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THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1954 



Fig. 3 — Discrimination versus O/iz for linear taper coupling. 



ward wave components of about 13 db. How can this discrimination be 

 improved? If the difference between the wavelengths of the desired and 

 undesired wave types is increased, the value of Q is increased and greater 

 discrimination results. In practical cases, however, there frequently is 

 very little that can be done about the wavelength difference because it is 

 inherent in a structure Avhich is fixed by other considerations. By increas- 

 ing the length of the coupling interval L the value of Q is also increased; 

 in the case of uniform coupling, (5) shows that a value of B/r equal to 

 about 8 is required to get 30 db discrimination. In the above example 

 this corresponds to L approximately equal to 125Xo . The latter coupling 

 length is probably impractical, and is certainly inconvenient. The final 

 alternative is to alter the distribution of coupling between the lines, and 

 considerable can be done in this manner. 



Suppose a Hnear taper of the strength of coupling is used, as sketched 

 in Fig. 3. Then the discrimination becomes 



Linear Taper Discrimination = 



g/2 

 sin ^/2 



(6) 



which is plotted in Fig. 3. The first peak in discrimination occurs at QJ-k 

 equals two, compared to a value of 0/t equals one for the first peak using 

 uniform coupling; however, for all values of ^/tt greater than about 3, 

 the linear taper provides superior discrimination. This illustrates a gen- 

 eral trend; tapering the coupling distribution improves the discrimina- 



