Open-Wire Crosstalk

F 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 unavoidable irregularities in the spacing of the wires and of the transposition poles. There is no point in reducing the type unbalances bytransposition 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. W i t h practical methods of locating transposition poles, the effect of the pole spacing irregularities m a y 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.