FEEDHORNS

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 2-43. 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 2-43.—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 2-44. RADAR SYSTEMS Now that you have a basic understanding of how radar antennas operate, we will introduce you to a few of the radar systems currently in use. Figure 2-44.—Offset feedhorn. AN/GPN-27(ASR-8) AIR SURVEILLANCE RADAR The AN/GPN-27(ASR-8) (fig. 2-45) antenna radiates a beam 1.5 degrees in azimuth and shaped in elevation to produce coverage of up to approxi- mately 32 degrees above the horizon. This provides a maplike presentation of aircraft within 55 nautical miles of an airport terminal. The antenna azimuth Figure 2-45.—AN/GPN-27(ASR-8) air surveillance radar. 2-26