WAVEGUIDE AS A COMMUXICATION MKDIUM 1229 



occur as the pulse travelled n\) ami down the line liad llic ^ciicral I'oiin 

 of an amplitude component which led oi' hiii^cd Ihc sif>;iial ])ulsc l)\- a 

 constant interval but which ^rachuUlx' inci'cascd in am))lilndc with in- 

 creasing distance that the signal jiulsc had Irax'ellcd. Note, loi- example, 

 the growth of spurious ]:)eaks b(>f()i'e and after the signal i)ulse in low 2 

 of Fig. 15 at 3,500, 7,000, 10,500 and 1 1,000 yards of travel. 



The explanation of this behavior involves transfer of energy from the 

 circular electric mode to one or more of the unused modes of propaga- 

 tion and reconversion of the same energy l)ack to the circular electric 

 mode. This process is one of the characteristic features of multimode 

 waveguide systems and is discussed at greater length in the following 

 sections. 



MODE CONVERSION AND RECONVERSION AS A SIGNAL-LOSS EFFECT 



In beginning discussion of the mode conversion-reconversion phe- 

 nomena, we take an idealized case of a dissipationless waveguide con- 

 taining two deformities. Fig. 16. We assume a c-w signal entirely in the 

 circular electric mode entering this waveguide. After passing the first 

 deformity there will be energy present in some other mode, and this is 

 designated as TXi . When the combination of TEoi + TXi strikes the 

 second deformity, another conversion takes place and the outi)ut will be 

 a large TEoi component, two smaller components in thv muised mode, 

 TXi and TX2 , and a still smaller circular electric wave component, TEo/ , 

 which is due to reconversion of energy- from TXi to the circular electric 

 wave in traversing the second deformity. It can be shown' that for the 

 proper distance between two identical symmetrical deformities, the wa\'e 

 emerging from the line may be purely circular electric; the two compo- 

 nents TXi and TX2 cancel each other under this condition. Another 

 separation between the deformities results in a maximum energy trans- 

 fer from circular electric to other modes. Therefore, it follows that any 

 mechanism which varies the effective spacing between conversion points 

 will produce conversion loss variations. This accounts for the change in 

 the attenuation of the circular electric wave pulses in Fig. 15 as a function 

 of the far end piston setting. 



FIRST SECOND 



DEFORMITY DEFORMITY 



TEoi+TX,+TX2+TE;, 



Fig. 16 — A distorted waveguide and the associated mode-coiivension signal- 

 loss effects. 



