Page:Electronics Technician - Volume 7 - Antennas and Wave Propagation - NAVEDTRA 14092.pdf/29

 Figure 2-11.—Goliath-type antenna.

examples of lf antenna design are shown in figures 2-13 and 2-14. The Pan polar antenna (fig. 2-13) is an umbrella, top-loaded monopole. It has three loading loops spaced 120 degrees apart, interconnected between the tower guy cables. Two of the loops terminate at ground, while the other is used as a feed. The NORD antenna (fig. 2-14), based on the the folded-unipole principle, is a vertical tower radiator grounded at the base and fed by one or more wires connected to the top of the tower. The three top loading wires extend from the top of the antenna at 120-degree intervals to three terminating towers. Each loading wire has a length approximately equal to the height of the main tower plus 100 feet. The top loading wires are insulated from ground and their tower supports are one-third the height of the transmitting antenna.

Figure 2-12.-Trideco-type antenna.

HIGH FREQUENCY (HF)

High-frequency (hf) radio antenna systems are used to support many different types of circuits, including ship-to-shore, point-to-point, and ground-to-air broadcast. These diverse applications require the use of various numbers and types of antennas that we will review on the following pages.

Yagi

The Yagi antenna is an end-fired parasitic array. It is constructed of parallel and coplaner dipole elements arranged along a line perpendicular to the axis of the dipoles, as illustrated in figure 2-15. The most limiting characteristic of the Yagi antenna is its extremely narrow bandwidth. Three percent of the center frequency is considered to be an acceptable bandwidth ratio for a Yagi antenna. The width of the array is determined by the lengths of the elements. The length of each element is approximately one-half 2-7