Antenna Installation And Connections; Com Antenna; Gps Antenna - Garmin GNS 480 Installation Manaul

Installation Procedures

2.9 Antenna Installation and Connections

The GNS 480 requires three antennas: COM antenna, GPS antenna, and NAV antenna. Structural
substantiation for mounting of antennas is byond the scope of the GNS 480 unit STC. For mounting the
GPS/WAAS antenna, refer to Garmin GA Antenna AML STC SA01695SE or other FAA approved data
(Section 1.8.1). For mounting the COM and NAV antennas, refer to the aircraft manufacturer's data.
2.9.1

COM Antenna

The GNS 480 requires a standard 50Ω vertically polarized antenna. Follow the manufacturer's installation
instructions for mounting the antennas.
The antenna should be mounted on a metal surface or a ground plane with a minimum area of 18 x 18
inches. The antenna should be mounted a minimum of two feet away from GPS antennas.
The COM antenna should also be mounted as far apart as practical from the ELT antenna, preferably one
on top and the other on the bottom of the aircraft fuselage. Some ELTs have exhibited re-radiation
problems generating harmonics that may interfere with GPS signals. This can happen when the COM
(GNS 480 or any other COM) is transmitting on certain frequencies such as 121.15 or 121.175 MHz,
which may cause the ELT output circuit to oscillate from the signal coming in on the ELT antenna coax.
The antenna coax cable should be made of RG-142B, RG-400 or a comparable quality 50Ω coax.
Assembly instructions for the rear coax connector are included in Figure 2-12.
2.9.2

GPS Antenna

The GPS antenna is a key element in the overall system performance and integrity for a GPS/WAAS
navigation system. The mounting location, geometry, and surroundings of the antenna can affect the
system performance and/or availability. The following guidance provides information to aid the installer
in ensuring that the most optimum location is selected for the installation of the GPS antenna. The
installation guidelines presented here meet the intent of AC 20-138A section 16. The greater the variance
from these guidelines, the greater the chance of decreased availability. Approach procedures with vertical
guidance are the most sensitive to these effects. LNAV only approaches, terminal operations, and enroute
operations may also be affected. Because meeting all of these installations guidelines may not be possible
on all aircraft, these guidelines are listed in order of importance to achieve optimum performance. Items
3a., 3b., and 3c. below are of equal importance and their significance may depend on the aircraft
installation. The installers should use their best judgment to balance the installation guidelines.
1. Mount the antenna as close to level as possible with respect to the normal cruise flight
attitude of the aircraft. If the normal flight attitude is not known, substitute the waterline,
which is typically referenced as level while performing a weight and balance check.
2. The GPS antenna should be mounted in a location to minimize the effects of airframe
shadowing during typical maneuvers. Typically mounting farther away from the tail section
reduces signal blockage seen by the GPS antenna.
3a. The GPS antenna should be mounted no closer than two feet from any VHF COM antenna
or any other antenna which may emit harmonic interference at the L1 frequency of 1575.42
MHz. An aircraft EMC check (reference VHF COM interference check in Post Installation
Checkout procedures, Section 3.3.2.1.2) can verify the degradation of GPS in the presence
of interference signals. If an EMC check reveals unacceptable interference, insert a GPS
notch filter in line with the offending VHF COM or the (re-radiating) ELT transmitter.
Note: This does not apply to GPS, XM and COM combination antennas, provided the
antenna model is TSO authorized and has been tested to meet Garmin's minimum
performance standards.
2-42 560-0982-01 Rev L GNS 480 (CNX80) Installation Manual