Page:Electronics Technician - Volume 7 - Antennas and Wave Propagation - NAVEDTRA 14092.pdf/38

 Center-Fed Dipole

The center—fed dipole (fig. 2—27) is designed for use at an average power rating of 100 watts. All major components of the radiating and support structures are aluminum. The central feed section and radiating elements are protected by a laminated fiberglass cover. Center—fed dipole antennas range from 29 to 47 inches in height and have a radiator diameter of up to 3 inches.

Coaxial Dipole

Figure 2-28 shows two types of coaxial dipoles. The coaxial dipole antenna is designed for use in the uhf range, with an rf power rating of 200 watts. The AT-150/SRC (fig. 2—28, view A) has vertical radiating elements and a balun arrangement that electrically balances the antenna to ground.

Figure 2-28, view B, shows an AS-390/SRC antenna assembly. This antenna is an unbalanced broadband coaxial stub antenna. It consists of a radiator and a ground plane. The ground plane (or counterpoise) consists of eight elements bent downward 37 degrees from horizontal. The lower ends of the elements form points of a circle 23 inches in diameter. The lower section of the radiator assembly contains a stub for adjusting the input impedance of the antenna. The antenna is vertically polarized, with an rf power rating of 200 watts, and a vswr not greater than 2:1.

Array Dipole

The array dipole antenna is designed for use in the 225-400 MHZ UHF range. It has a power rating of 2000 watts peak envelope power, with a VSWR not greater than 1.8:1. See figure 2-29.

The array consists of four broadband radiating dipole elements mounted an equal distance from each other and the mounting structure. It is fed from a single coaxial input and divided by an integral coaxial line matching transformer and feed line sections. All

Figure 2-27.—AS-2809/RC center-fed dipole antenna.

major supporting and radiating components are aluminum. It is designed to be mounted around masts of four to ten inches in diameter.

SATELLITE SYSTEMS

The Navy Satellite Communication System (SATCOM) provides communications links, via satellites, between designated mobile units and shore sites. These links supply worldwide communications coverage. The following paragraphs describe some of the more common SATCOM antenna systems to which you will be exposed. 2-16