matching device and as a directional radiator. Hornradiators may be fed by coaxial or other types of lines.Horns are constructed in a variety of shapes asillustrated in figure 2-43. The shape of the horn andthe dimensions of the length and mouth largelydetermine the field-pattern shape. The ratio of thehorn length to mouth opening size determines the beamangle and, thus, the directivity. In general, the largerthe opening of the horn, the more directive is theresulting field pattern.Figure 2-43.—Horn radiators.FEEDHORNSA waveguide horn, called a FEEDHORN, maybe used to feed energy into a parabolic dish. Thedirectivity of this feedhorn is added to that of theparabolic dish. The resulting pattern is a very narrowand concentrated beam. In most radars, the feedhornis covered with a window of polystyrene fiberglassto prevent moisture and dirt from entering the openend of the waveguide.One problem associated with feedhorns is theSHADOW introduced by the feedhorn if it is in thepath of the beam. (The shadow is a dead spot directlyin front of the feedhorn.) To solve this problem thefeedhorn can be offset from center. This locationchange takes the feedhorn out of the path of the rfbeam and eliminates the shadow. An offset feedhornis shown in figure 2-44.RADAR SYSTEMSNow that you have a basic understanding of howradar antennas operate, we will introduce you to a fewof the radar systems currently in use.Figure 2-44.—Offset feedhorn.AN/GPN-27(ASR-8) AIRSURVEILLANCE RADARThe AN/GPN-27(ASR-8) (fig. 2-45) antennaradiates a beam 1.5 degrees in azimuth and shapedin elevation to produce coverage of up to approxi-mately 32 degrees above the horizon. This providesa maplike presentation of aircraft within 55 nauticalmiles of an airport terminal. The antenna azimuthFigure 2-45.—AN/GPN-27(ASR-8) airsurveillance radar.2-26
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