Antenna Considerations; Gps Antenna Location - Garmin GTN 725 Installation Manual

2.4

Antenna Considerations

This section contains mounting location considerations for the antennas required for the GTN units.
Structural substantiation for mounting of antennas is beyond the scope of the GTN AML STC. For
mounting the GA GPS/WAAS antenna, refer to Garmin GA Antenna AML STC SA01695SE or other
FAA approved data. For mounting the COM and NAV antennas, refer to the aircraft manufacturer's data.
2.4.1

GPS Antenna Location

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 3
below are of equal importance and their significance may depend on the aircraft installation. The installer
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) 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: When mounting a combination antenna (ex. GPS and COM, GPS and XM), the
recommended distance of two feet or more is not applicable to the distance between
the antenna elements provided the combination antenna is TSO authorized and has
been tested to meet Garmin's minimum performance standards.
3b. The GPS antenna should be mounted no closer than two feet from any antennas emitting
more than 25 watts of power. An aircraft EMC check can verify the degradation of GPS in
the presence of interference signals.
3c. To minimize the effects of shadowing at 5 elevation angles, the GPS antenna should be
mounted no closer than 6" (edge to edge) from other antennas, including passive antennas
such as another GPS antenna or XM antenna.
4. To maintain a constant gain pattern and limit degradation by the windscreen, avoid mounting
the antenna closer than 3 inches from the windscreen.
5. For multiple GPS installations, the antennas should not be mounted in a straight line from the
front to the rear of the fuselage. Also varying the mounting location will help minimize any
aircraft shading by the wings or tail section (in a particular azimuth, when one antenna is
blocked the other antenna may have a clear view).
Figure 2-1 shows the recommended placement of antennas.
GTN 725/750 TSO Installation Manual
190-01007-02
Page 2-7
Rev. A