Uat Antenna Ground Plane; Uat Antenna Cable - FreeFlight ADS-B FDL-DB Dual Band Series Installation Information

be at least 6 ft and no further than 12 ft away from the UAT antenna. Also, there should
be a clear line-of-sight between the UAT antenna and transponder antenna with no
obstructions or projections.
•
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 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.
2.10.1

UAT Antenna Ground Plane

When a conventional aircraft monopole antenna is used it relies on a ground plane for correct
operation. For ideal performance, the ground plane should be very large compared to the
wavelength of the transmission, which is ~12 in. 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. Dimensions with 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 5.25 in per side; as the
size increases the performance may actually decrease, but will improve by increasing the
ground plane to 30.5 in 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 metallic mesh and grid solutions are available.
2.10.2

UAT Antenna Cable

The FDL-DB Series are designed to meet Class A1H/A1S requirements with an allowance of 2
maximum loss in the connectors and cable used to connect it to the antenna. Excessive loss
degrades transmitter output power, so it is recommended that the installation cable loss be
limited to the loss maximum of 2dB.
An acceptable cable then has:
•
A minimum of 1.5dB loss for the run length, but no greater than 2.0dB loss
•
A characteristic impedance of 50 Ohms
•
Double braid screens or has a foil and braid screen
Once the cable run length is determined, a cable type with the proper attenuation (loss) per foot
that meets the above requirements can be chosen. Longer runs require lower loss cable.
Consider moving the FDL-DB Series closer to the antenna to minimize the losses in the antenna
cable subject to the limits identified above.
The following table is a guide to the minimum and maximum usable lengths of some common
cable types. Actual cable loss varies between manufacturers and the table is based on typical
data. Use the table as a guide only and refer to the manufacturer's data sheet for the specific
cable chosen to calculate the minimum and maximum lengths.
FFS Copyright © 2020 All rights reserved.
Cable Attenuation
(dB/100 ft @ 1 GHz)
ADS-B FDL-DB Dual Band Series
Installation Manual
Document No. 88552 Rev A
Max Length
(ft)
2-35