UAV-1000711-001
ECCN 7A994
Page
17 | 24
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 other 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 1 meter of the ADF sense antenna
or any COMM antenna and 2 meters 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 minimize
the VSWR.
Electrical connection to the antenna should be protected to avoid loss of
efficiency due to exposure to liquids and moisture. All antenna feeders shall
be installed in such a way that a minimum of RF energy is radiated inside
the aircraft.
When a conventional aircraft monopole antenna is used it relies on a
ground plane for correct behavior. For ideal performance, the ground plane
should be large relative to the wavelength of the transmission, which is
275mm. In a metal, skinned aircraft this is usually easily 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 approx. 120mm per side; as the size
increases, the performance improves, until the ground plane is approx.
700mm on each side. Anything much larger than that size is unlikely to
result in 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 requirement, but obviously needs to be properly supported.
