OPEN-WIRE CROSSTALK 



29 



at the terminal of the Hne between these wires. If, however, one 

 wire has a bad joint, the two crosstalk currents become unequal and 

 there will be a current in such a receiver. 



Resistance unbalance crosstalk is of particular importance if two 

 pairs are used to create a phantom circuit in order to obtain three 

 transmission circuits from the four wires. The distribution of the 

 phantom transmission current I p in a short length of the two pairs is 

 indicated by Fig. 4B. Ideally, half the phantom current flows in 

 each of the four wires. The two currents in wires 1 and 2 are then 

 equal and in the same direction and there will be no current in terminal 

 apparatus connected between wires 1 and 2. In other words, trans- 

 mission over the phantom circuit results in no crosstalk in the side 

 circuit 1-2. The same may be said of side circuit 3-4. A bad joint 

 in any wire, such as 3, makes the two currents in wires 3 and 4 unequal 

 and results in a current in the side circuit 3-4. 



PHANTOM CURRENT = Ip 

 (A) CB) 



Fig. 4 — Effect of resistance unbalance on crosstalk. 



The phantom-to-slde crosstalk effect of resistance unbalance is 

 much more severe than the effect on crosstalk between two side 

 circuits or two non-phantomed circuits. The reason for this is evident 

 from Figs. 4B and 4A. In Fig. 4B the entire transmission current of 

 the disturbing phantom circuit normally flows in the two wires of the 

 disturbed side circuit and if a resistance unbalance causes a small 

 percentage difference in the currents in these two wires objectionable 

 crosstalk results. In Fig. 4A only crosstalk currents flow in wires 3 

 and 4 and a much larger percentage difference between these small 

 currents can be tolerated. 



In designing and operating phantom circuits, it is necessary to 

 exercise great care to minimize any dissimilarity between the two wires 

 of a side circuit, in order to avoid crosstalk from a phantom to its 

 side circuit or vice versa. Otherwise, the problem of crosstalk between 



