662 THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1954 



form is predicted by coupled transmission line theory. As a consequence, 

 analysis of the mode-conversion effects associated with circular-electric- 

 wave transmission in commercial round pipe has been aided materially 

 by applying the coupled-transmission-line concept.' In another problem, 

 the transmission of the circular-electric waves through bends,- the 

 coupled-wave theory of subsequent sections has also provided valuable 

 insight. 



Coupled transmission lines can be employed as circuit elements to 

 exchange power between one mode of a multi-mode line and a designated 

 mode of another transmission line. Consider Fig. 1, which shows a rec- 

 tangular Avaveguide having entries 1 and 2 coupled through a series of 

 apertures to a parallel round waveguide having entries 3 and 4. The 

 rectangular guide may be made single mode for convenience, and for the 

 configuration shown may be made to couple to any TE mode of the round 

 guide. Input power at entry 1 may be transferred in whole or in part to 

 the selected mode at entry 4, the remaining portion of the power appear- 

 ing at entry 2. Very little power in any mode will appear at entry 3 for 

 excitation at 1, and very little power in undesired modes will appear at 

 entry 4. Thus the structure has the h3^brid property in addition to being 

 mode selective. A matched impedance is presented at all entries to all 

 modes over a very broad frequency band. 



Recently, coupled transmission lines have found use as input and out- 

 put circuits for travelling-wave tubes. In this instance a helical input 

 (or output) line was electromagnetically coupled to the travelling-wave- 

 tube helix, with conditions adjusted for complete energy transfer be- 

 tween the helices. The result is an input-output circuit requiring no 

 metallic connection to the tube helix and requiring no connection through 



COUPLING 

 APERTURE 



3' 



Fig. 1 — Coupled transmission line transducer. 



