1212 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1954 



of brass rectangular waveguide used for TD-2 transmission lines has 

 attenuation in excess of 50 db per mile, and use of the very best copper 

 would only reduce the theoretical loss to about 40 db per mile. In order 

 to reduce the loss in ordinary waveguides, just as in coaxial or parallel 

 wire lines, one must go to lower frequencies. In particular, the theoreti- 

 cal loss at a carrier frequency near 1000 mc is about 2 db per mile, which 

 is about the same as the transmitter-to-receiver attenuation in our radio 

 relay systems. The waveguide in the 500-1000 mc region would have 

 cross-sectional dimensions on the order of one foot, would be cumber- 

 some to handle and would involve rather large material cost. In addi- 

 tion, it turns out that such a waveguide would be useful in signal band- 

 widths only a few mc wide as a result of delay distortion, which will be 

 discussed further in the ensuing discussion. Thus, we have concluded 

 that ordinary waveguide is not very attractive as a transmission medium 

 over distances on the order of a mile or more. 



It is true that the attenuation in any hollow metallic waveguide can 

 be reduced to any desired extent at a given frequency by making the 

 cross sectional area larger by a suitable factor. The penalty is that the 

 transmission medium becomes capable of propagating energy in several 

 characteristic ways, known as modes. The striking feature of a multi- 

 mode transmission medium is that energy in one mode is entirely in- 

 dependent and unaltered by the presence or absence of energy in one of 

 the other modes. This situation is sketched diagrammatically in Fig. 1. 

 Energy can theoretically propagate between 1 and 1', between 2 and 2', 

 and between 3 and 3' at the same time and in the same frequency hand 

 without mutual interference. The separate modes represent independent 

 transmission lines which occupy the same space. The distinguishing fea- 

 tures of the various modes in a multi-mode waveguide are: (1) ^>locity 

 of propagation or phase constant, (2) Attenuation coefficient, and (3) 

 Configuration of electric and magnetic field lines within the waveguide. 



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Fig. 1 — Diagram of multi-mode waveguide transmission. 



