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BELL SYSTEM TECHNICAL JOURNAL 



to fewer side-wall reflections and to a shorter over-all length of zig-zag 

 transmission line. Thus, in this frequency range, the attenuations con- 

 tributed both by the side walls and by the top and bottom plates de- 

 crease with increasing frequency. Proceeding to frequencies far above cut-off, 

 where 6 approaches 90 degrees, there will not only be very few reflections 

 but the over-all length of zig-zag line will approach as its limit a single, 

 straight two-conductor line made up of the top and bottom plates alone. 

 Thus the attenuation due to the side walls will approach zero and that 

 due to the top and bottom plates will increase as the square root of the 



0.0001 



8 



12 3 4 5 6 7 



Frequency — ttiousands of megacycles 



Fig. 6..v4. ComponeiU atlentuations contributed by the top and bottom plates and also 

 the two side walls of a rectangular waveguide. 



frequency. Since the attenuation contributed by the top and bottom plates 

 first decreases but later increases with frequency, we may expect, be- 

 tween these two ranges, a region of mininuun attenuation. The attenu- 

 ations contributed by the upi)er and lower plates and also by the side walls 

 of a 7.5 cm X 15 cm coi)per guide carrying the dominant mode have been 

 calculated. The results have beei^ ])lotted as curves ,1 and B in Fig. 

 6.5-4. They follow the courses jjredicted by the preceding qualitative 

 reasoning. 



The fact that the reflection type of attcituation, such as is c\-ident in the 

 side walls above, decreases with frequency, suggests that, if a kind of wave- 



