Transponder Antennas...again

[Kilo Charlie]

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

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.

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

Casey Lenox
KC
Phoenix

[Eric Greenwell]

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

[Kilo Charlie]

Thanks for the response Eric and to those that have written my privately.
It is good info. One pilot at Warner Springs has his antenna on the top and
has said that ATC and the airlines have painted him without problems.

It has been my experience that by flying outside the Mode C veil that we
generally are encountering departing aircraft below them since by 30 miles
out they have climbed to a fairly high altitude. Certainly not always true
but usually the case.

The other part of my logic is this and may be more specific to our local
environment.....As we leave the area we are circling to gain altitude to get
over the mountains to the north so with eyes outside and circling often see
the departing flights from Sky Harbor and radio the other folks about them.

The main worry for me are those long flat final glides from north to south
starting at 17,999 feet and 100 miles out....I can't even see the guys that
I know are half a mile in front of me so there is no way some 37 crew on
approach is going to see me on a final glide. I don't wish to end up with a
37 enema. But on a more serious note if it did happen you can bet that it
would change the face of glider flying overnight esp if it took down bunch
of passengers along with it. This is why I have been less interested in
requiring the ELT systems and more interested in figuring out a way to equip
all of us with transponders at cheaper than now available prices. Both
would be a good thing but the latter has the potential to impact far more
folks.....excuse the pun.

Thanks again!
Casey

[COLIN LAMB]

Since the aircraft that hits you must be close, the placement of the antenna
for TCAS may not make a lot of difference. Signal level decreases with the
square of the distance, so even attenuated by "poor" antenna placement, it
will provide a sufficient signal for a close range aircraft.

I have not played with carbon fiber as a reflecting plane, but since the
fuselage is round, I doubt whether the signal level on the opposite side of
the fuselage (the front to back ratio) would be more than 15-20 db. This
means that the signal level would be quite adequate no matter where the
attacking aircraft is located.

Assuming the antenna was placed on the bottom of the carbon fiber and the
blind spot was on the top, it would be a very narrow "cone" directly above
the aircraft - if carbon fiber is totally rf opaque. That is not likely
because the round fuselage is not a flat plane. But, even if there is a
null cone, it would not matter, because it would only be 15 - 20 db and the
attacking aircraft would have to be attacking at a high rate (going straight
down) to be a problem.

Since high speed aircraft must necessarily arrive from a point somewhere
other than directly overhead, any shielding would be irrelevant.

Sorry for the long explanation, I was just thinking out loud. My conclusion
is that for the purpose intended, it does not matter where you place it.

I have a mode C transponder and a "traffic watch" receiver.

Colin N12HS


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[Eric Greenwell]

Kilo Charlie wrote:

> The main worry for me are those long flat final glides from north to south
> starting at 17,999 feet and 100 miles out....I can't even see the guys that
> I know are half a mile in front of me so there is no way some 37 crew on
> approach is going to see me on a final glide. I don't wish to end up with a
> 37 enema. But on a more serious note if it did happen you can bet that it
> would change the face of glider flying overnight esp if it took down bunch
> of passengers along with it. This is why I have been less interested in
> requiring the ELT systems and more interested in figuring out a way to equip
> all of us with transponders at cheaper than now available prices.

I don't see any hope for cheaper transponders until the dollar gets a
lot stronger against the Euro (Becker, Filser) or the Australian dollar
(Microair), or China decides airplane owners have more bucks than
Wal-Mart or even Harbor Freight.

Another approach to the problem are the "TPAS" units that detect
transponders, such as sold by Proxalert, Monroy, and Surecheck. It's a
different solution, cheaper, but relies on the glider pilot to make the
evasive maneuvers. Four of us are presenting these options and others at
the SSA convention, and we plan to make the presentations available on
the web somewhere, for those that don't get to the convention.


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Eric Greenwell
Washington State
USA

[[email protected]]

If you want a reliable system there is a new system on the market by
the inventor's of TPAS called a TS Micro.
http://www.surecheckaviation.com/avionics/micro.html
it is under $300 and does the trick in my opinion.