Transponder Antenna Location

Jim Kellett wrote:

"Ken Kochanski" wrote in message
om...

Will the transponder system work satisfactorily with the antenna on
the top of the fuselage? ... understanding that the radiation pattern
would not be ideal for the ground based interrogation systems.


I've got two mounted topside facing up - one on the forward part of the
instrument cowling in an Open CIrrus, and the other inside the fuselage on a
fabricated GP strapped to some structural members just over the cg in an
ASW-20C. Both work just fine. Can be seen by ground based radar, although
it may not be "ideal". OTOH, my primary interest in xponders is to light up
TCAS units which are airborne airliners anyhow, and they're not on the
ground gr.

Are airliners more likely to be coming at you from below or above? I
decided it was a toss-up, so I put my antenna on the bottom to improve
the return for ground radar (that's on an all carbon fiber glider - on
your fiberglass gliders, it might not make a significant difference).
This will aid ATC in keeping non-TCAS equipped aircraft away from me, I
hope.


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Eric Greenwell
Washington State
USA

"Eric Greenwell" wrote in message

Are airliners more likely to be coming at you from below or above? I
decided it was a toss-up, so I put my antenna on the bottom to improve
the return for ground radar (that's on an all carbon fiber glider - on
your fiberglass gliders, it might not make a significant difference).
This will aid ATC in keeping non-TCAS equipped aircraft away from me, I
hope.


Modern airliners climb out much more steeply than they descend, so you are
more likely to see an airliner coming at you from above. (unless you are
within class B/C airspace)

mm wrote:
"Eric Greenwell" wrote in message

Are airliners more likely to be coming at you from below or above? I
decided it was a toss-up, so I put my antenna on the bottom to improve
the return for ground radar (that's on an all carbon fiber glider - on
your fiberglass gliders, it might not make a significant difference).
This will aid ATC in keeping non-TCAS equipped aircraft away from me, I
hope.

Modern airliners climb out much more steeply than they descend, so you are
more likely to see an airliner coming at you from above. (unless you are
within class B/C airspace)

Actually thinking about this leads me to change my mind: unless an
airport is accumulating airliners, there are just as many leaving it as
arriving at it, so, on average, the glider will encounter just as many
above as below. Based on mm's observation, the glider will encounter
"down-going" airliners further from the airport than the "up-going".

Maybe a pilot should do a survey of other pilots in the area he will be
flying in before picking an antenna location (just joking). I suspect
antenna position is irrelevant for alerting TCAS equipped aircraft, as
they will very close (a few miles) when it is needed, so a strong signal
isn't needed. Metal airplanes put them on the bottom, and I haven't
heard of any concerns about putting them there.

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Eric Greenwell
Washington State
USA