User's Manual

TT21/TT22 Transponder Installation Manual 13 July 2009

00560-00 Issue AF

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Page 22 Trig Avionics Limited

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 120mm per side; as the size increases

the performance may actually get worse, but will be better by the time the

ground plane is 700mm 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.

5.11.2 Antenna Cable

The TT21 is designed to meet Class 2 requirements with an allowance of 2 dB

for loss in the connectors and cable used to connect it to the antenna. The

TT22 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 1.5dB 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

metre that meets the above requirements can be chosen. Longer runs require

lower loss cable. Consider moving the TT21/TT22 closer to the antenna to

minimise the losses in the antenna cable – subject to the limits identified above,

the TT21/TT22 can be at any distance from the control head 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