ADS-B FDL-978 Series and
Lite Series Installation Manual
Document No. 87343 Rev G
FFS Copyright © 2016 All rights reserved.
2-19
In installations without a digital serial interface to the transponder, the FDL-978 and FDL-
978 Lite Series contains a Mode A receiver to receive Mode A/C Transponder control
data from the transponder antenna through the UAT antennas. Ensure the transponder
antenna is on the bottom of the aircraft (same as the UAT antenna). The transponder
antenna is recommended to be at least 6 ft and no further than 12 ft away from the UAT
antenna. The transponder antenna can be as close as 3 ft if there is at least 2-3 dB of
UAT antenna cable loss. 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 get worse, but will be better by the time the ground
plane is 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-978 Series and FDL-978 Lite Series are designed to meet Class A1H/A1S
requirements with an allowance of 2 – 6 dB for 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 minimum of 2-5.5dB.
Allowing 0.25dB loss for the connector at each end of the antenna cable assembly leaves an
allowance of 1.5 – 5.0 dB loss for the cable itself.
An acceptable cable then has:
A minimum of 1.5dB loss for the run length, but no greater than 5.0dB 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-978 Series and FDL-978 Lite Series closer to the antenna to minimize
the losses in the antenna cable subject to the limits identified above.
