Version 16.10 AF-6600 AF-5000 Series Install Manual 120
Antenna Ground Plane
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 very large compared to the wavelength of
reception, which is 305 mm. 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 130 mm per side; as the size increases the performance may
actually get worse, but will be better by the time the ground plane is 780 mm 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.
Antenna Cable
An acceptable cable:
• Has less than 3.0 dB loss for the run length needed. For runs less than 17 feet, RG58 is
acceptable.
• Has a characteristic impedance of 50 Ohms
• If running the antenna cable in a bundle with or in close proximity to the transponder antenna
cable:
Has double braid screens or has a foil and braid screen. For runs less than 17 feet,
RG400 is acceptable.
When routing the cable, ensure that you:
• Route the cable away from sources of heat.
• Route the cable away from potential interference sources such as ignition wiring, 400Hz
generators, fluorescent lighting and electric motors.
• Keep the cable run as short as possible.
• Avoid routing the cable round tight bends.
• Avoid kinking the cable even temporarily during installation.
• Secure the cable so that it cannot interfere with other systems.
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