INSTALLATION MANUAL
CMA-5024 GLSSU
This document includes Proprietary Information and shall not be reproduced or communicated to third party
without prior written permission by CMC Electronics Inc.
Page 107
November 21, 2008
LNA
GPS
SENSOR
Fixed loss =
Lmax to Lmin dB
041201
Figure 103. Cable Loss Considerations
Notes: These requirements allow using an antenna with a nominal preamplifier gain of 29.5 dB
(tolerance of ±3 dB i.e. minimum gain of 26.5 dB and maximum gain of 32.5 dB), with very
short antenna cables, as some aircraft retrofit installations may require.
If the antenna preamplifier gain is expected to exceed 32.5 dB, the excess gain must be
compensated by using an antenna cable of suitable length and RF loss, in order to
avoid overloading the RF/IF circuits in the receiver. In this case, follow the computation
example below.
The following example shows the computation for an antenna with a nominal
preamplifier gain of 33 dB. An estimate of the cable length is provided based on the
nominal cable loss; refer to the cable manufacturer for detailed information on cable
loss tolerance.
EXAMPLE: Assume that the specified preamplifier gain of the antenna is 33 dB ±3 dB. Then L
min is (33
+3 dB - 33 dB) = 3.0 dB and L
max is (33 - 3 dB - 13.5 dB - 1 dB) = 15.5 dB. That is, the
overall cable insertion loss must stay within 3.0 to 15.5 dB at L1 carrier frequencies over the
life of the installation and the temperature range.
Selecting for example the E.C.S. cable part number 311201, which has nominal losses of
6.8 dB per 100 feet, the minimum/maximum lengths, 44 and 225 feet respectively in this
sample case, are estimated as follows, assuming that there are only two RF connector pairs
along the RF cable (one at each end):
- On one hand, the antenna cable must not be too short. Neglecting the connector
losses, the 3.0 dB minimum loss is divided by the E.C.S. cable loss of 6.5 dB per 100
feet. Rounded to the nearest foot, this gives a minimum cable length of 44 feet.
- On the other hand, the antenna cable must not be too long. Allocating 0.1dB per
connector leaves a maximum loss of 15.3 dB for the cable itself (15.5 dB - 0.2 dB).
When divided by the E.C.S. cable loss of 6.5 dB per 100 feet and rounded to the
nearest foot, this gives a maximum cable length of 225 feet.
The document reference is online, please check the correspondence between the online documentation and the printed version.
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