Com Antenna Location - Garmin GPS 400W Installation Manual

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).
Figure 2-1 shows the recommended placement of antennas.
2.4.2

COM Antenna Location

The GNC 420W/GNS 430W COM antenna should be well removed from all projections, 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 antenna should be mounted a minimum of six feet
from any DME or other COM antennas, four feet from any ADF sense antennas, and two feet from the
400W Series and its GPS 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 (400W Series 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 coax.
If simultaneous use of two COM transceivers is desired (Split-COM or simul-comm), use of the TX
interlock function is mandatory. In addition, the COM antennas should be spaced for maximum
isolation. A configuration of one topside antenna and one bottom side antenna is recommended.
GNS 400W Series Installation Manual
190-00356-08
Page 2-5
Rev. G