5&4 



BELL SYSTEM TECHNICAL JOURNAL 



That is, the effect of connecting an impedance Z, to the end of a 

 line of characteristic impedance Z© is to return toward the source a 



7 7 



current whose value is ° ' times the current that would exist at 

 Zo+Z/ 



the terminus if the line were regularly terminated. The ratio between 



— TTOT' i-^ror^ 



3= 



— nm^-' — 'im^- 

 oouoooooooooo^n 



Fig. 2 



the rc'tk'Ctcd and ihf incident current is known as ihc "rcllcclicjn 

 coefficient," the value of which is expressed as follows: 



/. Zo+z: 



(7) 



This ratio can also be expressed in transmission units (TU). When 

 expressed in TIT this relation will be referred to in this paper as the 

 "transmission loss of the returned current," or, briefly, as the "return 

 loss." 



If a condition occurs in a line which causes the impedance at any 

 point to differ from the characteristic impedance it has the same effect 

 as an irres^ular termination. 



Return Loss .\t .\ Repis.mkr Duk td .\ .Singlk Irki;gularity 



Fig. 2 shows a No. 21-type repeater connected between a line and 

 a network whose impedance is exactly equal to the characteristic 

 impedance Zo of the line. If the line is perfectly regular the repeater 

 will be perfectly balanced and the gain can be increased indefinitely 

 witht)ut causing the repeater to sing. 



Assume now that the line is terminateil by some apparatus having 

 an impedance Z» at a distance from the repeater such that the trans- 

 mission loss of the intervening line is 7" TU. If a wave of current 

 having a certain magnitude leaves the repealer, it is reduced in 

 strength by T TU when it reaches the terminus. Of this current, a 

 certain amount is transmitted back toward the re|)eater, suffering a 



