190-01004-02
GTN 625/635/650 TSO Installation Manual
Rev. F
Page 4-13
4.11.2 COM Antenna (GTN 635/650 Only)
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 3.5.2 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.
4.11.3 NAV Antenna (GTN 650 Only)
The NAV antenna is a standard 50
Ω
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 50
Ω
coax for the NAV
antenna(s).
Summary of Contents for GTN 625
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