Com Antenna; Nav Antenna - Garmin GTN 725 Installation Manual

3.10.2 COM Antenna

The GTN unit requires a standard 50 vertically polarized antenna. Follow the antenna manufacturer's
installation instructions for mounting the antenna.
The antenna should be mounted on a metal surface or a ground plane with a minimum area of
18 inches x 18 inches. Refer to Section 2.4.2 for installation location considerations.
The antenna coax cable should be made of RG-142B, RG-400 or a comparable quality 50 coax.
Check for insertion loss and VSWR (voltage standing wave ratio). VSWR should be checked with an in-
line type VSWR/wattmeter inserted in the coaxial transmission line between the transceiver and the
antenna. The VSWR should be inserted as close to the transceiver as possible. When rack and harness
buildup is performed in the shop, the coax termination may be provisioned by using a 6-inch inline BNC
connection. This would be an acceptable place to insert the VSWR. Any problem with the antenna
installation is most likely seen as high reflected power. A VSWR of 3:1 may result in up to a 50% loss in
transmit power.

3.10.3 NAV Antenna

The NAV antenna is a standard 50Ω horizontally polarized NAV/VOR/Localizer/Glideslope antenna (the
glideslope may be a separate antenna in some aircraft) 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.
It is recommended that the installer use RG-142B, RG-400 or equivalent 50Ω coax for the NAV
antenna(s).
The GTN has a single VOR/LOC/Glideslope input requiring some installations to use a standard external
diplexer or triplexer. See Section TBD.
GTN 725/750 TSO Installation Manual Page 3-9
190-01007-02 Rev. A