INRF.CLLIRITIES !N LOADED TELEPHONE CIRCUITS 565 



further loss of 7'TU on the way; consequently, the relation expressed 

 in Tl' between the strength of the currents leaving and returning 

 to the re^HMter, that is, the return loss at the repealer, is given by 

 the eciuation 



If the gain of the repeater, expressed in Tl', is ecjual to or greater 

 than 5 the repeater will sing provided the returning current has the 

 correct phase relation to reinforce the original wave. For this reason 

 the term "singing point" has frequently been applied to the quantity 

 5, which is called returned loss in this paper. 



If the line is shortened until the impedance Zi is connected directly 

 to the repeater terminals, the transmission loss 7' between the repeater 

 and the irregularit\- is reduced to zero and the return loss becomes 



5 = 20 1og,„f^'. (9) 



Return Loss of Irregular Lines 



In practice, lines are never perfectly regular. Not only is it im- 

 practicable to build apparatus which would form a perfectly regular 

 termination for a line, but there are numerous causes of irregularity 

 in the lines themselves, each one of which is capable of reflecting a 

 portion of the waves which traverse the line. These irregularities 

 can be kept smaller than any specified amount if sufficient care is 

 used in building and maintaining the line but they cannot be entirely 

 eliminated; consequently, if a length of actual line is terminated 

 regularly by a network of impedance Zo, the return loss will be high 

 if the line is carefully built and low if it contains large irregularities. 

 The return loss of such a line, when terminated regularly by a network 

 is a measure of the quality of the line from the standpoint of repeater 

 performance. In measuring the return loss of a line it is necessary 

 that a rather long section of the line be available so as to include 

 all irregularities near enough to have an appreciable effect upon the 

 result. If the section measured is too short, the result will be too 

 high because only a few irregularities will be included. 



Calculation of the Return Loss of Coil Lo.^ded Lines 



Owing to the facts that the inductance of coil loaded lines is con- 

 centrated principally in the loading coils and the capacity is divided 

 into elements of finite size by the loading coils and, further, that the 



