1230 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1954 

 MODE CONVERSION AND RECONVERSION AS AN INTERFERENCE EFFECT 



The mode eoiiverision-reconversion phenomena can also produce a 

 signal distortion or interference effect. Fig. 17 shows another idealized 

 waveguide containing two deformities, but in this case we assume a 

 short pulse in the circular electric wave as the input signal. The ampli- 

 tude-time plots below the waveguide show the energy present in the 

 circular electric and unused waves respectively at various physical points 

 along the line. The key item in this diagram is the time displacement 

 between the converted energy TX and the circular electric wave energy 

 at the input to the second deformity. This time difference appears as a 

 result of propagation over an identical line length at two different group 

 velocities which, in general, the circular electric and unused modes will 

 possess. Since the signal and unused mode pulses strike the second de- 

 formity at different times the second conversion process results in energy 

 appearing back in the circular electric wave at a time separated from the 

 signal pulse itself. When the distance between deformities is too short 

 for the pulses to be resolved at the second deformation, the result will 

 be a distortion of the signal pulse rather than the appearance of a separate 

 pulse. 



The above very much simplified picture of the mode conversion and 

 reconversion effects allows one to visualize several general properties of 

 this phenomenon: 



1. In general, there will be a large number of unused modes which will 

 be coupled to the signal mode through the various imperfections in the 

 transmission line. Since these unused modes have unequal phase con- 

 stants, and since the imperfections will be randomly spaced along the 

 line, the reconverted signal pulses in a time-division system w^ill be spread 



WAVEGUIDE 

 DEFORMATIONS 



PURE 

 TF° 



Fig. 17 — Signal interference effects due to mode conversion and reconversion. 



