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 arrays
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 approxi-
mately 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