Release 1.7
© XCOM Avionics 2005
www.xcom760.com Page
17
The most suitable location for the antenna is generally on top of the aircraft in a
location which can provide the best unobstructed line-of-sight view of the
surrounding areas. The antenna should be positioned so as to provide the
maximum clearance from obstacles and other antennas on the aircraft to limit
reflection and cross talk interference. In particular, position the antenna as far as
reasonably possible from high power transmitter aerials, such as that of the
transponder.
To provide the best performance from your antenna, a suitable ground must be
established. Without an adequate ground, transmission patterns may be difficult to
predict and performance will be significantly reduced. On metal aircraft with
certain antennas it is necessary to remove the paint from the inner surface to
provide a good metal-to-metal contact with the aircraft’s skin. On composite
aircraft it is necessary to install a ground plane unless already installed by the
aircraft manufacturer or unless using a dipole antenna.
Where the antenna requires a ground plane, the ground plane should be at least
20” (500mm) diameter or larger. Best performance will be gained with a large
ground plane. In composite aircraft, it is generally possible to epoxy or RTV
(silicone) a thin piece of aluminum sheet inside the structure to provide an
adequate ground plane.
Ground plane independent antennas are suitable for use with the XCOM. However,
in our experience, a fully grounded antenna with a large ground plane provides
superior performance.
Antenna construction is also a factor in determining the best installation. Most VHF
antennas used on sport aircraft are ¼ wave whip type antennas made from either
metal or fiberglass. The cheaper wire whip antennas are prone to deformation and
bending in flight and this impacts the tuned characteristics, reducing performance
of the radio installation. The enclosed fiberglass antennas generally do not suffer
this shortfall.
As a result, many of the more expensive antennas provide far greater receive and
transmission ranges due to their broad bandwidth characteristics.
Most antennas sold today are pre-tuned to the middle of the aircraft frequency
communications band. Although they provide adequate performance, these
antennas are generally not optimum for every installation as the characteristics
mentioned above vary in every aircraft. We therefore recommended that a SWR
test be performed at 118.000 MHz. and 137.00 MHz. and the antenna tuned to try
and get the SWR reading as low as possible at both of these frequencies. An ideal
VSWR reading should be below 1.5:1 but any value up to 3:1 is still considered
acceptable.
Finally, try to keep the antenna cable as short as possible and keep it clear of other
interference producing wiring such as electrical cables, strobe supplies, other
antenna cables and the like. Never coil excess cable, always cut the antenna cable
to the shortest possible length.
