334 GENERIC TYPES OF RADAR SYSTEMS AND TECHNIQUES 



X/j Radians (Theoretical) 



Near Zone-c — ^Far Zone 



Fig. 6-29 Antenna Beamwidth Pattern. 



shown in Fig. 6-29. As can be seen, the concept of angular beamwidth holds 

 only for the so-called far zone (Fraunhofer zone) where the range R is 

 greater than d"^ l\P At closer ranges, the effective pattern width is variable, 

 but as a general rule it can be considered equal to the antenna dimension. 

 Thus the resolution obtainable with an antenna aperture of (S' feet cannot be 

 better than d feet regardless of what is implied by the angular beamwidth 

 expression. 



For a given antenna aperture and operating frequency, certain techniques 

 such as monopulse and sidelobe cancellation are useful for "sharpening" the 

 beam and thereby obtaining better definition (that is to say the transition 

 of the signal return as the beam crosses an isolated target will be sharper). 

 Improved resolution has been claimed from the use of these techniques. 

 Such claims rest upon relatively shaky theoretical grounds and are based 

 more upon the appearance of better resolution resulting from sharper 

 definition than upon a rational repudiation of the basic laws governing the 

 formation of interference patterns. 



In certain cases where prior knowledge of the target characteristics exists, 

 velocity resolution may be employed to give the appearance of better angular 

 resolution than one would predict from the beamwidth. Such a case is 

 shown in Fig. 6-30 where an antenna points straight down from an airborne 



V^2 = \^FSinQ: f^2 



2Vf sino : 

 X 



Fig. 6-30 Improvement of Apparent Angular Resolution by Doppler Filtering. 



l^See S. Silver, Microwave Antenna Theory and Design, Chap. 6, McGraw-Hill Book Co. 

 Inc., 1949. 



