272 



BELL SYSTEM TECHNICAL JOURNAL 



On the other hand, because paraboloidal antennas are simple and common 

 it is natural that many cosecant designs should be based on them. These 

 designs can be classified into two grouj)s, those in which the reflector is 

 modified and those in which the feed is modified. 



Some early cosecant antennas were made by introducing discontinuities 

 in paraboloidal reflectors as illustrated in Fig. i3. These controlled the 

 radiation more or less as desired over the desired cosecant pattern but pro- 



NORMAL 

 PARABOLOID / 

 SURFACE / 



PARABOLOID 

 SURFACE 



Fig. 33 — Some Cosecant Antennas Based on the Paraboloid (Cosecant Energy Down- 

 ward). 



duced rather serious minor lobes elsewhere. This difficulty can be overcome 

 through the use of a continuously distorted surface as illustrated in Fig. 34. 

 This reflector, flrst used at the Radiation Laboratories, is a normal parabo- 

 loid in the lower part whereas the upper part is the surface that would be 

 obtained by rotating the parabola through the vertex of the upper part about 

 its focal j)oint. 



Several types of feed have been used in combination with paraboloids to 

 produce cosecant patterns. These are usually arrays which operate on the 

 princij)lc that each element is a feed which contributes principally to one 



