WAVEGUIDE AS A COMMUNICATION MEDIUM 



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Fig. 11 — Photograph of cathode-ray tube presentation during the time inter- 

 val immediately following the transmitted pulse. 



group velocity. Pulses immediately following this first received pulse 

 represent energy travelling in other modes whose velocities are lower 

 and which therefore recjuire more time for the one round trip of travel. 

 At the time 2Af, we begin to observe pulses which have made two round 

 trips in the Une. If the transmitted pulse width were short enough, we 

 could theoretically identify the mode in which the energy travelled by 

 observing the time of arrival, since the velocities of propagation and the 

 distance are known parameters. The ^o microsecond pulse used in 

 these experiments is not short enough to allow this kind of resolution on 

 an individual mode basis. Something on the order of five or six modes 

 have velocities so nearly the same that they cannot be resolved as sep- 

 arate pulses with the Ho microsecond pulse after a single round trip in 

 the 500-foot line. 



Fig. 12 represents the same condition as Fig. 1 1 , except that the 

 horizontal time base has been changed to display the interval to 14Af 

 instead of the interval to 2At. Fig. 12 shows fewer pulses in the time 

 interval 6At to 12Ai than in the time interval to 6A^. This is because 

 energy travelling in some modes is attenuated more rapidly than that 

 in other modes. For time delays greater than 10 At the received pulses 



