A METHOD OF IMPEDANCE CORRECTION 829 



If a perfect match were secured at the filter terminals then, by the 

 reciprocity principle, a perfect match should be secured at the line 

 terminals also. In order to evaluate the performance of the networks, 

 therefore, the impedances they present to the line were computed. 

 The results are shown in Figs. 18 and 19. 



Comparison of Direct and Reverse Networks 



At first glance the curves of Figs. 18 and 19 seem to show that while 

 networks of the reverse type produce a good impedance match over a 

 moderate fraction of band they will be much less successful than the 

 structures previously described at frequencies very near the cutoff. 

 This apparent advantage in favor of the networks first described is 

 discounted considerably however by the economy of elements resulting 

 from the relative simplicity of the reactance or susceptance controlling 

 networks used with terminations of the second type. If we adopt as 

 our standard in comparing the two types of networks the total number 

 of elements each requires, rather than the number of branches they 

 contain, the advantage of networks of the first type becomes much 

 less impressive, if it does not actually disappear. More important 

 considerations recommending the first type of terminations in prefer- 

 ence to the second for most practical designs appear to be the greater 

 ease with which they can be designed to meet a given reflection coefficient 

 requirement, resulting from the relatively smaller number of branches 

 they contain, the greater ease with which they can be adapted to 

 filters which must operate in parallel, and the fact that the attenuation 

 they contribute to the total filter suppression is usually more useful 

 than that furnished by terminations of the second type. 



Under certain circumstances, however, the second type of terminat- 

 ing sections have a definite advantage over the others. When a 

 filter operates in conjunction with a modulating device a high modulator 

 efficiency with low distortion demands that the impedance of the filter 

 to the untransmitted side band be low (or high) and nearly constant. 

 In spite of their poor characteristics within the transmitting band it 

 has hitherto been necessary to use mid-shunt image impedance termina- 

 tions of the "constant-^" type in these circuits. Impedance correcting 

 sections of the first type are not suitable for this service because the 

 complicated susceptance and reactance annulling networks at their 

 line terminals produce sharp changes in reactance in the attenuating 

 region. The outermost branch of terminations of the second type, 

 however, is of simple configuration and if we choose it to resemble the 

 final branch of a mid-shunt terminated "constant-^" type filter, as 

 has been done in the sections shown in Figs. 16 and 17, we will secure 



