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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 for the GPS receiver installation) 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.
This check can only be performed if a GPS receiver exists on the aircraft or one
is being installed.
When mounting a combination antenna, the recommended distance of two feet
or more is not applicable to the distance between the antenna elements in a
combination antenna (ex. GPS and SiriusXM) 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 achieve the best possible low-elevation antenna gain (by minimizing pattern degradation due
to shadowing and near-field interaction), the GPS antenna shall be mounted with clearance from
other antennas, including passive antennas such as another GPS antenna or SiriusXM
antenna. When practical, installers should use 12 inch center-to-center spacing between
antennas. If 12 inch spacing is not practical use a maximum center-to-center spacing from
adjacent antennas, but never less than 9 inch center-to-center spacing. Spacing less than 9 inches
center-to-center results in unacceptable GPS/WAAS antenna pattern degradation.
4. To limit degradation by windscreen effects, avoid mounting the antenna closer than 3 inches from
the windscreen.
Antennas on certain airplanes with a maximum speed over 250 KIAS should be
installed at least 24 inches aft of the windscreen, unless another location has been
certified.
5. For multiple GPS installations, the antennas should not be mounted in a straight line from the
front to the rear of the fuselage to prevent a lightning strike from damaging both antennas. 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).
Page 2-2
Rev. F
NOTE
NOTE
NOTE
GA 35, GA 36, and GA 37 Antenna Installation Instructions
190-00848-00
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