empennage fairing all the time, with excellent results. I put mine inside the canopy, just aft of the seat support beam, which is what Van's has done with at least one demonstrator (RV-14, I think).
 
Most legacy Mode A/C transponder antennas are on the bottom, because they link with ATC on the ground.

Airline Mode S transponders traditionally have an antenna on the top and the bottom, because Mode S was originally implemented with TCAS II in the early 90s.

The top antenna supports the link with aircraft above.

Every Mode S transponder transmission has the 24 bit ICAO address that's unique to the aircraft registration.

That code is how TCAS sorts out multiple "intruder" aircraft located along the same azmuth.

Sometimes they are called diversity antennas.

The diversity antenna has to do with ensuring top and bottom antennas don't transmit the same signal simultaneously at the same power, because it could create large null fields around the aircraft.
 
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