60 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1956 



an arrangement having oval sections deliberately placed in the wave- ' 

 guide in order to explain the method. Pure TEoi excitation is vised, and 

 the converted TE21 mode observed with a coupled line transducer giv- ; 

 ing an output for that mode alone. ; 



Let us consider first what would happen with the far-end squeezed; 

 section alone, omitting the near-end squeezed section from considera- • 

 tion. The injected TEoi mode signal would then travel down the 250 , 

 feet of 3" diameter round waveguide to the far end with substantially, 

 no mode conversion at the level being measured. At this point it goes 

 through the squeezed section. Conversion now takes place from the TEou, 

 mode to the TE21 mode. Both these modes after reflection from the piston 

 travel back up the waveguide to the sending end. The group velocity 

 of the TE21 mode is higher than the group velocity of the TEoi mode, so 

 energy in these two modes separates, and if a coupling system were 

 used to receive energy in both modes, two pulses would appear, with at 

 time separation between them. In this case, since the receiver is con- 

 nected to the line through the coupled line transducer which is responsive 

 only to the TE21 mode, only one pulse is seen, that due to this mode 

 alone. This is the center pulse in the trace photograph at the top of 

 Fig. 17. If only one mode conversion point at the far end of the guide 

 exists, only this one pulse is seen at the receiver. It would be spaced a 

 distance away from the injected outgoing pulse that corresponds m:^ 

 time to one trip of the TEoi mode down to the far end and one trip of || 

 the TE21 mode from the far end back to the receiver. 



Now let us consider what would happen if the near-end squeezed sec- 

 tion alone were present. When the TEqi wave passes the oval section! 

 just beyond the coupled line transducer, conversion takes place, andi 

 the energy travels down the line in both the TEoi and the TE21 modes,:; 

 at a higher group velocity in the TE21 mode. These two signals are re- 

 flected by the piston at the far end and return to the sending end. The 

 TE21 signal comes through the coupled line transducer and appears as 

 the pulse at the left of the photo shown on Fig. 17. Now the TEoi energy 

 has lagged behind the TE21 energy, and when it gets back to the near- 

 end squeezed section, a second mode conversion takes place, and TE21 

 mode energy is produced which comes through the coupled line trans-: 

 ducer and appears at the receiver at the time of the right hand pulse. 

 The spacing between these two pulses is equal to the difference in round 

 trip times between the two modes. 



In general, for a single conversion source occurring at any point in 

 the line, two pulses will appear on the scope. The spacing between these 

 pulses corresponds to the difference in group velocity between the modes. 



