Transponder Antenna Location

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."