SINGING ON TWO-WIRE CABLE CIRCUITS 60S 



The various quantities are like those enumerated above except that 

 they are for the active return loss in the other direction and include 

 the gain of the repeater in the east to west direction. 



Figure 2 shows the value of Sp as derived in Appendix II plotted 

 with Sf- By comparison of these curves, it may be seen that the 

 spread of active return losses around a repeater will be somewhat 

 larger than the spread of active return losses in one direction. The 

 Sp curve approaches more nearly to a so-called normal law than the 

 Sp curve does. 



The previous discussion has been confined to the returned currents 

 obtained from intermediate points in the circuit and also at a single 

 pre-selected frequency. In addition to these currents, there is at 

 each end of the circuit a current returned through the path called 

 the terminal return loss. In a four-wire cable circuit this is the only 

 path for circulating currents. From field data it appears that these 

 terminal return losses approximately follow the Sp distribution curve 

 also. The end path toward the west may be written as: 



• ^21 + 5f = 7 -f £i + Sp, (3) 



where 7 db is assumed as the terminal return loss for Sp = 0. In 

 this case £i is the net loss at the particular frequency from the repeater 

 in question (from the west side of repeater C) to the far end of the 

 circuit and from that end of the circuit back to the repeater minus the 

 gain of the receiving repeater on the latter side of the circuit (the 

 W-E repeater). The end path toward the east is 



S^2 + Sp = 1 +E, + Sp. (4) 



If the losses of each cable section are equal and the gains of each inter- 

 mediate repeater are equal, the repeater in question is at the center of 

 the circuit, and £i = £2, and at 1000 cycles £1 would be equal to the 

 nominal circuit net loss. 



The active return loss at a given frequency toward the west which 

 includes the end path (both including the west to east repeater gain) 

 will be 



^31 + Sp= (5n X 52i) + Sp (5) 



V 



and toward the east will be 



Sz2 -\- Sp — (Sn X S22) + Sp. (6) 



p 



Where X means that the quantities so connected are combined as 



