Antenna Considerations - Garmin GTX 3x5 Series Installation Manual

3.8 Antenna Considerations

Mounting location considerations for the antenna(s) is provided in this section. The recommended antenna
placements are illustrated in figure 3-1.
3.8.1 Transponder Antenna
NOTE
For installations on composite aircraft, ground planes must be considered. Conductive
wire mesh, radials, or thin aluminum sheets embedded in the composite material provide
the ground plane allowing the antenna pattern (gain) to be maximized for improved
transponder performance.
NOTE
Note that penetration of the pressure vessel on pressurized aircraft requires additional
data not included in this manual.
The following guidance provides information to aid the installer in ensuring the most optimum location is
selected for the transponder antenna.
The antenna mounting should utilize the aircraft manufacturers type certificated antenna location and style.
The antenna should be installed in accordance with manufacturer instructions and/or AC 43.12-2A
Chapter 3.
Transponder antenna considerations include the following.
• Be mounted away from major protrusions, such as engines, nacelles, propellers, and antenna masts
• Be as far as practical from landing gear doors, access doors, or other openings that could affect its
radiation pattern
• Must be mounted vertically on the bottom of the aircraft
• Must not be mounted within three feet of the ADF sense antenna or any other communication
antenna
• Must not be mounted within six feet of a DME antenna
• To prevent RF interference, the antenna must be mounted a minimum of three feet from the
GTX 3x5
3.8.2 GPS Antenna
The installation guidelines meet the intent of AC 20-138A Section 16. The greater the variance from these
guidelines, the greater the chance of decreased signal quality and availability. Meeting all of these
installation guidelines may not be possible on all aircraft. These guidelines are listed in order of
importance to achieve optimum performance. Items 3 to 5 below are of equal importance and their
significance may depend on the aircraft installation. The installer should use best judgment to balance the
installation guidelines.
1. The GPS antenna should be mounted 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 with the
waterline, which is typically referenced as level while performing a weight and balance check. A
shim may be used to level the antenna.
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.
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Rev. 1
GTX 3x5 TSO Installation Manual
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