Table 3-11 Gtx 335 Gps Antennas; Gps Antenna Location - Garmin GTX 335 w GPS Installation Guidance

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Model Description

GA 35, GPS/SBAS

GA 36, GPS/SBAS

GA 37, GPS/SBAS/XM

A33W, WAAS

GPS / VHF

GPS / XM / VHF

GPS / WAAS

GPS / XM

3.11.5 GPS Antenna Location

When a combination antenna is attached, the recommended distance of two feet or more is not applicable to

the distance between the antenna elements (e.g., GPS and COM, GPS and SiriusXM). This is provided that

the combination antenna is TSO authorized and has been tested to meet Garmin's minimum performance

standards.

These installation guidelines meet the intent of AC 20-138B Chapter 12, Section 12-1. Meeting all of the

installation guidelines is not possible on some aircraft. These guidelines are listed in order of importance to

achieve optimum performance. The installer should use their best judgment to balance the installation

guidelines.

GPS antennas should be installed as follows:

As close to level as possible. If the normal flight attitude is not known, substitute with the

waterline, which is typically referenced as level while a weight and balance check is done.

Mount in a location to minimize the effects of airframe shadowing during typical maneuvers.

Mounting farther away from the tail section decreases signal blockage.

Mounted no closer than two feet from any VHF COM antenna.

Mounted no closer than two feet from any antennas emitting more than 25 watts of power.

Mounted no closer than nine inches (center to center) from other antennas.

Maintain a constant gain pattern and limit degradation by the windscreen, avoid attaching the

antenna closer than 3 inches from the windscreen.

For installations with more than one antenna, the antennas should not be attached in a straight line

from the front to the rear of the fuselage. This is so a single lightning strike does not damage all

antennas.

When possible, 12-inch center-to-center spacing between GPS antennas must be used. If 12-inch spacing is

not possible, the maximum center-to-center spacing possible must be used, but never less than 9-inch

center-to-center spacing.

An aircraft EMC check can show if there is a degradation of GPS in the presence of interference signals.

Refer to section 7. If an EMC check reveals unsatisfactory interference, select a different GPS Antenna

location or insert a GPS notch filter in line with the VHF COM or the (re-radiating) ELT transmitter that is

causing interference.

190-00734-17

Rev. 1