TT21/TT22 Transponder Installation Manual 22 August 2017
00560-00 Issue AP
______________________
Trig Avionics Limited 23
the ground plane for the antenna.
The antenna should be mounted on the bottom surface of the aircraft
and in a vertical position when the aircraft is in level flight.
Avoid mounting the antenna within 3 feet of the ADF sense antenna
or any COMM antenna and 6 feet from the transponder to the DME
antenna.
Where practical, plan the antenna location to keep the cable lengths as
short as possible and avoid sharp bends in the cable to minimise the
VSWR.
Electrical connection to the antenna should be protected to avoid loss of
efficiency as a result of the presence of liquids or moisture. All antenna feeders
shall be installed in such a way that a minimum of RF energy is radiated inside
the aircraft.
5.11.1 Antenna Ground Plane
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
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.
The ground plane should be as large as you can sensibly make it. Because it is
a function of the wavelength of the transmission, the smallest practical ground
plane for a transponder is a square around 120mm per side; as the size
increases the performance improves until the ground plane is around 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.
To Next Page
Loading...
