TT21/TT22 Transponder Installation Manual
22 August 2017
00560-00
AP
______________________
22
Trig Avionics Limited
The distance between the TT21/TT22 transponder and the TC20 controller is
limited by the impedance of the wire between them. The TC20 is powered
from the TT21/TT22, not from aircraft power, and therefore the acceptable
voltage drop in the power line is what limits the distance. The TC20 needs an
impedance of less than 0.5 ohm in the power line for satisfactory operation.
The following table gives guidelines for typical aircraft hook-up wire. Note
that different brands may vary – check your supplier for details.
Gauge
Ohm/km
Length for 0.5 Ohm
20 AWG
35
14.2 m
22 AWG
64
7.8 m
24 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
