Kilo Charlie wrote:

Has anyone had experience with installing a transponder antenna on the top
of their glider?

Several people - Tom Knauff did it that way on his Duo Discus - I can't
remember the others. Also, a few have installed them on top of the
instrument panel, not quite the same thing, but similar.


With as little level flight that we do and with most threats that have TCAS
coming from above it would seem logical that having it on top would not be a
detriment to the functionality of the instrument.

I've wondered about this, and decided just as many must come from the
bottom. They have to take off from somewhere and climb up to be above
you! I suppose if you only fly where airliners are descending, putting
it on top might possibly be an advantage. I doubt that it's really
helpful, since the big majority of transponder antennas on GA aircraft
are mounted on the bottom of the airplane.

The main (only?) reason given for the bottom mount is a better signal to
ATC, which uses ground based radars. I'm guessing, but maybe your
antenna location isn't important to a TCAS equipped airplane, since it
has to be close (within a few miles of you) before it needs to receive
your signal, and this close distance could easily make up for a poor
location.


We have a carbon fiber fuselage (Nimbus 3D) which would prevent penetration
to any receivers directly below the aircraft.

THat may be acceptable, since those aircraft aren't the ones that will
hit you: it's ones at angles from slightly above (and descending) or
below (and ascending) you that could potentially hit you, and those
shallow angles ought to put your antenna in sight.

I've not seen any studies about this, just the usual recommendation to
put the antenna on the bottom.


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Eric Greenwell
Washington State
USA