User's Manual

TT21 Transponder Installation Manual 19 January 2009
00560-00 Issue AA
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connectors and cable used to connect it to the antenna. 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 closer
to the antenna to minimise the losses in the antenna cable – subject to the limits identified above, the
TT21 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 from too-tight bends or from momentary kinking during installation.
Once kinked, these cables do not return to full performance when straightened.
The following table is a guide to the maximum usable lengths of some common cable types. Actual
cable loss varies between manufacturers, there are many variants, and the table is therefore based on
typical data. Use it as a guide only and refer to the manufacturer’s data sheet for your specific chosen
cable for accurate values.
Max Length
in Metres
Max Length
in Feet
Insertion Loss
dB/metre at
1090MHz
MIL-C-17 Cables Electronic Cable
Specialists Type
2.54 8’ 4” 0.59 M17/128 (RG400)
3.16 10’ 4” 0.47 3C142B
3.81 12’ 6” 0.39 M17/112 (RG304)
5.25 17’ 3” 0.29 M17/127 (RG393) 311601
6.42 21’ 1” 0.23 311501
8.22 26’ 11” 0.18 311201
12.59 41’ 3” 0.12 310801
Contact Electronic Cable Specialists on [phone] or www.ecsdirect.com for their data sheets.
When routing the cable, ensure that you:
 Route the cable away from sources of heat.
 Route the cable away from potential interference sources such as ignition wiring, 400Hz
generators, fluorescent lighting and electric motors.
 Allow a minimum separation of 300mm (12 inches) from an ADF antenna cable.
 Keep the cable run as short as possible.
 Avoid routing the cable round tight bends.
 Avoid kinking the cable even temporarily during installation.
 Secure the cable so that it cannot interfere with other systems.
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