MICROWAVE RADAR TESTING 



461 



For an untuned or multi-resonant echo box, rectangulaf shape is con- 

 venient. The box should be large enough to make it highly probable that 

 over the operating band one or more modes will be present within any fre- 

 quency inter\'al of width equal to the main concentration of the transmitter 

 spectrum. For a given rectangular volume a cube gives the largest number 

 of modes. The total number of modes up to a frequency of wave length 

 X is 



N^f = 8.38 F/Xo 



(8) 



where T' is the volume. However, because of the cubical shape many dif- 

 ferent modes tend to coincide in wavelength, a condition referred to as 



i^^**u.;^^ 



Fig. 14 — A grouji of echo boxes of various types. 



degeneracy. To spread out the modes, the box is made slightly off cube and 

 one or more corners sliced off. At the longer microwaves the size of box 

 is determined by the nimiber of modes, and the size becomes quite awkward. 

 For the shorter microwaves the size is determined by the required value 

 of Q. Hence tlie use of untuned echo boxes has been limited to the fre- 

 quency range from about 9000 mc upwards with sizes of the order of 12 to 

 24 inches on a side. Even with an extraordinarily high probability of find- 

 ing modes within the radar band, substantial differences in response are 

 found for relatively small changes in frequency. Accordingly untuned echo 

 boxes are more useful for rough tune-up than for precise measurement. 

 A number of specific designs of echo boxes for different microwave bands 

 are shown in Fig. 14. 



