Page:Electronics Technician - Volume 7 - Antennas and Wave Propagation - NAVEDTRA 14092.pdf/28

 Figure 2-9.—Ground screen and counterpoise.

The ground screen in view A is composed of a series of conductors arranged in a radial pattern and buried 1 or 2 feet below the surface of the earth. These conductors, each usually 1/2 wavelength long, reduce ground absorption losses in the vicinity of the antenna.

A counterpoise (view B) is used when easy access to the base of the antenna is necessary. It is also used when the area below the antenna is not a good conducting surface, such as solid rock or ground that is sandy. The counterpoise serves the same purpose as the ground screen but is usually elevated above the earth. No specific dimensions are necessary for a counterpoise, nor is the number of wires particularly critical. The primary requirement is that the counter-poise be insulated from ground and form a grid of reflector elements for the antenna system.

COMMUNICATIONS ANTENNAS

Some antennas can be used in both shore-based and ship-based applications. Others, however, are designed to be used primarily in one application or the other. The following paragraphs discuss, by frequency range, antennas used for shore-based communications.

VERY LOW FREQUENCY (VLF)

The main difficulty in vlf and lf antenna design is the physical disparity between the maximum practical size of the antenna and the wavelength of the frequency it must propagate. These antennas must be large to compensate for wavelength and power handling requirements (0.25 to 2 MW). Transmitting antennas for vlf have multiple towers 600 to 1500 feet high, an extensive flat top for capacitive loading, and a copper ground system for reducing ground losses. Capacitive top-loading increases the bandwidth characteristics, while the ground plane improves radiation efficiency.

Representative antenna configurations are shown in figures 2-10 through 2-12. Variations of these basic antennas are used at the majority of the Navy vlf sites.

LOW FREQUENCY (LF)

Antennas for lf are not quite as large as antennas for vlf, but they still occupy a large surface area. Two

Figure 2-10.—Triatic-type antenna. 2-6