Page 3-2 GDU 37X Installation ManualRevision C 190-01054-01*For each 1 dB gain over 24 dB, add 1 dB of attenuation into the antenna cable path between the antennaand the GDU 375.It is the installer’s responsibility to ensure that their choice of antenna meets FAA standards according tothe specific installation. This installation manual discusses only the antennas listed in Tables 3-1 and 3-3.Other antennas may be acceptable but their installation is not covered by this manual.There are several critical factors to take into consideration before installing an antenna for a satellitecommunications system. These factors are addressed in the following sections.3.3 Antenna Mounting ConsiderationsThe information in this section does not pertain to in-cabin (internal) mounted antennas such as the GA26C, refer to the accompanying installation instructions (190-00082-00).No special precautions need be taken to provide an electrical bonding path between the GPS Antenna andthe aircraft structure.3.3.1 VHF COM/GPS InterferenceOn some installation VHF COM transceivers, Emergency Locator Transmitter (ELT) antennas, andDirection Finder (DF) receiver antennas can re-radiate through the GPS antenna. The GDU 37X does notinterfere with its own GPS receiver. However, placement of the GPS antenna relative to a COMtransceiver and COM antenna, ELT antenna, and DF receiver antenna is critical.Use the following guidelines, in addition to others in this document, when locating the GDU 37X and itsantennas.• GPS Antenna—Locate as far as possible from all COM antennas and all COM transceivers, ELTantennas, and DF antennas. The GPS antenna is less susceptible to harmonic inter-ference if a 1.57542 GHz notch filter is installed on the COM transceiver antennaoutput.• Locate the GDU 37X as far as possible from all COM antennas.If a COM antenna is found to be the problem, a 1.57542 GHz notch filter (Garmin P/N 330-00067-00) maybe installed in the VHF COM coax, as close to the COM as possible.If a COM is found to be radiating, the following can be done:1. Replace or clean the VHF COM rack connector to ensure good coax ground.2. Place grounding straps between the GDU 37X unit, VHF COM and a good ground.3. Shield the VHF COM wiring harness.Table 3-4 XM Satellite Radio Antenna Minimum RequirementsCharacteristics SpecificationsFrequency Range 2332.5 to 2345 MHzGain (Typical) 24 dB*Noise Figure <1.2 dBNominal Output Impedance 50 ohmsSupply Voltage 3.6 to 5.5 VDCSupply Current (maximum) 55 mAOperating Temperature Gain -50 to +85°C
