RADAR ANTENNAS 



275 



A lobing antenna which is to provide information concerning one angle only, 

 azimuth for example, is capable of producing two beams, one at a time, 

 and of switching rapidly from one to the other. This process is called 

 Lobe Stvitclniig. The two beams are nearly coincident, differing in direction 

 by about one beam width. When the signals from the two beams are com- 

 pared, they will be equal only if the target lies on the bisector between the 

 beams (Fig. 35). The two signals can be compared visually on an indicator 

 screen of the radar or they can be compared electrically and fed directly 

 into circuits which control the direction of fire. 



ANTENNA DIRECTED 

 TO LEFT OF TARGET 



ANTENNA DIRECTED 

 AT TARGET 



ANTENMA DIRECTED 

 TO RIGHT OF TARGET 



RELATIVE SIGNALS FROM TWO BEAMS 



Fig. 35 — Lobe Switching. 



When two perpendicular directions are to be determined, such as the 

 elevation and azimuth required by an anti-aircraft battery, four or in prin- 

 ciple three discrete beams can be used. Radar antennas designed for solid 

 angle coverage more commonly, however, produce a single beam which ro- 

 tates rapidly and continuously around a small cone. This rotation is 

 known as conical lobing. A comparison of amplitudes in a vertical plane 

 can then be used to give the elevation of the target and a similar comparison 

 in a horizontal plane to give its azimuth. Here too the electrical signals 

 can be compared visually on an indicator screen, but an electrical comparison 

 will provide continuous data which can be used to aim the guns and at the 

 same time to cause the radar antenna to follow the target automatically. 



11.1 Lobe Sivitching 



Two methods of lobe switching are common. In one of these the lobing 

 antenna is an array of two equally excited elements. Each of these ele- 



