Garmin GPS 400W Installation Manual

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Garmin GPS 400W Installation Manual Manual pdf 52 page image

Page 3-8 GNS 400W Series Installation ManualRev. G 190-00356-083.10 Antenna Installation and Connections3.10.1 GPS AntennaThis section provides information on the antenna cable installation. Refer to 2.4.1 herein for installationlocation considerations.NOTEThe internal 400W Series COM does not interfere with its own GPS receiver. However,placement of the 400W Series antenna relative to other COM transceivers and antennas(including the 400W Series COM antenna) is critical.Suggestion: Temporarily locate the GPS antenna with coax connected to the 400W Series unit and checkthe GPS performance as described in Section 5.5.2 and Section 5.5.3. Once a suitable location has beenverified, then permanently mount the antenna.Once the antenna mounting position has been prepared, route the coax cable from the antenna to the400W Series unit. Proper selection of coax cable and assembly of connectors is critical to GPS signalperformance. The cable loss from the GPS antenna shall be between 1.5 dB and 6.5 dB in order tomaintain proper rejection to interference signals.The coaxial connectors and adapters, such as TNC to BNC, add additional loss to the cable and should beconsidered when computing the cable loss. A typical loss of 0.2 dB can be used for each connection. Tomaintain integrity of the WAAS signal, the GPS antenna coaxial cable must have a minimum of twoshields (e.g. RG-400 or RG-142B).NOTEIf RG-142B or RG-400 is used, 1.5 dB equates to a length of approximately 6.5 feet ofcable with a connector on each end. RG-142B or RG-400 cable can be used as long as thelength is less than 35 feet. For longer lengths, use low-loss double or triple shielded 50coax.For very short runs, where the loss is less than 1.5 dB, additional cable should be used to increase the lossto within 1.5 dB to 6.5 dB. This additional cable may be coiled, taking into account the minimum bendradius of the cable.During the post-installation checkout, susceptibility to harmonics of VHF COM transmitters will beevaluated. If problems arise, then better isolation, or distance, may be required between the GPS andCOM antennas, or a 1575.42 MHz notch filter may be installed in series with the antenna coax of the VHFCOM transceiver to reduce or eliminate the harmonic interference. A notch filter for this use(P/N 330-00067-00) is available from Garmin.If a VHF COM transmitter causes problems with the GPS on the selected frequencies as listed in the post-installation checkout, the problem may be due to the ELT. This can be verified by disconnecting the ELTantenna coax at the ELT unit. If the ELT is found to cause the problem, then contact the ELTmanufacturer or replace the ELT.