RADAR ANTENNAS 255 



used for a particular job. They may help in choosing a focal length and in 

 determining which tinite portion of a theoretically infinite parabolic curve 

 should be used. Finally they may assist in determining whether reflector 

 technique is really the best for the purpose at hand or whether we could do 

 better with a lens or an array. 



In designing a parabolic antenna it must obviously be decided at an early 

 stage whether a paraboloid or one or more parabolic cylinders are to be 

 employed. This choice must be based on a number of mechanical and elec- 

 trical considerations. Paraboloids are more common in the radar art than 

 parabolic cylinders and are probably to be preferred, yet a categorical a 

 priori judgment is dangerous. It will perhaps be helpful to compare the 

 two alternatives by the simple procedure of enumerating some features in 

 which each is usually preferable to the other. 

 Paraboloidal antennas 



(a) are simpler electrically, since point sources are simpler than line 

 sources. 



(b) are usually lighter. 



(c) are more efficient. 



(d) have better patterns in the desired polarization. 



(e) are more appropriate for conical lobing or spiral scanning. 

 Antennas employing parabolic cylinders 



(a) are simpler mechanically since only singly curved surfaces are 

 required. 



(b) have separate electrical control in two perpendicular directions. 

 This last advantage of parabolic cylinders is important in special antennas, 



many of which will be described in later sections. It is useful where an- 

 tennas with very large aspect ratios (ratio of dimensions of the aperture in 

 two perpendicular directions) are desired. It is highly desirable where con- 

 trol in one direction is to be achieved through some special means, as in 

 cosecant antennas, or in antennas which scan in one direction only. 



Let us suppose that we have selected the aperture dimensions and have 

 decided whether the reflector is to be paraboloidal or cylindrical. The 

 reflector is not yet completely determined for we are still free in principle to 

 use any portion of a parabolic surface of any focal length. In order to 

 obtain economy in physical size the focal length is generally made between 

 0.6a and 0.25a where 'a' is the aperture. For the same reason a section of 

 the reflecting surface which is located symmetrically about the vertex is 

 often chosen (Figures 3 and 19). 



When a symmetrically located section of the reflector is used certain diffi- 

 culties are introduced. These difficulties, if serious enough so that their 

 removal justifies some increase in size can be bypassed through the use of an 



