}riCKO]]AVE RADAR TESTING 441 



near misses, and the combinations of shock and vibration connoted by the 

 requirement of "transportation over all types of terrain in any Army vehi- 

 cle." Test and experience have made it possible to translate these general 

 requirements into two specific requirements, namely the ability to withstand 

 (1) vibration at frequencies from 10 to v^3 cycles per second with yg" excur- 

 sion for 30 minutes in each of three axes and (2) the shock produced by a 400 

 lb. hammer falling through distances of 1, 2 and 3 ft. in each of three axes, 

 and striking an anvil to which the set is attached. These requirements 

 have been met without using shock and vibration mounts, which are unde- 

 sirable in test sets because they increase size and weight. 



Range Capability 



The range capability of a radar, like that of any radio system, depends 

 upon three things; the transmitted power, the loss in the medium, and the 

 minimum perceptible received signal. Two of these can be combined by 

 taking the ratio of the radiated signal to the minimum perceptible received 

 signal. This ratio, ordinarily expressed as a level difference in db, is vari- 

 ously termed the "system performance," "over-all performance" or merely 

 the "level difference." It may be determined by separate measurement of 

 transmitter power and receiver sensitivity, or by a single overall measure- 

 ment. With the powers and sensitivities commonly employed in radar, 

 the level difference is of the order of 150 to 180 db. 



The actual range that can be spanned for a given performance ratio varies 

 considerably. For a given transmitted power, the echo power received by 

 a radar theoretically varies inversely as the fourth power of the range. The 

 reason for this is simple. In free space the power intercepted by a target 

 which is small in comparison with the area of the radar beam in its vicinity 

 will var>^ according to the inverse square of the distance from the radar. 

 Similarly, that fraction of the energy reflected from the target which is 

 intercepted by the receiving antenna will var>- as the inverse square law. 

 Since the received power involves the product of these two factors, the rela- 

 tion becomes: 



P, = x|;, or, R-{K^y (1) 



where Pi and Pr represent, respectively, the transmitted and received power, 

 R the range and K a constant determined by antenna design, character of 

 target, etc. 



Under operating conditions considerable departure from the ahovQ rela- 

 tionship m.ay be experienced, due to such factors as (1) the curvature of the 

 earth, (2) interference between the direct beam and single or multiple re- 

 flections, and (3) attenuation due to atmospheric absorption. Except under 



