3.12.2 Gps Antenna - Garmin GTX 3X5 Installation Manual

3.12.2 GPS Antenna

NOTE
When attaching a combination antenna, the recommended distance of two feet or more is
not applicable to the distance between the antenna elements of the combination antenna
(ex. GPS and COM, GPS and XM) provided the combination antenna is TSO authorized
and has been tested to meet Garmin's minimum performance standards. Refer to table 1-
10 for approved antennas.
NOTE
If twelve inch spacing is not practical, the maximum center-to-center spacing must be
used, but never less than nine inches. Spacing less than nine inches results in
unacceptable antenna pattern degradation.
The installation guidelines meet the intent of AC 20-138A section 16. The greater the deviation from these
guidelines, the greater the chance of decreased signal quality and availability. It is possible that all of the
installation guidelines cannot be met. These guidelines are listed in order of importance to get the best
performance. The installer should use best judgment to balance the installation guidelines.
The GPS antenna should:
• Be installed as near to level as possible with respect to the normal cruise flight attitude of the
aircraft.
• Be installed in a location to minimize the effects of airframe shadowing during typical maneuvers.
• Be installed a minimum of two feet from any VHF COM antenna or any other antenna which may
emit harmonic interference at the L1 frequency of 1575.42 MHz.
• Be installed a minimum of two feet from any antennas emitting more than 25 watts.
• Be installed a minimum of nine inches (center to center) from other antennas, including passive
antennas such as another GPS or XM antenna.
• Be installed a minimum of three inches from the windscreen.
• Have a twelve inch center to center spacing between GPS antennas.
An aircraft EMC check can find the source of GPS degradation in the presence of interference signals. If
an EMC check shows unacceptable interference, either insert a GPS notch filter in line with the offending
VHF COM or the (re-radiating) ELT transmitter, or select a different GPS antenna location.
190-01499-02
Rev. 4
GTX 3X5 Transponder TSO Installation Manual
Page 3-22