WAVEGUIDE AS A COMMUNICATION MEDIUM 1233 



TluMi we know the distaiico (s.j — ^i) from 



1.15 



(--.. - zi) = 



The correspoiuling "upper limil" on lime dohiy hclwccn tlic signal and 

 tlie reconverted energy in the fsignal mode i.s 



t= fe-^x)(---) (1) 



where Vx and Vs are the groiij) velocities in the mode x and tlie signal 

 mode. 



It is well known that the unused modes of a circular electric waveguide 

 have attenuation coefficients which are appreciably larger than attenua- 

 tion coefficients for the circular electric wave itself. In the light of this 

 attenuation to the unused modes plus the fact that the reconverted 

 energy has undergone two mode conversion losses before it reaches the 

 signal mode again, one might wonder whether the mode conversion- 

 reconversion phenomenon wn)uld really be an important effect. The first 

 indication that the magnitudes of the reconversion amplitudes are sig- 

 nificant came during experimental work on the 5'' diameter 9,000-mc 

 line, described above in connection with Fig. 15. A more quantitative 

 theoretical discussion which follows shows that the reconversion phe- 

 nomena will continue to be important even when mode filters are intro- 

 duced into the line. 



The effects of the mode conversion-reconversion process are very simi- 

 lar to the effects of multipath transmission through the atmosphere. In 

 microwave radio there are under unusual fading conditions 2 or 3 sub- 

 sidiary signals, and these are representative of propagation over different 

 path lengths in space but at the same velocity of propagation. In the 

 waveguide, there will in general be a large number of subsidiary trans- 

 mission paths, each of which corresponds to the identical distance of 

 propagation but at velocities of propagation which are different for 

 the \arious modes. The radio multipath phenomenon exists only occa- 

 sionally, whereas the waveguide multipath phenomenon is a steady 

 characteristic present 100 per cent of the time. Long-distance radio trans- 

 mission in the 6-20 mc region by way of the ionosphere encounters mul- 

 ti-path effects more like those expected in the waveguide, with the excep- 

 tion that wa\-eguide multi-path effects are expected to have far greater 

 short-time stability. 



Quantitative relations describing the conversion-reconversion process 

 may be derived by considering an infinitely long waveguide composed of 



