Garmin GDL 90 Installation Manual

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Installation Procedures2-8 560-1049-00 Rev B GDL 90 Installation ManualFigure 2-6. Antenna Placement2.6.1 GPS ANTENNAThe mounting location and cable connections for the GPS antenna are very important. The antennashould be mounted no closer than two feet from VHF comm transmitter antennas, six inches from otherantennas emitting less than 25 watts, and two feet from higher power antennas. See A-33 GPS AntennaInstallation Manual, p/n 560-0949-xx for more information. Special care should be taken to ensure thatthe GPS antenna is not mounted in close proximity to antennas that may emit harmonic interference at theL1 frequency of 1575.42 MHz. Refer to AC 20-138A Airworthiness Approval of Global NavigationSatellite System (GNSS) Equipment for additional information and guidelines.The connectors and RG-400 coax cable are included in the installation kit. Assembly instructions for theconnectors are included in Figure 2-7. RG-400 cable can be used as long as the length is less than 35 feet. Forlonger lengths, use low-loss 50Ω coax.Suggestion: Temporarily locate the GPS antenna with coax connected to the GDL 90 and check the GPSperformance as described in the GPS Operation and Position test in the Post Installation CheckoutSection. Once a suitable location has been verified, permanently mount the antenna.NOTEIf using a GPS antenna that was already on the aircraft, or if mounting the antenna closerthan recommended distance from a comm antenna, conduct the GPS Operation andPosition test in the Post Installation Checkout. If the GDL 90 passes the test, then furthermeasures are not necessary.Once the antenna mounting position has been prepared, route the coax cable from the antenna to the GDL90. Proper selection of coax cable and assembly of connectors is critical to GPS signal performance. Thecable loss from the GPS antenna shall not be greater than 7 dB. If SatCom is installed on the aircraft, thecable loss shall be 3 dB minimum to ensure proper interference rejection from SatCom. The coaxialconnectors and adapters, such as TNC to BNC and GPS splitter, add additional loss to the cable and