2 BELL SYSTEM TECHNICAL JOURNAL 



obtained by combining sections of wave-filters having equivalent 

 characteristic impedances but different propagation constants. 

 Among others, a superior advantage of composite over uniform wave- 

 filters is shown to be their greater flexibility of design, as a result of 

 which composite wave-filters are often the only means of meeting 

 severe design requirements. Many of the methods here used are 

 found to have further application in general recurrent network design. 

 The ideal toward which wave-filter design is usually directed is a finite 

 network having any preassigned transmitting and attenuating bands, 

 zero attenuation and a terminal characteristic impedance equal to any 

 one preassigned constant resistance in all transmitting bands, and in- 

 finite attenuation throughout all attenuating bands. Due to such an 



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Fig. 1 — Ladder Type Recurrent Network 



impedance characteristic, at frequencies in the transmitting bands 

 there would be no loss of transmitted energy if the network were in- 

 serted between two resistances, a generator and a receiver, each having 

 a constant resistance of this same magnitude, approximately the case 

 of two transmission lines. The infinite attenuation of the network 

 to currents of all other frequencies would effectively prevent energy 

 transmission through it. 



Practically, such an ideal has not been attained but the methods 

 developed here lead to designs which can approximate it rather 

 closely. No attempt will be made to give the construction of wave- 

 filter elements minimizing energy dissipation, as we shall be con- 

 cerned mainly with a determination of the magnitude and locations 

 of the elements in the network. It may be stated, however, that the 

 less dissipative the elements the more nearly will the ideal of free 

 transmitting bands be reached. 



Part I. Theory of Design 



The uniform recurrent network specifically considered in this design 

 method is the ladder type of Fig. 1 having identical series impedances 



