User's Manual

TT21 Transponder Installation Manual 19 January 2009

00560-00 Issue AA

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AWG

99 5.0 m

An alternative to a harness built from individual wires, particularly for a long cable run, is to use a

multi-core cable. Aviation grade cable with 6 or more cores is often more expensive than the

individual wires, and therefore is not generally a good choice for a certified aircraft. For aircraft where

those considerations do not apply however, an attractive alternative solution may be to use 3 pair or 4

pair data cable.

Whilst appropriate cables may exist, please note that not all data cable is suitable for this application.

Cables with solid cores should not be used, and cables should be selected based on the wear

characteristics of their insulation material, including temperature rating, resistance to solvents and oils,

and flammability. Most inexpensive commercial data cables have poor flammability properties.

5.11 Antenna Installation

The antenna should be installed according to the manufacturer’s instructions.

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 minimise 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.

5.11.1 Antenna Ground Plane

When a conventional aircraft monopole antenna is used it relies on a ground plane for correct

behaviour. For ideal performance the ground plane should be very large compared to the wavelength

of the transmission, which is 275mm. 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 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

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