52 



THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1956 



Table II — Calculated Probe Coupling Factor, Line Loss and 

 Pulse Height for Each Mode 



Since the TE12 pulse is so weak, it would not show on the trace even if 

 it were resolved on a time basis. Coupling to the TM02 mode is rather 

 weak, and the gain was increased somewhat at its position on the trace 

 to show its time location. 



8. DELAY distortion 



Another effect of the wide bandwidth of the pulses used with this 

 equipment can be observed in Fig. 12. The pulses that have traveled 

 for a longer time in the guide are in the modes closer to cutoff, and are 

 on the right-hand side of the oscilloscope trace. They are broadened 

 and distorted compared with the ones on the left-hand side. This effect 

 is due to delay distortion in the guide. This can be explained by refer- 

 ence to Fig. 13. On this figure the ratio of group velocity to the velocity 

 in an unbounded medium is shown plotted as a function of frequency 

 for each of the modes that can be propagated. The bandwidth of the 

 transmitted pulse is indicated by the vertical shaded area. It will he 

 noticed that the spacing of the pulses on the oscilloscope trace on Fig. 

 12 from left to right in time corresponds to the spacing of the group 

 velocity curves in the bandwidth of the pulse from top to bottom. De- 

 lay distortion on these curves is shown by the slope of the line across 

 the pulse bandwidth. If the line were horizontal, showing the same group 

 velocity at all points in the band, there would be no delay distortion. 

 The greater the difference in group A-elocity at the two edges of the 

 band, the greater the delay distortion. The curves of Fig. 13 indicate 



