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THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1956 



narrow pulse like the transmitted one is all that appears on the indicator 

 oscilloscope. It is not possible from this to get any information about 

 the location or extent of the conversion points in the line. Moving the 

 far end piston does not change the relative phases of the modes, so no 

 changes are seen in indicator pattern or pulse level as the piston is 

 moved. For the Holmdel waveguides, which are about 500 feet long, 

 the total round trip T]\In mode converted level varies from 32 to 36 db 

 below the input TEoi mode level over a frequency range from 8,800 to 

 9,600 mc per second. 



All the other modes have velocities that are different than that of 

 the TEoi mode. ^Vhen mode conversion takes place at many closely 

 spaced points along the waveguide, the pulses from the various sources 

 overlap, and phasing effects take place. In general, a filled-in pulse 

 much longer than the injected one is observed. The maximum possible, 

 but not necessary, pulse length is equal to the difference in time re- 

 quired for the TEoi mode and the converted mode to travel the total 

 waveguide length being observed. The phasing effects within the broad- 

 ened pulse change its height and shape as a function of frequency and 

 line length. 



Measurements of mode conversion from TEoi to TE31 in these wave- 

 guides illustrate distributed sources and piston phasing effects. The 

 TE3, mode has a group velocity 1.4 per cent slower than the TEoi mode. 

 For a full round trip in the 500-foot lines, assuming conversion at the 

 imput end, this causes a time separation of about two and one half 

 pulse widths between these two modes. The received pulse is about two 

 and a half times as long as the injected pulse, indicating rather closely 

 spaced sources over the whole line length. For one far-end piston posi- 

 tion, the received pattern is shown as the upper trace in Fig. 19. As 

 the piston is moved, the center depressed part of the trace gradually 



ImK. 10 — Hocoivcd pulsr patterns willi llic .irraiijicnuMit of Fig. IS used for 

 studying conversion to tlie Tlvn mode. 



