Installation Instructions

GTN 6XX/7XX AML STC Installation Manual 190-01007-A3
Page 2-20 Rev. 4
6. A 12-inch center-to-center spacing between GPS antennas is needed to achieve the best possible
low-elevation antenna gain by minimizing pattern degradation due to shadowing and near-field
interaction. When practical, 12-inch center-to-center spacing between GPS antennas must be
used. If 12-inch spacing is not practical, the maximum center-to-center spacing possible must be
used, but never less than 9-inch center-to-center spacing. Spacing less than 9 inches center-to-
center results in unacceptable antenna pattern degradation.
2.4.4.2 COM Antenna Location
The GTN 635/650/750 COM antenna should be mounted well away from all projections that could
interfere with RF communications, engines and propellers. The ground plane surface directly below the
antenna should be a flat plane over as large an area as possible (18 inches square, minimum). The COM
antenna must be grounded to the ground plane. The antenna should be mounted a minimum of six feet
from other COM antennas for best performance, four feet from any ADF sense antennas, and two feet
from the GTN unit and its GPS/WAAS antenna. The COM antenna should also be mounted as far apart as
practical from the ELT antenna.
Some ELTs have exhibited re-radiation problems generating harmonics that may interfere with GPS
signals. This can happen when the COM (GTN COM or any other COM) is transmitting on certain
frequencies such as 121.15 or 121.175 MHz, which may cause the ELT output circuit to oscillate from the
signal coming in on the ELT antenna coaxial. In addition, the COM antennas should be spaced for
maximum isolation. A configuration of one topside antenna and one bottom side antenna is
recommended.
2.4.4.3 NAV Antenna Location
The GTN 650/750 NAV antenna should be mounted well away from all projections, engines and
propellers. It should have a clear line of sight if possible. The antenna must be mounted along the
centerline of the aircraft, minimizing the lateral offset.