GNS 400W Series Installation Manual Page 2-5190-00356-08 Rev. Gaircraft shading by the wings or tail section (in a particular azimuth, when one antenna is blockedthe 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.
