Page:Electronics Technician - Volume 7 - Antennas and Wave Propagation - NAVEDTRA 14092.pdf/32

 cover a frequency range of 2 to 30 MHZ requires one tower approximately 140 feet high and an antenna length of around 500 feet, with a ground system that covers approximately 3 acres of land in the immediate vicinity of the antenna.

Sector Log-Periodic Array

This version of a vertically polarized fixed-azimuth LPA consists of four separate curtains supported by a common central tower, as shown in figure 2-17. Each of the four curtains operates independently, providing antennas for a minimum of four transmit or receive systems, and a choice of sector coverage. The four curtains are also capable of radiating a rosette pattern of overlapping sectors for full coverage, as shown by the radiation pattern in figure 2-17. The central supporting tower is constructed of steel and may range to approximately 250 feet in height, with the length of each curtain reaching 250 feet, depending on its designed operating frequencies. A sector antenna that uses a ground plane designed to cover the entire hf spectrum takes up 4 to 6 acres of land area.

Figure 2-17.—Sector LPA and its horizontal radiation pattern.

Figure 2-18.—Rotatable log-periodic antenna.

Rotatable LPA (RLPA)

RLPAs (fig. 2-18) are commonly used in ship-to-shore-to-ship and in point-to-point communications. Their distinct advantage is their ability to rotate 360 degrees. RLPAs are usually constructed with either tubular or wire antenna elements. The RLPA in figure 2-18 has wire elements strung on three aluminum booms of equal length, spaced equally and arranged radially about a central rotator on top of a steel tower approximately 100 feet high. The frequency range of this antenna is 6 to 32 MHz. The gain is 12 dB with respect to isotropic antennas. Power handling capability is 20 kw average, and vswr is 2:1 over the frequency range.

INVERTED CONE ANTENNA

Inverted cone antennas are vertically polarized, omnidirectional, and have an extremely broad bandwidth. They are widely used for ship-to-shore and ground-to-air communications. Inverted cone antennas are installed over a radial ground plane system and are supported by poles, as shown in figure 2-19. The equally-spaced vertical radiator wires terminate in a feed ring assembly located at the bottom center, where a 50-ohm coaxial transmission line feeds the antenna. Inverted cones usually have