IMPEDANCE CORRECTION OF WAVE FILTERS 



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to bring out the departure of the image impedance from its ideal value 

 more clearly. A low pass filter to which the termination has been 

 applied is shown on Fig. 14. The terminating sections are enclosed by 

 the broken lines. The resulting reflection coefficient is given on the 

 same figure. It will be observed that the reflection coefficient over 

 93 per cent of the transmitting band is less than 2.5 per cent, or only 

 about one-fourth that of the analogous m-type terminated filter shown 

 in Fig. 6. The application of the structure to band-pass filters is illus- 

 trated in Fig. 15, which represents a portion of the redesigned type 



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Fig. 15 — Band-pass filters with 2- branch terminations modified for parallel 

 operation. The reflection coefficient of these filters is given by Curve III, Fig. 11. 



"Z>" system. The configuration shown in Fig. \2-B has been modified 

 in these networks to adapt the filters for parallel operation. There are 

 no separate terminations at the receiving ends of the filters since with 

 these very simple structures reflection at the receiving ends could be 

 taken into account by a slight adjustment of the terminating network 

 facing the line. The improvement produced by the new networks in 

 the reflection coefficient characteristics of the system is evident from a 

 comparison of Curves III and I of Fig. 11. 



The terminating section shown in Fig. 12-B which is one step beyond 

 the "m-type" section in complexity, is adequate in most situations. 

 When a severe reflection coefficient requirement must be met almost up 

 to the filter cutoff, however, it is necessary to resort to the more com- 

 plicated configuration of Fig. 12-C The image impedance character- 

 istic of this section, when its parameters are adjusted for an operating 

 range extending over 97.5 per cent of the theoretical transmitting band, 

 is shown on Fig. 16. The reflection coefficient actually obtained when 

 sections of this type, but with somewhat different values of the design 

 parameters, were applied to a high-pass filter is shown by Fig. 17. 

 The filter configuration is given on the same figure. In this figure the 



