56 



THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1956 



switch was then taken through the directional coupler to the receiver 

 as shown by the output arrow. The series of pulses at the left-hand 

 photograph of the oscilloscope traces was taken with this waveguide i 

 shorting switch closed. The top pulse shows the direct leakage across 

 the inside of the end plate before it has traveled through the 3" round 

 guide. The next pulse is marked one round trip, having gone therefore 

 214 feet in the TEoi mode in the round waveguide. The successive pulses 

 have traveled more round trips as shown by the number in the center 

 between the two photographs. The effect of increased delay distortion 

 broadening and distorting the pulse can be seen as the numbers increase. 

 The values of fB from the Darlington formula in the previous section 

 for these lengths are given in Table IV. 



It will be noticed that pulse broadening, and eventually severe dis- 

 tortion, occurs as fB decreases much below the 175-mc pulse band- 

 width. The effect is gradual, and not too bad a pulse shape is seen until 

 fB is about half the pulse bandwidth, although broadening is very 

 evident earlier. 



When the waveguide short-circuiting switch was opened so that the 

 tapered delay distortion equalizer was used to reflect the energy, in- 

 stead of the switch, the series of pulses at the right was observed on 

 the indicator. It will be noted that there is much less distortion of these, 

 pulses, particularly toward the bottom of the series. The ones at the top, 

 have less distortion than would be expected, probably because of fre-, 

 quency modulation of the injected pulse. The equalizer consists of a 

 long gradually tapered section of waveguide which has its size reduced 

 to a point beyond cutoff for the frequencies involved. Reflection takes 

 place at the point of cutoff in this tapered guide. For the high frequency 

 part of the pulse bandwidth, this point is farther away from the short- 

 ing switch than for the low frequency part of the bandwidth. Conse- 

 quently, the high frequency part of the pulse travels farther in one round 

 trip into this tapered section and back than the low frequency part of 



Table IV — Values of fB from the Darlington Formula 

 FOR the Arrangement Show^n in Fig. 14 



li 



