264 BELL SYSTEM TECHNICAL JOURNAL 



of elements excited with the proper phase and amplitude can also be 

 used. Some discussion of alternative approaches will appear in the section 

 on scanning techniques. 



8.7 Tolerances in Parabolic Antcinias 



The question of tolerances will always arise in practice. Ideal dimensions 

 are only approximated, never reached. The ease of obtaining the required 

 accuracy is an important engineering factor. 



The tolerances in paraboloidal antennas or in parabolic cylinders illu- 

 minated by line sources can be divided into three general classes: 



(a) Tolerances on reflecting surfaces. 



(b) Tolerances on spacial relationships of feed and reflector. 



(c) Tolerances on the feed. 



When the actual reflector departs from the ideal parabolic curve deviations 

 in the phase will result. These will tend to reduce the gain and increase the 

 minor lobes. The effects of such deviations on the gain can be estimated 

 with the help of Fig. 17. We should recall that an error of a in the reflector 

 surface will produce an error of about 2<j in the phase front. Based on this 

 kind of argument and on experience reflector tolerances are generally set in 



X X 



practice to about ± 77 or ± ~ dependmg on the amount of beam deteriora- 

 tion that can be permitted. 



In Fig. 28 are compared some electrical characteristics of two paraboloidal 

 antennas, one employing a precisely constructed paraboloidal searchlight 

 mirror and the other a carefully constructed wooden paraboloidal reflector 

 with the same nominal contour. This comparison is revealing for it shows 

 the harm that can be done even by small defects in the reflector surface. 

 Although the two patterns are almost identical in the vicinity of the main 

 beam, the general minor lobe level of the wooden reflector remains higher 

 at large angles and its gain is less. 



It must not, however, be assumed that a solid reflecting surface is neces- 

 sary to insure excellent results. Any reflecting surface which reflects all 

 or most of the power is satisfactory provided that it is properly located. Per- 

 forated reflectors, reflectors of woven material and reflectors consisting of 

 gratings with less than half wavelength spacing are commonly used in radar 

 antenna practice. These reflectors tend to reduce weight, wind or water 

 resistance and visibility. Many of them will be described in Part III of 

 this paper. 



The feed of a parabolic reflector should be located so thai its i)hase center 

 coincides with the focus of the reflector. If it is located at an incorrect dis- 



