Figure 2-19.—Inverted cone antenna.
gains of 1 to 5 dB above isotropic antennas, with a vswr not greater than 2:1. They are considered medium-to high-power radiators, with power handling capabilities of 40 kW average power.
CONICAL MONOPOLE ANTENNA
Conical monopoles are used extensively in hf communications. A conical monopole is an efficient broadband, vertically polarized, omnidirectional antenna in a compact size. Conical monopoles are shaped like two truncated cones connected base-to-base. The basic conical monopole configuration, shown in figure 2-20, is composed of equally- spaced wire radiating elements arranged in a circle around an aluminum center tower. Usually, the radiating elements are connected to the top and bottom discs, but on some versions, there is a center waist disc where the top and bottom radiators are connected. The conical monopole can handle up to 40 kW of average power. Typical gain is -2 to +2 dB, with a vswr of up to 25:1.
RHOMBIC ANTENNA
Rhombic antennas can be characterized as high-power, low-angle, high-gain, horizontally-polarized, highly-directive, broadband antennas of simple, inexpensive construction. The rhombic antenna (fig. 2-21) is a system of long-wire radiators that depends on radiated wave interaction for its gain and directivity. A properly designed rhombic antenna presents to the transmission line an input impedance insensitive to frequency variations up to 5:1. It maintains a power gain above 9 dB anywhere within a 2:1 frequency variation. At the design-center frequency, a gain of 17 dB is typical. The radiation pattern produced by the four radiating legs of a rhombic antenna is modified by reflections from the earth under, and immediately in front of, the antenna.
Figure 2-20.-Conical monopole antenna.
Because of the importance of these ground reflections in the proper formation of the main lobe, the rhombic should be installed over reasonably smooth and level ground. The main disadvantage of the rhombic antenna is the requirement for a large land area, usually 5 to 15 acres.
QUADRANT ANTENNA
The hf quadrant antenna (fig. 2-22) is a special-purpose receiving antenna used in ground-to-air-to-ground communications. It is unique among horizontally-polarized antennas because its
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