Garmin 010-00859-00 Manuel d'installation - Page 28
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4.7 Equipment Mounting
4.7.1 Unit Replacement
4.8 Antenna Installation and Connections
This section provides information on the antenna cable installation. Refer to 3.1 herein for installation
location considerations.
SUGGESTION: Temporarily locate the GPS/SBAS antenna with coax connected to the GTN unit and
check the GPS/SBAS performance as described in Section. Once a suitable location has been verified,
then permanently mount the antenna.
Once the antenna mounting position has been prepared, route the coax cable from the antenna to the GTN
unit. Proper selection of coax cable and assembly of connectors is critical to GPS signal performance.
Cable loss from the GPS/SBAS antenna must be between 1.5 dB and 6.5 dB in order to maintain
proper rejection to interference signals.
Coaxial connectors and adapters, such as TNC to BNC, add additional loss to the cable and should be
considered when computing the cable loss. A typical loss of 0.2 dB can be used for each connection. To
maintain integrity of the SBAS signal, the GPS antenna coaxial cable must have a minimum of two
shields (e.g. RG-400 or RG-142B).
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 coax.
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 to 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, then 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 coax 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.
If a VHF COM transmitter causes problems with the GPS on the selected frequencies as listed in the post-
installation checkout, the problem may be due to the ELT. This can be verified by disconnecting the ELT
antenna coax at the ELT unit. If the ELT is found to cause the problem, then contact the ELT
manufacturer or replace the ELT.
Page 4-6
Rev. A
NOTE
GDL 88 TSO Installation Manual
190-01122-00