Garmin GNC 420A Installation Manual page 18

2.2.6 ANTENNA LIMITATIONS
Garmin's GA 56 Antennas are recommended for installations where the airspeed of the aircraft is subsonic.
In such installations, GA 56—Mod 1 or later—must be used. See the COM, VOR/LOC, and Glideslope
antenna specifications for their limitations.
2.2.7 VHF COM INTERFERENCE OF GPS
On many panel-mounted installations, VHF COM transceivers can radiate strong harmonics from the
transceiver and its antenna. The 400 Series COM does not interfere with its own GPS section. However,
placement of the GA 56 GPS antenna relative to a COM transceiver and COM antenna, including the GNC
420 or GNS 430 COM antenna, is critical.
Use the following guidelines, in addition to others in this document, when locating the 400 Series unit and
its antennas.
•
GPS Antenna—Locate as far as possible from all COM antennas and all COM transceivers
(including the 400 Series COM). The GPS antenna is less susceptible to harmonic interference
if a 1.57542 GHz notch filter is installed on the COM transceiver antenna output.
•
Locate the 400 Series unit as far as possible from all COM antennas.
Figure 2-1. GPS Antenna and Unit Installation Considerations
If a COM antenna is found to be the problem, a 1.57542 GHz notch filter (Garmin P/N 330-00067-00) may
be installed in the VHF COM coax, as close to the COM as possible. This filter is not required for the
GNC 420 and GNS 430 transmitters.
If a COM is found to be radiating, the following can be done:
1. Replace or clean the VHF COM rack connector to ensure good coax ground.
2. Place grounding straps between the 400 Series unit, VHF COM and a good ground.
3. Shield the VHF COM wiring harness.
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400 SERIES INSTALLATION MANUAL
P/N 190-00140-02