On 2019-08-17, at 04:14, Jeffrey Williams <jeffwill55@sbcglobal.net> wrote:
> I am installing instruments in my Europa XS and was told all transponders need
a ground plane. My research indicates a 12 inch circle of thin copper or
aluminum
will do, but that the ground plane and the antenna should be outside the
aircraft so things like the engine and other metal stuff doesnt interfere with
the signal. I have a Mode C and a Mode S to deal withand a GPS and Comm antenna
too, but those dont seem to need a ground plane. I am being informed that
these antennas need to be at least 40 inches apart as well. Also, I have been
told that putting the antenna itselfoutside the aircraft and the ground plane
inside is a bad idea that screws up the signal.
Jeff - my Trig manual says:
When a conventional aircraft monopole antenna is used it relies on a ground
plane
for correct behaviour. For ideal performance the ground plane should be very
large compared to the wavelength of the transmission, which is 275mm. In a metal
skinned aircraft this is usually easy to accomplish, but is more difficult
in a composite or fabric skinned aircraft. In these cases a metallic ground
plane should be fabricated and fitted under the antenna.
As the ground plane is made smaller, the actual dimensions of the ground plane
become more critical, and small multiples of the wavelength should be avoided,
as should circles. Rectangles or squares are much less likely to create a
critical
dimension that resonates with the transmissions. The smallest practical
ground plane is a square around 120mm per side; as the size increases the
performance
may actually get worse, but will be better by the time the ground plane
is 700mm on each side. Anything much larger than that size is unlikely to show
significant further improvement.
The thickness of the material used to construct the ground plane is not
critical,
providing it is sufficiently conductive. A variety of proprietary mesh and
grid solutions are available. Heavyweight cooking foil meets the technical
requirements,
but obviously needs to be properly supported.
You can see what I did at:
http://www.rowlandcarson.org.uk/aviation/europa_435/transponder_installation.php
I made the minimum-size ground plane (120mm x 120mm) from 0.4mm copper sheet.
Caveat emptor - this has not yet been used in anger as the whole thing hasnt
flown
yet!
in friendship
Rowland
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