Installation; Introduction; Antenna Considerations - Garmin GNC 420A Installation Manual

2.

INSTALLATION

2.1

INTRODUCTION

Careful planning and consideration of the suggestions in this section are required to achieve the desired
performance and reliability from the 400 Series unit. Any deviations from the installation instructions
prescribed in this document shall be accomplished in accordance with the requirements set forth in FAA
AC 43.13-2A, and 14 CFR Part 43 Maintenance, Preventive Maintenance, Rebuilding, and Alteration.
2.2

ANTENNA CONSIDERATIONS

Antenna installations on pressurized cabin aircraft require FAA approved installation design and
engineering substantiation data whenever such antenna installations incorporate alteration (penetration) of
the cabin pressure vessel by connector holes and/or mounting arrangements. For needed engineering
support pertaining to the design and approval of such pressurized aircraft antenna installations, it is
recommended that the installer proceed according to any of the following listed alternatives:
1. Obtain approved antenna installation design data from the aircraft manufacturer.
2. Obtain an FAA approved STC, pertaining to, and valid for the antenna installation.
3. Contact the FAA Aircraft Certification Office in the appropriate Region and request identification of
FAA Designated Engineering Representatives (DERs) who are authorized to prepare and approve the
required antenna installation engineering data.
4. Obtain FAA Advisory Circular AC-183C and identify a DER from the roster of individuals in it.
5. Contact an aviation industry organization such as the Aircraft Electronics Association for assistance.
2.2.1 GPS ANTENNA LOCATION
The GA 56 Antenna must be mounted on top of the aircraft. For best performance, select a location with
an unobstructed view of the sky above the aircraft when in level flight. Figure 2-1 illustrates a typical GPS
antenna installation. The antenna should be located at least three feet from transmitting antennas such as
VHF COM, HF transmitter, DME, Transponder and Radar.
For rotorcraft, locate the GA 56 Antenna as far as possible from the main rotor hub. This reduces the
percentage of time the blade blocks the antenna. Also mount it as far below the blade surface as possible if
installing the antenna under the blade. This reduces signal distortion caused by the blades.
2.2.2 COM ANTENNA LOCATION
The GNC 420 or GNS 430 COM antenna should be well removed from all projections, engines and
propellers. The ground plane surface directly below the antenna should be a flat plane over as large an area
as possible (18 inches square, minimum). The antenna should be mounted a minimum of six feet from any
DME or other COM antennas, four feet from any ADF sense antennas, and three feet from the 400 Series
and its GPS antenna.
If simultaneous use of two COM transceivers is desired (spit- COM or simulcomm), use of the TX
interlock function is mandatory. In addition, the COM antennas should be spaced for maximum isolation.
A configuration of one topside antenna and one bottom side antenna is recommended.
2.2.3 VOR/LOC ANTENNA LOCATION
The GNS 430 VOR/LOC antenna should be well removed from all projections, engines and propellers. It
should have a clear line of sight if possible. The antenna must be mounted along the centerline of the
aircraft, minimizing the lateral offset.
2.2.4 GLIDESLOPE ANTENNA LOCATION
The GNS 430 Glideslope antenna should be well removed from all projections, engines and propellers. It
should have a clear line of sight if possible.
2.2.5 ELECTRICAL BONDING
No special precautions need to be taken to provide a bonding path between the GPS antenna and the
aircraft structure. Follow the manufacturers' instructions for the COM, VOR/LOC and Glideslope
antennas.
400 SERIES INSTALLATION MANUAL
P/N 190-00140-02
Page 2-1
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