3-6] FACTORS AFFECTING ANGULAR RESOLUTION 173 



Very large degradations of resolution can often be attributed to non- 

 linearities in the receiving system. The dynamic range of many search 

 radar systems is less than 10 db above the average noise level, and 20 db 

 is rare. The apparent beamwidth when scanning a very strong target with 

 a system which has limited dynamic range can be as great as twice the 

 normal beamwidth. In such cases, it is quite possible for large targets to 

 completely blank out smaller adjacent targets which might have been 

 resolved with a linear system. 



Two other minor factors might be noted, the effects of pulsing and 

 the system bandwidth. When only a limited number of pulses compose 

 the envelope generated by a scan over the target, the exact form of the 

 continuous envelope is somewhat indeterminate. As an extreme example, 

 if only two pulses are received during a scan over a target, the question 

 arises as to whether these are two pulses from a single strong target or from 

 two weaker targets. The effect of pulsing can be regarded as a widening of 

 the effective beamwidth. Equation 3-65 gives a simple and useful approxi- 

 mation for the equivalent effective beamwidth in terms of the actual 

 beamwidth and the angular interval between pulses: 



Effective beamwidth = V©' + ^^" (3-65) 



where 9 = antenna beamwidth 



A^ = angular interval between pulses. 



The antenna pattern described by Equation 3-63 and illustrated in Fig. 

 3-15 assumed monochromatic radiation. In some applications where very 

 wide bandwidths are required, the antenna beamwidth will be modified. 

 Such an application might be the use of microwave radiometers for map- 

 ping. When there is no chromatic aberration (approximately true when 

 a parabolic reflector is used) and the average frequency is maintained 

 constant, the increase in beamwidth with bandwidth is small. A maximum 

 beamwidth increase of about 5 per cent is given for a bandwidth of 15 

 per cent of the average frequency. 



