Nav Antenna; Figure 2-11 - Rear Coaxc - Garmin GNS 480 Installation Manual

Installation Procedures
Once the antenna mounting position has been prepared, route the coax cable from the antenna to the GNS
480. Proper selection of coax cable and assembly of connectors is critical to GPS signal performance.
The cable loss from the GPS antenna shall not be greater than 7 dB. If SatCom is installed on the aircraft,
the cable loss shall be 3 dB minimum to ensure proper interference rejection from SatCom. The coaxial
connectors and adapters, such as TNC to BNC, add additional loss to the cable and should be considered
when computing the cable loss. A typical loss of 0.2 dB can be used for each connection. The typical
cable loss for 20 feet of RG-142B coax with a connector on each end is 4 dB.
During the post-installation checkout, susceptibility to harmonics of VHF comm transmitters will be
evaluated. If problems arise, then better isolation, or distance, may be required between the GPS and
comm antennas, or a 1575.42 MHz notch filter may be installed in series with the antenna coax of the
VHF comm transceiver to reduce or eliminate the harmonic interference. A notch filter for this use (part
#162-1059) is available from Garmin AT.
GNS 480 performance has been verified in typical installations and has not shown
problems of the built-in comm interfering with the GPS when installed according to the
recommended installation guidelines.
If a VHF comm 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
ELT antenna coax at the ELT unit. If the ELT is found to cause the problem, then contact the ELT
manufacturer or replace the ELT.
2.9.3 NAV A
NTENNA
The NAV antenna is a standard 50Ω horizontally polarized NAV/VOR/Localizer/Glideslope antenna that
receives VOR frequencies between 108 and 117.95 MHz, and localizer frequencies between 108 and 112
MHz, and glideslope information between 328.6 and 335.4 MHz. Follow the antenna manufacturer's
installation instructions for mounting antennas. The Nav antenna should be mounted as far apart as
2-42
NOTE
Figure 2-11 - Rear Coax Connector Assembly
560-0982-01 Rev A GNS 480 (CNX80) Installation Manual