open- Wire Crosstalk * 



By A. G. CHAPMAN 



Effect of Constructional Irregularities 



IF the cross-sectional dimensions of an open-wire line were exactly 

 the same at all points and if the transpositions were located at 

 exactly the theoretical points, the crosstalk could be reduced by huge 

 ratios by choosing a suitable transposition arrangement and interval 

 between the transposition poles. 



Practically, however, the crosstalk reduction is limited by un- 

 avoidable irregularities in the spacing of the wires and of the trans- 

 position poles. There is no point in reducing the type unbalances by 

 transposition design beyond the point where the constructional 

 irregularities control the crosstalk. 



Transposition Pole Spacing Irregularities 



The following discussion covers the method of estimating the 



crosstalk due to irregularities in the spacing of transposition poles and 



the derivation of rules for limiting such irregularities. With practical 



methods of locating transposition poles, the effect of the pole spacing 



irregularities may ordinarily be calculated by considering only the 



transverse crosstalk. Special conditions for which attention must be 



paid to interaction crosstalk are discussed later. The simplest case, 



that of transverse far-end crosstalk due to pole spacing irregularities, 



will be discussed first. 



A transposition section is divided into segments by transposition 



poles which in practice vary in number from four to 128. Each 



segment causes an element of crosstalk current at a circuit terminal 



and this element is about proportional to the segment length. For 



far-end crosstalk between similar circuits all these crosstalk current 



elements would add almost directly if there were no transpositions. 



The function of the transpositions is to reverse the phase of half the 



current elements. The segments corresponding to the reversed current 



elements may be called the minus segments. If the other half of the 



* This is the second half of a paper which was begun in the January 1934 issue 

 of the Technical Journal, giving a comprehensive discussion of the fundamental 

 principles of crosstalk between open-wire circuits and their application to the trans- 

 position design theory and technique which have been developed over a period of 

 years. 



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