RADAR ANTENNAS 251 



7.2 Amplitude Distribution 



Except where special, in particular cosecant, patterns are desired the 

 principle factors affecting amplitude distribution are efficiency and required 

 minor lobe level. The amplitude distribution or taper of an ideal uniphase 

 rectangular wave front affects the minor lobe level as indicated by Figures 1 1 , 

 12 and 13. Practical antennas tend to fall somewhat below this ideal picture 

 because of non-uniform phase and because of variations from the ideal 

 amplitude distribution due to discontinuities in the aperture and undesired 

 leakage or spillover of energy. Nevertheless a commonly used rule of thumb 

 is that minor lobes 20 db or more below the peak radiation level are tolerable 

 and will not be exceeded with a rounded amplitude taper of 10 or 12 db. 



7.3 Phase Control 



Uniphase wave fronts are used whenever a simple pattern with prescribed 

 gain, beam widths and minor lobes is to be obtained with minimum aperture 

 dimensions. When special results are desired such as cosecant patteri^s or 

 scanning beams the phase must be varied in special ways. 



Mechanical tolerances in the antenna structure make it impossible to hold 

 phases precisely to the desired values. The accuracy with which the phases 

 can be held constant in practice varies with the technique, the antenna size 

 and the wave length. Undesired phase variations increase the minor lobes 

 and reduce the gain of an antenna. The extent to which phase variations 

 can be expected to reduce the gain is indicated in Fig. 17. 



8. Parabolic Antennas 



The headlights of a car or the searchlights of an antiaircraft battery use 

 reflectors to produce beams of light. Similarly the majority of radar anten- 

 nas employ reflectors to focus beams of microwave energy. These reflectors 

 may be exactly or approximately parabolic or they may have special shapes 

 to produce special patterns. If they are parabolic they may be paraboloids 

 which are illuminated by point sources and focus in both directions, or they 

 may be parabolic cylinders which focus in only one direction. If they are 

 parabolic cylinders they may be illuminated by line sources or they may be 

 confined between parallel conducting plates and illuminated by point sources 

 to produce line sources. 



8.1 Control of Phase 



A simple and natural way to distribute energy smoothly in space is to 

 radiate it from a relatively nondirectional 'primary' source such as a dipole 

 array or an open ended wave guide. This energy will be formed into a direc- 

 tive beam if a reflector is introduced to bring it to a plane area or wave front 

 with constant phase. If the primary source is effectively a point as far as 



