34 BELL SYSTEM TECHNICAL JOURNAL 



impedance and attenuation, the effect of surrounding circuits is taken 

 care of in practice by using a capacity per unit length sHghtly higher 

 than the value which would exist with only one circuit on the line. 

 The proper capacity to use is determined in practice by measurements 

 on a short length of a multi-wire line. 



The effect on the propagation constant of the transverse crosstalk 

 paths indicated by n and / of Fig. 7 cannot be suppressed by trans- 

 positions. As explained later, if the two circuits marked a were 

 actually different circuits, the effect could be largely suppressed by 

 transposing one circuit at certain points and leaving the other circuit 

 untransposed at these points. Since the disturbing and disturbed 

 circuits indicated by Fig. 7 are actually the same circuit, they must 

 be transposed at the same points and, therefore, the transverse cross- 

 talk effect cannot be suppressed by frequent transpositions. 



Figure 7 also shows a crosstalk path marked r. This is one of the 

 possible interaction crosstalk paths. The effect of such paths on the 

 impedance and attenuation of the circuit may be largely suppressed 

 by suitable transpositions. The difference between the two curves of 

 Fig. 6 is due to lack of this suppression in the case of the upper curve. 



Such an extreme effect of crosstalk reacting back into the primary 

 or initiating transmission circuit and thus affecting direct transmission 

 is seldom important in practical transposition design. A marked 

 reaction on the primary circuit would necessitate such large crosstalk 

 currents in neighboring communication circuits as to make them unfit 

 for communication service at the frequency transmitted over the 

 primary circuit. Therefore, it is only when the neighboring circuits 

 are not to be used at this frequency that transposition design to control 

 simply the direct transmission becomes of practical importance. 

 When many circuits on a line are used for carrier operation, the 

 crosstalk currents must be made so weak (by transpositions, physical 

 separation of circuits, etc.) that their reactions back into the primary 

 circuits are very small. 



The effect of transpositions on crosstalk from one circuit into another 

 different circuit will now be considered. The discussion of the control 

 of this effect is the main object of this paper. 



Figure 8A shows a short segment of a parallel between two long 

 circuits and a near-end crosstalk coupling marked n. The segment 

 could be divided into a series of thin slices and theoretically there 

 would be interaction crosstalk between different slices. The segment 

 length is, however, assumed to be short enough to neglect interaction 

 crosstalk. The coupling n is, therefore, due either to direct or indirect 

 transverse crosstalk in the short segment or to both of these types of 



