RADAR ANTENNAS 



265 



tance from the vertex a circular curvature of phase results and the system 

 is said to be 'defocussed' (Sec. 3.9). As the feed is moved off the axis of 

 the reflector the first effect is a shifting of the beam due to a linear variation 

 of the phase (Sec. 3.8). For greater distances off axis a cubic component of 

 phase error becomes effective (Sec. 3.10). Phase error, whether circular, 

 cubic or more complex, results in a reduction in gain and usually in an in- 

 crease of minor lobes. Although the effects of given amounts of phase curva- 

 ture on the radiation characteristics of an antenna can be estimated by theo- 

 retical means, it is usually easier and quicker to find them experimentally. 



5 



S 25 



UJ 



in 

 10 30 



45 

 50 



55 



30 25 20 15 10 5 5 10 15 20 25 30 



HORIZONTAL ANGLE IN DEGREES 



Fig. 28 — Effect of Small Inaccuracies in Reflector. 



The tolerances on the feed itself appear in various forms, many of which 

 can be examined with the aid of transmission line theory and most of which 

 are too detailed for discussion in this paper. It is generally true here also 

 that experiment is a more effective guide than theory. 



Experience has shown that when parallel plate systems are used, either 

 as complete antennas or as line feeds for other elements, tolerances on the 

 parallel conducting plates must be considered carefully. It is obvious that 

 when the TEoi mode is used the plate spacing must be held closely, since 

 the phase velocity is related to the spacing. This spacing can be controlled 

 through the use of metallic spacers perpendicular to the plates. These 



