Transponder Antenna Location

We have been researching transponder installation options for our
ASW-27B (carbon fuselage) and have seen or heard about antenna
installations on the bottom of the fuselage in front and behind the
gear ... on top of the deck ... and with internal or external ground
planes ... and without ground planes. All appear to work, although I
have not seen any hard empirical data or testing results that shows
one type of installation is marginal ... or another superior.

Is a ground plane required on a carbon fuselage ... I understand some
German manufacturers don't install GPs during the factory install.

If a GP is required, should it be outside the fuselage ... or will it
also work as well if it is inside?

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.

Gracias'

KK

Ken Kochanski wrote:
We have been researching transponder installation options for our
ASW-27B (carbon fuselage) and have seen or heard about antenna
installations on the bottom of the fuselage in front and behind the
gear ... on top of the deck ... and with internal or external ground
planes ... and without ground planes. All appear to work, although I
have not seen any hard empirical data or testing results that shows
one type of installation is marginal ... or another superior.

Is a ground plane required on a carbon fuselage ... I understand some
German manufacturers don't install GPs during the factory install.

Start off with a query to the dealer, and if that doesn't work, ask the
factory directly. They install antennas routinely. On the ASH 26 E, they
install it about 2" behind the left gear door, no ground plane, and the
carbon fiber fuselage is apparently satisfactory. I installed one on my
ASH 26 behind the other gear door because it was easier to route the
cable to it. Seems to work fine.

If a GP is required, should it be outside the fuselage ... or will it
also work as well if it is inside?

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 don't know specifically about 27s, but people have put them there on
other gliders, and on top of the instrument cowling, so it appears to
work. I read Knauff put one on top behind the canopy on his Duo Discus.

On my 26, there is plenty of clearance between the antenna and the
trailer, and it won't hit the pavement in a gear up landing. I use the
cheap "ball on a stick" type that bends easily, rather than damage the
fuselage, if something does hit it.

I know a fellow that mounted a dipole inside the nose (which is
fiberglass) of his 26, and says it works well. A clean install, if you
don't mind having the antenna transmitting so close to you.

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Eric Greenwell
Washington State
USA

We have been researching transponder installation options for our
ASW-27B (carbon fuselage) and have seen or heard about antenna
installations on the bottom of the fuselage in front and behind the
gear ... on top of the deck ... and with internal or external ground
planes ... and without ground planes. All appear to work, although I
have not seen any hard empirical data or testing results that shows
one type of installation is marginal ... or another superior.

How about in the tail -- not carbon -- right next to the radio
antenna? It's certainly easier before they glue the fuse together, but
one might be able to get the transponder antenna in there after the
fact too.

John Cochrane

On 8 Jan 2004 06:47:02 -0800, (John
Cochrane) wrote:

We have been researching transponder installation options for our
ASW-27B (carbon fuselage) and have seen or heard about antenna
installations on the bottom of the fuselage in front and behind the
gear ... on top of the deck ... and with internal or external ground
planes ... and without ground planes. All appear to work, although I
have not seen any hard empirical data or testing results that shows
one type of installation is marginal ... or another superior.

How about in the tail -- not carbon -- right next to the radio
antenna? It's certainly easier before they glue the fuse together, but
one might be able to get the transponder antenna in there after the
fact too.

John Cochrane


Probably too much loss in the extra length of co-ax in volved.

Mike Borgelt

"Mike Borgelt" wrote in message
. ..

On 8 Jan 2004 06:47:02 -0800, (John





Probably too much loss in the extra length of co-ax in volved.



Mike Borgelt



There are several reasons, including Mike and Eric's, that the underside of
fuselage near the gear bay doors is a good spot.

The high frequency means that coax loss is a factor for longer runs. IIRC
Becker recommends less than 15' and if greater distance is needed, then low
loss coax should be used.

The high frequency means the RF radiation behaves much like light, in that
metalic or conductive objects near and within the antenna radiation pattern
will shadow or block the signal. This is why the rudder would not be such a
good location, as looking forward would be blocked by the carbon vertical
fin. And is also why the belly is often preferred so the antenna has an
unobstructed view of ground radar sites.

As Eric mentioned, some pilots prefer not to have the antenna near them due
to concerns about the long term effects of exposure to high freq. radiation.
Cockpit is not so good - - the choices are limited. The belly location, just
aft of the gear doors, works well in the carbon fiber fuselage of the
ASH26E, and with apparantly no additonal ground plane needed. I'm sure it
wouldn't hurt to add one, and I'd do it with aluminum or copper foil plate
if I was doing a new install.
--
bumper ZZ (reverse all after @)
"Dare to be different . . . circle in sink."

Ken,

Check with the Baron. I think he gave a lot of thought to all
potential options before settling on the underside install.

"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.

Jim Kellett, Resident Curmudgeon

"We cannot guarantee success, but we can do something better. We can deserve
it." . . . John Adams

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