Figure 2-1. Gps Antenna And Unit Installation Considerations - Garmin GNC 420A Installation Manual

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 GPS INTERFERENCE
On some installations VHF COM transceivers, Emergency Locator Transmitter (ELT) antennas, and
Direction Finder (DF) receiver antennas can re-radiate through the GPS 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), ELT antenna,
and DF receiver 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), ELT antennas, and DF receiver antennas. 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