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THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1954 



periodic energy exchange of the type described by equations (26) and 

 (27) is observed. ]\Ioreover, he also showed that if one line were made 

 very lossy the energy exchange phenomena disappeared and, despite 

 sufficient coupling to cause complete power transfer when both lines 

 were loss-free, power passed through the coupling region in the low-loss 

 line with less than 0.25 db attenuation. This verified the predictions of 

 equations (35) and (36). 



Other implications of the coupled wave theory can also be utilized in 

 dielectric waveguides. If the two lines (Fig. 54) are made of materials 

 having different dielectric constants and their cross-sectional dimensions 

 set so as to secure identical phase constants at a frequency fm , then a 

 frequency-selective coupled-wave filter results and the selectivity charac- 

 teristic of Fig. 53 applies. As an alternative to using materials having 

 different dielectric constants, the same dielectric may be used for both 

 lines by making one line solid and the other hollow. 



If both lines are made of the same material and the cross-sectional 

 dimensions are set so as to obtain a known difference between their phase 

 constants, the result is a directional coupler having a region of flat trans- 

 fer loss (of any desired magnitude) and equations (42), (43) and (44) 



apply. 



Both of the preceding appUcations can be carried out in dielectric 

 waveguides having arbitrary cross-sectional shapes. 



Fig. 54 — Coupled dielectric waveguides. 



