User's Manual

TT21/TT22 Transponder Installation Manual 13 July 2009
00560-00 Issue AF
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Page 20 Trig Avionics Limited
The TT21/TT22 /TC20 was tested and certified using unshielded, untwisted
wiring, and that is sufficient for a certified installation. There may however be
technical benefits of improved electromagnetic emissions and susceptibility to
and from the transponder system if the two wires of the TMAP pair are lightly
twisted together – one twist per 1 to 2 inches is appropriate. This may reduce
interference and break-through on adjacent audio wiring if it is not possible to
route them separately. For tidiness and consistency the other pairs in the
bundle can also be twisted, but there will be no particular difference in
behaviour.
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.