1222 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1954 



base having a duration of a few microseconds. In order to observe the 

 received pulse after a selected number of trips back and forth down the 

 waveguide, a variable delay was placed between the trigger and the 

 oscilloscope horizontal deflection. The discussion which follows will 

 refer to photographs of the oscilloscope under different conditions. By 

 way of preparation, it may be stated that the pulse transmitted through 

 the small hole in the end plate of the waveguide will excite a large num- 

 ber of modes. There are, at 9,000 mc, approximately 40 modes which 

 can propagate in this waveguide. Also, the coupling through the holes is 

 so weak and the energy lost due to dissipation in the shorting plates is 

 so small as to represent an attenuation which is small compared with the 

 theoretical wall loss in the 500-foot long line. Therefore, as the pulse 

 shuttles back and forth in the line, it will decay as though it had trav- 

 elled on a straight long section of waveguide made up of 500-foot long 

 segments identical to the single 500-foot section actually constructed. 



Fig. 11 shows a photograph of the oscilloscope displaying the time 

 interval immediately following the transmitted pulse. The pulse at the 

 extreme left represents the transmitted pulse which passes directly from 

 the transmitter hole to the receiver hole on the end plate of the wave- 

 guide. The blank time interval immediately following the transmitted 

 pulse is about one microsecond long and represents the time of travel of 

 energy down to the far end of the 500-foot line and back to the sending 

 end. During this interval no pulses were received because the joints in 

 the line produce little reflection. The first pulse after the transmitted 

 pulse represents energy travelling in the mode which has the highest 



[* - 500 FT --^ 



Fig. 10 — Diagram of equipment used for pulse tests of waveguide transmission. 



