OPEN-WIRE CROSSTALK 39 



per mile were added to both circuits at the same points which are 

 indicated by x on Fig. 10. The large effect of these transpositions 

 shows the practical importance of the interaction type of far-end 

 crosstalk. 



In connection with Fig. 9B, there arises the question of how far 

 apart the transpositions can be placed without serious crosstalk, in 

 other words, how long is it permissible to make the segment d. If 

 this length is increased the transpositions at B and .0 become less 

 effective in suppressing the near-end crosstalk between a and c in 

 length CE and between c and h in length AC. The degree to which 

 the interaction crosstalk path r' must be suppressed is, therefore, 

 important in determining the maximum permissible length of d. If d 

 is increased the transposition at C becomes less effective in controlling 

 the near-end crosstalk between a and h and, therefore, the length d 

 also depends on the permissible near-end crosstalk. 



It may be noted that transpositions at B and D in but, one of the 

 circuits a or h will help to suppress r' , but the suppression is less 

 effective than if both circuits are transposed at these points. If a is 

 transposed at B and D the near-end crosstalk between a and c in 

 length CE is reduced but the near-end crosstalk between c and h in 

 length AC IS not reduced. The product of these two near-end crosstalk 

 values is greater, therefore, than if they had both been reduced by 

 transposing both circuits at B and D. 



Crosstalk Coefficients 



The crosstalk between any two long open-wire circuits may be 

 calculated by dividing the parallel into a succession of thin transverse 

 slices and summing up the crosstalk for all these slices. To calculate 

 the crosstalk in any slice it is necessary to know certain "crosstalk 

 coefficients." The discussion below defines these coefficients and 

 describes briefly how they are measured or computed. 



Figures 2 A and 2B indicate both near-end and far-end crosstalk 

 coupling of both the direct and indirect transverse types in a thin 

 transverse slice. Any of these couplings may be expressed in crosstalk 

 units and the value of the coupling in a short length divided by the 

 length in miles is called the crosstalk per mile. Since, as shown in 

 the previous section, the crosstalk may not increase directly as length, 

 strictly speaking, the crosstalk per mile is the limit of the ratio of 

 coupling to length as the length approaches zero. The crosstalk per 

 mile includes both the direct and indirect types of transverse crosstalk 

 coupling. In the frequency range of interest (i.e., above a few hundred 

 cycles for near-end crosstalk and above a few thousand cycles for 



