274 BELL S YSTEM TECH NIC A L JOURNA L 



10.2 Cylindrical Cosecant Antennas 



Harmful cross polarized radiation is produced by doubly curved reflectors. 

 This radiation is dillicult to control and therefore undesirable where a closely 

 controlled cosecant characteristic at high angles is required. Although not 

 at first evident, it seems natural now to bypass polarization difficulties 

 through the use of singly curved cylindrical reflectors. These reflectors if 

 illuminated with a line source of closely controlled linear polarization provide 

 a beam which is linearly polarized. This beam has also in azimuth approxi- 

 mately the directivity of the line source at all vertical angles. It is thus 

 superior in two significant respects to cosecant beams produced by doubly 

 curved reflecting surfaces. 



A cylindrical cosecant antenna consists of a cylindrical reflector illumi- 

 nated by a line source. Part of the cylinder is almost parabolic and con- 

 tributes chiefly to the strong part of the beam which lies closest to the hori- 

 zontal. This part is merged continuously into a region which departs 

 considerably from the parabolic and contributes chiefly to the radiation at 

 higher angles. 



Although wave front principles can be used and certainly must not be 

 violated, the principles of geometrical optics have been particularly effective 

 in the determination of cosecant reflector shapes. The detailed application 

 of these principles will not be discussed here. While applying the geo- 

 metrical principles the designer must be sure that the over-all size and con- 

 figuration of the antenna can produce the results he wants. He must design 

 a line source with the desired polarization and horizontal pattern and a 

 vertical pattern which fits in with the cosecant design. In addition he must 

 take particular care to reduce sources of pattern distortion to a level at 

 which they cannot interfere significantly with the lowest level of the cose- 

 cant 'tail'. 



11. LOBING 



In many of the tactical situations of modern war radar can be used to 

 provide fire control information. Radar by its nature determines range and 

 microwave radar with its narrow well defined beams is a natural instrument 

 for finding directions to a target, whether the missile to be sent to that 

 target is a shell, a torpedo or a bomb. In fire control radar, as opposed to 

 search or navigational radar, two properties of the antenna deserve par- 

 ticular attention. These are the accuracy and the rate with which direc- 

 tion to a target can be measured. 



Lobing is a means which utilizes to the fullest extent the accuracy avail- 

 able from a given antenna aperture and which increases, usually as far as 

 is desired, the rate at which this information is provided and corrected. 



