OPEN-WIRE CROSSTALK 



209 



the calculated near-end crosstalk for ideal point transpositions. 

 Curves B and C show the calculated and observed near-end crosstalk 

 for drop bracket transpositions. The curves show that the drop 

 bracket effect can be calculated quite accurately and that it may 

 reduce the total crosstalk. In the general case, it is impractical to 

 take much advantage of this reduction effect because a marked re- 

 duction for one combination of circuits is likely to result in an increase 

 for some other combination and because a reduction of crosstalk in 

 one part of the line may increase the vector sum of crosstalk elements 

 from all parts of the line. 



Wire Configurations 



The crosstalk coefficients for the various pair combinations may be 

 altered by changing the configuration of the wires. Therefore, the 

 crosstalk for a given transposition design and a given accuracy of 

 transposition pole spacing irregularity may also be altered. The 

 crosstalk due to sag differences also depends on the wire configuration. 

 It is important, therefore, to choose a configuration most desirable 

 from the crosstalk standpoint. Such an optimum configuration 

 requires the fewest transpositions and least accuracy of pole spacing 

 for a given maximum frequency and given permissible values of 

 crosstalk coupling. 



Various "non-inductive" arrangements of wire configurations have 

 been suggested and tested. Such arrangements may appear to have 

 possibilities but their study to date has indicated that they are im- 

 practicable for more than a few pairs on a line. 



Fig. 25 — "Non-inductive" arrangements for two pairs of wires. 



Fig. 25 illustrates several suggested arrangements for two pairs. 

 Arrangement A is often called a square phantom. If pair 1-2 is the 

 disturber and there are equal and opposite currents in wires 1 and 2 

 there will be no voltages induced in either wire 3 or wire 4 because 

 either of these wires is equally distant from wires 1 and 2. Since 

 wires 1 and 2 are not equally distant from the ground, the currents 



