On Friday, December 27, 2019 at 11:34:32 AM UTC-8, wrote:
Hi all,
Planning to install a Transponder Antenna on my Ventus CM.
Schempp Hirth indicates the possibilty of installing on top of the fuselage , behind the engine compartment doors.
Regarding being seen by ATC and Airlines, would that location perform better/worse/same as the standard location on side of fuselage - above gear doors ?
Dan

The preferred location should be below. That is more visible to ATC SSR, and indirectly providing ADS-R/TIS-B services to other aircraft. Close by aircraft with TCAS (or ADS-B In if you have ADS-B Out) that are a collision threat to you usually have no problem seeing your glider transponder. And arguably below is more a blind spot for gliders so maybe putting more signal there even if everything else did not matter makes sense (and it should also provide good coverage behind).

"on the side above the gear doors??? I've never seen a transponder antenna installed "on the side". Many are behind the gear doors and offset slightly from the centerline. The antenna needs to be mostly vertical since transponders, SSR, and TCAS use vertical polarization. Yes aircraft bank, but you need to start with the antenna somewhat vertical, slightly off centerline is fine.

Pay attention to the proper ground plane installation with any of the standard external transponder 1/4 wave antenna. That is very important.

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Interesting things may happen in Canada and other countries with Aerion satellite based 1090ES reception is being used. The satellites (much further away than threat aircraft) need good top antennas. That *might* lead to aircraft (including gliders?) using diversity (top and bottom) transponder antennas at some future time... not supported by the transponders we use today (GA works so well with just a bottom antenna it's not been needed until...)..