The Bell System Technical Journal 



Devoted to the Scientific and Engineering Aspects 

 of Electrical Communication 



April, 1923 



Impedance of Smooth Lines, and Design of 

 Simulating Networks 



By RAY S. HOYT 



Introduction 



THE transmission of alternating currents over any transmission 

 line between specified terminal impedances depends only on the 

 propagation constant and the characteristic impedance of the line 

 (at the particular frequency contemplated). In this sense, then, the 

 properties of transmission lines may be classed broadly as propagation 

 characteristics and impedance characteristics. In telephony we are 

 primarily concerned with the dependence of these characteristics on 

 the frequency, over the telephonic frequency range. 



Prior to the application of telephone repeaters to telephone lines the 

 propagation characteristics of such lines were moie important than 

 their impedance characteristics, because the received energy depended 

 much more on the former than on the latter. 



The application of the two-way telephone repeater greatly altered 

 the relative importance of these two characteristics, decreasing the 

 need for high transmitting efficiency of a line but greatly increasing 

 the dependence of the results on the impedance of the line. As well 

 known, this is because the amplification to which a two-way repeater 

 can be set without singing, or even without serious injury to the 

 intelligibility of the transmission, depends strictly on the degree of 

 impedance-balance between the lines or between the lines and their 

 balancing-networks. In the case of the 21-type repeater the two lines 

 must have such impedances as to closely balance each other throughout 

 the telephonic frequency range. In the case of the 22-type repeater, 

 which for long lines requiring more than one repeater is superior to the 



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