Antenna Cable; Table 2-3. Maximum Usable Cable Lengths - Honeywell BendixKing KT 74 Installation Manual


Where practical, plan the antenna location to keep the cable lengths as
short as possible and avoid sharp bends in the cable to minimize the
VSWR.

To prevent RF interference, the antenna must be physically mounted a
minimum distance of 3 feet (91.4 cm) from the KT 74 Mode S transponder.
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.

2.8.1 Antenna Cable

The KT 74 is designed to meet Class 1 requirements with an allowance of 2
dB for loss in the connectors and cable used to connect it to the antenna.
Excessive loss will degrade both transmitter output power and receiver
sensitivity.
Allowing 0.25 dB loss for the connector at each end of the antenna cable
assembly leaves an allowance of 1.5 dB maximum loss for the cable itself.
An acceptable cable:

Has less than 1.5 dB 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
meter that meets the above requirements can be chosen. Longer runs require
lower loss cable.
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.
Table 2-3 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.

Table 2-3. Maximum Usable Cable Lengths

MAX MAX
Length in Length in
Feet-Inches Meters
8-4 2.54
10-4 3.16
© Honeywell International Inc. Do not copy without express permission of Honeywell.
INSTALLATION MANUAL
89000007
Insertion
Loss
dB/meter at MIL-C-17
1,090 MHz Cables
0.59 M17/128
(RG400)
0.47
Electronic
Cable
Specialists
Type
-
3C142B
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15 Nov 2013