RADAR ANTENNAS 277 



It is sometimes necessary to obtain accurate and rapid information from 

 all regions within an agular sector. It may be necessary to watch a certain 

 region of space almost continuously in order to be sure of picking up fast 

 moving targets such as planes. To accomplish any of these ends we must 

 use a rapid scanning radar. A rapid scanning radar antenna produces a 

 beam which scans continuously through an angular sector. The beam may 

 sweep in azimuth or elevation alone or it may sweep in both directions to 

 cover a solid angle. An azimuth or elevation scan may be sinusoidal or it 

 may occur linearly and repeat in a sawtooth fashion. Solid angle scanning 

 may follow a spiral or flower leaf pattern or it might be a combination of two 

 one way scans. A combination of scanning in one direction and lobing in 

 the other is sometimes used. 



Scanning antennas must, unfortunately, be constructed in obedience to 

 the same principles which regulate ordinary antennas. The same attention 

 to phase, amplitude, polarization and losses is necessary if comparable 

 results are to be obtained. When scanning requirements are added to 

 these ordinary ones new problems are created and old ones made more 

 difficult. 



An antenna in order to scan in any specified manner must act to produce 

 a wave front which has a constant phase in a plane which is always normal 

 to the required beam direction. This can be done in several different ways. 

 The simplest of these, electrically, is to rotate a fixed beam antenna as a 

 whole in the required fashion. This can be called mechanical scanning. 

 Alternatively an antenna array can be scanned if it is made up of suitable 

 elements and the relative phases of these elements can be varied appro- 

 priately. This can be called array scanning. Thirdly, optical scanning 

 can be produced by moving either the feed or the focussing element of a 

 suitably designed optical antenna. 



12.1 MecJianical Scanning 



Electrical complexities of other types of rapid scanners are such that it 

 is probably not going too far to say that the required scan should be accom- 

 plished by mechanical means wherever it is at all practical. This applies 

 to radar antenna scans which occur at a slow or medium rate. Search 

 antennas, whether they rotate continuously through 360° or back and forth 

 over a sector are scanners in a sense but the scan is usually slow enough to 

 be performed by rotating the antenna structure as a whole. As the scan 

 becomes more rapid, mechanical problems become more severe and elec- 

 trically scanning antennas appear more attractive. 



Mechanical ingenuity has during the war extended the range in which 

 mechanical scanners are used. One important and eminently practical 

 mechanical rapid scanner, the 'rocking horse' is now in common use (Fig. 

 36). This antenna is electrically a paraboloid of elliptical aperture illu- 



