User's Manual

TT21/TT22 Transponder Installation Manual 13 July 2009
00560-00 Issue AF
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Trig Avionics Limited Page 21
5.11 Antenna Installation
The antenna should be installed according to the manufacturer’s instructions.
The following considerations should be taken into account when siting the
Antenna.
 The antenna should be well removed from any projections, the
engine(s) and propeller(s). It should also be well removed from
landing gear doors, access doors or others openings which will break
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
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