Page 2-4 400W Series Installation ManualRev. E 190-00356-021. Mount the antenna as close to level as possible with respect to the normal cruise flightattitude of the aircraft. If the normal flight attitude is not known, substitute the waterline,which is typically referenced as level while performing a weight and balance check.2. The GPS antenna should be mounted in a location to minimize the effects of airframeshadowing during typical maneuvers. Typically mounting farther away from the tailsection reduces signal blockage seen by the GPS antenna.3a. The GPS antenna should be mounted no closer than two feet from any VHF COM antennaor any other antenna which may emit harmonic interference at the L1 frequency of 1575.42MHz. An aircraft EMC check (reference VHF COM interference check in Post InstallationCheckout procedures) can verify the degradation of GPS in the presence of interferencesignals. If an EMC check reveals unacceptable interference, insert a GPS notch filter inline with the offending VHF COM or the (re-radiating) ELT transmitter.Note: When mounting a combination antenna (ex. GPS and COM, GPS and XM), therecommended distance of two feet or more is not applicable to the distance between theantenna elements provided the combination antenna is TSO authorized and has been testedto meet Garmin’s minimum performance standards.3b. The GPS antenna should be mounted no closer than two feet from any antennas emittingmore than 25 watts of power. An aircraft EMC check can verify the degradation of GPS inthe presence of interference signals.3c. To minimize the effects of shadowing at 5° elevation angles, the GPS antenna should bemounted no closer than 6 inches (edge to edge) from other antennas, including passiveantennas such as another GPS antenna or XM antenna.4. To maintain a constant gain pattern and limit degradation by the windscreen, avoidmounting the antenna closer than 3 inches from the windscreen.5. For multiple GPS installations, the antennas should not be mounted in a straight line fromthe front to the rear of the fuselage. Also varying the mounting location will help minimizeany aircraft shading by the wings or tail section (in a particular azimuth, when one antennais blocked the other antenna may have a clear view).Figure 2-1 shows the recommended placement of antennas.2.4.2 COM Antenna LocationThe GNC 420W/GNS 430W COM antenna should be well removed from all projections, engines andpropellers. The ground plane surface directly below the antenna should be a flat plane over as large anarea as possible (18 inches square, minimum). The antenna should be mounted a minimum of six feetfrom any DME or other COM antennas, four feet from any ADF sense antennas, and two feet from the400W Series and its GPS antenna. The COM antenna should also be mounted as far apart as practicalfrom the ELT antenna. Some ELTs have exhibited re-radiation problems generating harmonics that mayinterfere with GPS signals. This can happen when the COM (400W Series or any other COM) istransmitting on certain frequencies such as 121.15 or 121.175 MHz, which may cause the ELT outputcircuit 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 TXinterlock function is mandatory. In addition, the COM antennas should be spaced for maximumisolation. A configuration of one topside antenna and one bottom side antenna is recommended.
