Page:Electronics Technician - Volume 7 - Antennas and Wave Propagation - NAVEDTRA 14092.pdf/49

 Figure 2-44.—Phased array.

mit as a group on one frequency with each element having a different phase of that frequency. The different phases are combined at a focal point from the antenna face, which results in the beam being reinforced in the desired direction and suppressed in undesired directions, as shown in figure 2-44. The phase shift is accomplished by computer controlled phase shifters located in series with each radiating element. The computer is pre-programmed or operator overridden to achieve the required direction.

HORN RADIATORS

Horn radiators, like parabolic reﬂectors, may be used to obtain directive radiation at microwave frequencies. Because they do not involve resonant elements, horns have the advantage of being usable over a wide frequency band.

The operation of a horn as an electromagnetic directing device is analogous to that of acoustic horns. However, the throat of an acoustic horn usually has dimensions much smaller than the sound wavelengths for which it is used, while the throat of the electromagnetic horn has dimensions that are comparable to the wavelength being used.

Horn radiators are readily adaptable for use with waveguides because they serve both as an impedance-matching device and as a directional radiator. Horn radiators may be fed by coaxial or other types of lines.

Horns are constructed in a variety of shapes as illustrated in figure 2-45. The shape of the horn and the dimensions of the length and mouth largely determine the field-pattern shape. The ratio of the horn length to mouth opening size determines the beam angle and, thus, the directivity. In general, the larger the opening of the horn, the more directive is the resulting field pattern.

Figure 2-45.-Horn radiators.

FEEDHORNS

A waveguide horn, called a FEEDHORN, may be used to feed energy into a parabolic dish. The directivity of this feedhorn is added to that of the parabolic dish. The resulting pattern is a very narrow and concentrated beam. In most radars, the feedhorn is covered with a window of polystyrene fiberglass to prevent moisture and dirt from entering the open end of the waveguide.

One problem associated with feedhorns is the SHADOW introduced by the feedhorn if it is in the path of the beam. (The shadow is a dead spot directly in front of the feedhorn.) To solve this problem the feedhorn can be offset from center. This location change takes the feedhorn out of the path of the rf beam and eliminates the shadow. An offset feedhorn is shown in figure 2-46.

Figure 2-46.—Offset feedhorn. 2-27