User's Manual

Table Of Contents
SYSTEM DESCRIPTION AND INSTALLATION MANUAL
T
3
CAS/Part No.9005000
(13) An additional procedure for troubleshooting TCAS antenna system failures is to
determine the RF insertion loss in the TCAS antenna coaxial cables. RF insertion
loss is equivalent to RF IxR (voltage) drop through the coax and is measured in
dB rather than volts. Each coax cable is required to have 2.5 dB (±0.5 dB) of
insertion loss and the loss in all the coax cables must be within 0.5 dB of each
other. Many VSWR meters have a fixed RF source and can be used to measure
RF insertion loss. In this test, the VSWR meter is used to measure the loss up
one cable and down the other. The total loss should not exceed 6 dB.
NOTE:
The VSWR/insertion loss meter should be set up for L-band
frequencies (approximately 1.0 GHz).
To perform an RF insertion loss test, do the following:
(a) Connect two of the TCAS antenna coax cables to each other at the
antenna end with a dc-shorted coupler.
(b) Before any measurements are taken on the antenna system, the meter
and any connectors/cables that are used to connect the meter to the
TCAS antenna system must be zeroed out. This is typically done with a
calibration adjustment on the VSWR meter.
(c) After the VSWR meter (and associated connectors/cables) have been
calibrated for 0 dB, connect the VSWR meter to the TCAS antenna
coaxes under test (at the LRU end). Measure and record the RF loss.
(d) Test different combinations of the TCAS antenna coax cables by
connecting the coupler (at the antenna end) to different coax cables. By
process of elimination, it can be determined if one of the TCAS antenna
coax cables has excessive RF insertion loss.
(e) As an example, suppose the following insertion losses have been
measured:
Antenna Port
Antenna Port
Measured Loss
0
90
8.5 dB
0
180
5.5 dB
90
180
8.0 dB
It can be seen that when the coax connected to the 90 degree port is
included in the measurement, there is excessive loss. Simple algebra
can be used to determine that this coax has 5.5 dB of insertion loss.
NOTE:
1 dB of excessive loss in one cable can result in about 3.5
degrees of intruder bearing error.
(14) If the VSWR tests and RF insertion loss test comply, return the directional
antenna to the manufacturer for further testing. Install a new directional antenna
in accordance with AMM procedures.
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04 Nov 2014
34-45-29
Pub. No. 8600200-001, Revision 004
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