Garmin GNC 255 Installation Manual - page 38

GTR 225/GNC 255 TSO Installation Manual

190-01182-02

Page 4-6

Rev. B

4.7.2 GTR/GNC Unit Insertion and Removal

It may be necessary to insert the hex drive tool into the access hole and rotate the drive tool 

counterclockwise until it completely stops in order to ensure correct position of the retention mechanism 

prior to placing the unit in the rack. The GTR/GNC unit is installed in the rack by sliding it straight in until 

it stops, about 3/8 inch short of the final position. A 3/32-inch hex drive tool is then inserted into the access 

hole at the bottom of the unit face. Rotate the hex tool clockwise while pressing on the left side of the bezel 

until the unit is firmly seated in the rack. 

To remove the unit from the rack, insert the hex drive tool into the access hole on the unit face. Rotate 

counterclockwise until the unit is forced out about 3/8 inch and the hex drive tool completely stops. This 

will allow the unit to be freely pulled from the rack.

Be sure not to over tighten the unit into the rack. The application of hex drive tool torque exceeding 

15 in-lbs can damage the locking mechanism.

4.7.3 Unit Replacement

Whenever the GTR/GNC unit is removed or reinstalled, verify that the unit powers up successfully.

4.8 Antenna Installation and Connections

4.8.1 COM Antenna

The GTR/GNC 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 3.2.1 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 Voltage Standing Wave Ratio (VSWR). 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.