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GTN 6XX/7XX AML STC Installation Manual 190-01007-A3
Page 3-3 Rev. 4
3.3 Antenna Installation and Connections
3.3.1 GPS/WAAS Antenna
This section provides information on the antenna cable installation. Refer to Section 2.4.4.1 for antenna
installation location considerations.
NOTE
The internal GTN unit COM does not interfere with its own GPS receiver. However,
placement of the GTN unit antenna relative to other COM transceivers and antennas
(including the GTN unit COM antenna) is critical.
It is permissible to temporarily locate the GPS antenna with a coaxial cable connected to the GTN and
check the GPS performance as described in Section 5.7.2. Permanently mount the antenna once a suitable
location has been verified.
Once the antenna mounting position has been established, route the coaxial cable from the antenna to the
GTN. Proper selection of coaxial cable and assembly of connectors is critical to GPS signal performance.
NOTE
GPS/WAAS antenna cable loss must be between 1.5 dB and 6.5 dB in order to
maintain proper rejection of interference signals.
Additional loss from coaxial connectors and adapters, such as TNC to BNC, should be considered when
computing cable loss. A typical loss of 0.2 dB can be used for each connection. To maintain integrity of
the WAAS signal, the GPS antenna coaxial cable must have a minimum of two shields (e.g. RG-400 or
RG-142B).
NOTE
GPS antennas listed in Section 3.3 require a cable loss between 1.5 dB and 6.5 dB. If
RG-142B or RG-400 is used, 1.5 dB equates to a length of approximately 6.5 feet of
cable with a connector on each end. RG-142B or RG-400 cable can be used as long as
the length is less than 35 feet. For longer lengths, use low-loss double or triple-
shielded, 50 coaxial.
For very short runs, where the loss is less than 1.5 dB, additional cable should be used to increase the loss
to within 1.5 dB and 6.5 dB. This additional cable may be coiled, taking into account the minimum bend
radius of the cable.
During the post-installation checkout, susceptibility to harmonics of VHF COM transmitters will be
evaluated. If problems arise, better isolation or distance may be required between the GPS and COM
antennas, or a 1575.42 MHz notch filter may be installed in series with the antenna coaxial of the VHF
COM transceiver to reduce or eliminate the harmonic interference. A notch filter for this use (P/N 330-
00067-00) is available from Garmin.
83


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