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mast and the fuselage with a suitable sealer, such as zinc chromate paste or equivalent. 

The mounting bases of antennas vary in shape and sizes; however, the aforementioned installation procedure is typical of mast-type antenna installations. 

## Transmission Lines 

A transmitting or receiving antenna is connected directly to its associated transmitter or receiver by a transmission line. This is a shielded wire also known as coax. Transmission lines may vary from only a few feet to many feet in length. They must transfer energy with minimal loss. Transponders, DME and other pulse type transceivers require transmission lines that are precise in length. The critical length of transmission lines provides minimal attenuation of the transmitted or received signal. Refer to the equipment manufacturer’s installation manual for the type and allowable length of transmission lines. 

To provide the proper impedance matching for the most efficient power transfer, a balun may be used in some antenna installations. It is formed in the transmission line connection to the antenna. A balun in a dipole antenna installation is illustrated in Figure 11-163 . 

Coax connectors are usually used with coax cable to ensure a secure connection. Many transmission lines are part of the equipment installation kit with connectors previously installed. The aircraft technician is also able to install these connectors on coax. Figure 11-164  illustrates the basic steps used when installing a coax cable connector. 

When installing coaxial cable, secure the cables firmly along their entire length at intervals of approximately 2 feet. To assure optimum operation, coaxial cables should not be routed or tied to other wire bundles. When bending coaxial cable, be sure that the bend is at least 10 times the size of the cable diameter. In all cases, follow the equipment manufacturer’s instructions. 

## Maintenance Procedure 

Detailed instructions, procedures, and specifications for the servicing of avionics equipment are contained in the manufacturer’s operating manuals. Additional instructions for removal and installation of the units are contained in the maintenance manual for the aircraft in which the equipment 

