290 BELL SYSTEM TECHNICAL JOURNAL 



in the four most important specific classes, namely, low pass, high pass, 

 low-and-high pass and band pass, are also shown. 



Finally, two by-products obtained from a further use of these fixed 

 network designs will be added. One is the ready design of networks to 

 simulate the mid-point image impedances of "constant ^" wave- 

 filters. The other leads to the design of networks which simulate the 

 impedances of a loaded line, approximately a low pass wave-filter, over 

 the greater part of its transmitting band. 



It need hardly be mentioned that these terminal transducers may 

 be used to terminate a lattice or other type of wave-filter which has a 

 standard image impedance or, vice versa, that of a derived wave-filter 

 such as the Mill'-type. In this manner the terminal image impedance 

 can be altered efficiently from one characteristic to another. The 

 lattice type (zi, S2) is itself a symmetrical structure. 



The procedure for the design of a wave-filter network to meet 

 specific requirements may even begin with the choice of terminal wave- 

 filter impedance characteristics, which are physical and not in general 

 the same at both ends. The terminal, or reflection, losses due to 

 resistance or other known terminating impedances would thus be 

 definitely known. With these taken into account the internal part 

 would be designed using any type or types so as to fit in between the 

 chosen image impedances without impedance irregularity, as in a 

 composite structure, and give the remainder of the desired transmission 

 characteristic. 



Part 1. Derivation of Wave-Filters Which Possess Desirable 



Image Impedances 



1.1 General Ladder Type Structure 



Of the three simple general types of recurrent or iterative structures, 

 the ladder, lattice and bridged- T types, only the ladder type which has 

 alternate series and shunt impedances, Si and S2, respectively, has two 

 different image impedances per periodic interval and these are Wi and 

 W'2. at the two mid-points, mid-series and mid-shunt. The ladder type 

 can therefore be separated on the image basis into either of two kinds 

 of symmetrical sections with two pairs of terminals, mid-series or mid- 

 shunt sections, or into one kind of dissymmetrical section, a mid-half 

 section. The existence of two different image impedances for a section, 

 the general property of all mid-half sections, is a necessary condition 

 for the proper combination of mid-half sections of different related 

 structures to give the desirable terminal impedance results obtained in 

 this paper. Definitions of these three kinds of sections which have 

 been considered in previous papers will be reviewed here. 



