THE BELL SYSTEM 



TECHNICAL JOURNAL 



VOLUME XXXIII NOVEMBER 1954 numberG 



Copyright, 19BJ,, American Telephone and Telegraph Company 



Waveguide as a Communication Medium 



By S. E. MILLER 



(Manuscript received March 23, 1954) 



The circular electric wave in round metallic tuhing has an attenuation 

 coefficient which decreases as the freciuency of operation is increased. A corol- 

 lary to this behavior is the fact that any preselected attenuation coefficient can 

 in theory be obtained in any predetermined diameter of pipe through the 

 choice of a suitably high carrier frequency. The attenuation which is charac- 

 teristic of microwave radio repeater links, about 2 db/mile, is in theory 

 attainable in a copper pipe of about 2" diameter using a carrier frequency 

 near 50,000 mc. 



Scale-model transmission experiments, conducted at 9,000 mc, showed 

 average transmission losses about 50 per cent above the theoretical value. 

 These extra losses were due to (1) roughness of the copper surface and (2) 

 transfer of power from the low-loss mode to other modes which can also 

 propagate in the pipe. 



The latter effect may have serious consequences on signal fidelity because 

 power will transfer (at successive waveguide imperfections) from the signal 

 mode to unused modes and, after a time delay, back to the signal mode. 

 This effect has been studied experimentally and theoretieally , and it is con- 

 cluded that (1) either mode fillers must be inserted periodically to absorb the 

 power in the unused modes of propagation, or (2) the medium itself 7nust be 

 modified to continuously provide large attenuations for the unused modes of 

 propagation. The latter approach is attractive in that it also provides a solu- 

 tion to the problem of bending this form of low-loss guide. 



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