64 



THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1956 



gone further in the higher loss TE21 mode. Conversions from the piston 

 end of the guide return in the center of the pulse, and only in this re- 

 gion do piston phasing effects appear. As the frequency is changed the ' 

 pattern changes, until it reaches the extreme shape shown in the next- 

 to-the-bottom trace, with this narrower pulse coming at a time corre- 

 sponding to the center of the broadened pulse at the top. Further fre- 

 quency change in the same direction returns the shape to that of the 

 top traces. At the frequency giving the received pulse shown on the 

 next-to-the-bottom trace, moving the far-end piston causes a gradual 

 change to the shape shown on the lowest trace. This makes it appear 

 as if the mode conversion were coming almost entirely from the part of 

 the guide near the piston end at this frequency. The upper traces appear 

 to show that more energy is converted at the transducer end of the 

 waveguide at that frequency. It would seem that at certain frequencies 

 some phase cancellation is taking place between conversion points 

 spaced closely enough to overlap within the pulse width . At frequencies 

 between the ones giving traces like this, the appearance is more like 

 that shown for the TE31 mode on Fig. 19 except for the slope across the 

 top of the pulse being reversed. The highest part of this TEoi pulse is 



Fiff. 20 — Received pulse patterns witli the urrangemeiit of Fig. 18 used for 

 studying conversion to the TE21 mode. 



