WAVEGUIDE TESTING WITH MILLIMICROSECOND PULSES 



49 



in Fig. 12. In this arrangement energy was sent into the round line from 

 a probe inserted in the side of the guide. This couples to all of the 12 

 modes which can be supported, with the exception of the TEoi circular 

 electric mode. The sending end of the round guide was terminated. A 

 directional coupler is connected to the sending probe so that the return 

 from the piston at the far end can be observed on the receiver. Because 

 of the different time that each mode takes to travel one round trip in 

 this waveguide, which was 258 feet long, separate pulses are seen for 

 each mode. The pulses in this figure have been marked to show which 

 mode is being received. 



The time of each pulse referred to the outgoing pulse was measured 

 and found to check very well with the calculated time. The formula for 

 the time of transit in the waveguide for any mode is: 



T = 



0.98322V'1 - VnJ 



[where T = time in millimicroseconds 



L = length of pulse travel in feet 



Vnm ^^ A /Ac 



X = operating wavelength in air 



Ac = cutoff wavelength of guide for the mode involved. 



[ Table I — Calculated and Measured Value of Time for One 



Round Trip 





