Advanced AF-5000, Installation Manual

Page: 104 / 267

Version 16.10
AF-6600 AF-5000 Series Install Manual
104
Antenna Installation
The antenna should be installed according to the manufacturer’s instructions.
In our RV-10 Aircraft we used a Comant CI 105 DME/Transponder Antenna.
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 minimize the VSWR (voltage standing wave ratio).
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.
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 the transmission, which is 275
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 120
mm per side; as the size increases the performance may actually get worse, but will be better by the time the ground
plane is 700 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
The SV-261 is designed to meet Class 1 requirements with the same 2 dB allowance. Excessive loss will degrade both
transmitter output power and receiver sensitivity.
Allowing 0.25dB loss for the connector at each end of the antenna cable assembly leaves an allowance of 1.5dB
maximum loss for the cable itself.
An acceptable cable:
•
Has less than 1.5 dB loss for the run length needed
•
Has a characteristic impedance of 50 Ohms
•
Has double braid screens or has a foil and braid screen
Once the cable run length is known, a cable type with low enough loss per meter that meets the above requirements
can be chosen. Longer runs require lower loss cable. Consider moving the SV -261 closer to the antenna to minimize
the losses in the antenna cable
– subject to the limits identified above, the SV- 261 can be at any distance from the AF-
5000 display without affecting performance in any way.
Note : Low loss cable typically uses foamed or cellular dielectrics and foil screens. These make such cables especially
prone to damage from too-tight bends or from momentary kinking during installation. Once kinked, these cables do
not return to full performance when straightened.