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 Figure 2-13.—Pan polar antenna.

wavelength, depending on its intended use (driver, reflector, or director). The required length of the array depends on the desired gain and directivity. Typically, the length will vary from 0.3 wavelength for three—element arrays, to 3 wavelengths for arrays with numerous elements. For hf applications, the maximum practical array length is 2 wavelengths. The array's height above ground will determine its vertical radiation angle. Normally, array heights vary from 0.25 to 2.5 wavelengths. The dipole elements are usually constructed from tubing, which provides for better gain and bandwidth characteristics and provides sufficient mechanical rigidity for self—support. Yagi arrays of four elements or less are not structurally complicated. Longer arrays and arrays for lower frequencies, where the width of the array exceeds 40 feet, require elaborate booms and supporting structures. Yagi arrays may be either fixed-position or rotatable.

LOG-PERIODIC ANTENNAS (LPAs)

An antenna arranged so the electrical length and spacing between successive elements causes the input impedance and pattern characteristics to be repeated periodically with the logarithm of the driving frequency is called a LOG-PERIODIC ANTENNA (LPA). The LPA, in general, is a medium-power, high-gain, moderately-directive antenna of extremely broad bandwidth. Bandwidths of up to 15:1 are possible, with up to 15 dB power gain. LPAs are rather complex antenna systems and are relatively expensive. The installation of LPAs is normally more difficult than for other hf antennas because of the tower heights involved and the complexity of suspending the radiating elements and feedlines from the towers.

Vertical Monopole LPA

The log-periodic vertical monopole antenna (fig. 2-16) has the plane containing the radiating elements in a vertical field. The longest element is approximately one-quarter wavelength at the lower cutoff frequency. The ground system for the monopole arrangement provides the image equivalent of the other quarter wavelength for the half-dipole radiating elements. A typical vertical monopole designed to 2-8