THEORY AND DESIGN OF WAVE-FILTERS 27 



wave-filter sections can be changed without changing one of their 

 mid-point characteristic impedances. Obviously then it should be 

 possible to combine such sections so as to give a non-uniform network 

 which introduces a number of different propagation constants. 



The composite wave-filter is a network of serially connected wave-filter 

 sections some or all of which are different in propagation constants, but 

 adjacent sections of which are equivalent in characteristic impedance at 

 their junction. The latter condition ensures the absence of impedance 

 irregularities within the network. Consequently the composite wave- 

 filler is specified by the sum of the propagation constants of the individual 

 sections and the characteristic impedances of the end sections. 



The advantage of composite over uniform wave-filters is in their 

 flexibility of design by means of which it is easier and more economical 

 to meet the attenuation and impedance requirements in many wave- 

 filter networks. For example, to utilize the frequency range as 

 completely as possible the attenuation of the network should in 

 general rise rapidly upon entering the attenuating bands and remain 

 high. It is also often desirable that the network have an approxi- 

 mately constant resistance terminal impedance in the transmitting 

 bands. No uniform wave-filter possesses all these properties as it was 

 found that the attenuation constant of any section varies markedly 

 with frequency over the attenuating bands, being much higher in 

 some parts then in others; then, too, the impedances of most wave- 

 filters are not the best available. To give high attenuation at fre- 

 quencies where the attenuation constant of a section is low requires 

 a relatively large number of uniform sections and this means a sur- 

 plus of attenuation at other frequencies. Aside from economic con- 

 siderations this number is practically limited by the amount of at- 

 tenuation introduced in the transmitting bands due to dissipation 

 in the elements. In a composite wave-filter, however, it is possible 

 to distribute the low and high attenuations of the individual sections 

 over the frequency bands so that an efficient use is made of these 

 attenuation properties and a more uniform high attenuation is pro- 

 duced; a desirable impedance characteristic is obtainable by M-type 

 section terminations. 



In the case of ladder types, for example, we may look upon the 

 composite wave-filter as having been originally a number of sections 

 of the general mid-series or mid-shunt equivalent wave-filters wherein 

 now the propagation constants of the sections have been changed 

 without changing their characteristic impedances. The mid-series 

 and mid-shunt wave-filters may also both be included since their 

 junction can be made through the intermediate use of the "constant 



