380 BELL SYSTEM TECHNICAL JOURNAL 



repeaters have practically the same phase shift and the propagation 

 characteristics of the two coaxials are nearly identical. One way to 

 prevent this direct addition is to transpose one section against another 

 or one group of sections against another group along the line. In the 

 case of unbalanced circuits these "transpositions" take the form of 

 transformers or extra tube stages in one of the systems at repeaters, 

 either of which will produce a 180-degree phase reversal. 



If the far-end crosstalk in one transposition section is Fu and that in 

 another is F12 the total in the two sections, exclusive of interaction cross- 

 talk between sections, is inherently Fn + Fn. With a transposition in 

 one coaxial at the junction the total becomes Fn — Fn- Hence, if 

 Fn = F12 it is possible to eliminate this crosstilk component entirely. 

 However, due to irregularities in the cable and the practical impos- 

 sibility of locating repeater points exactly, Fn will not, in general, 

 equal F12 and even after transposing a small residual may remain. 



This residual, however, may be negligible compared with the near- 

 end near-end and far-end far-end interaction crosstalk components 

 Fnn and F/f between repeater sections (that is, around repeaters), 

 unless transmission along the tertiary circuits from one repeater section 

 into another is suppressed at repeater points. The interaction cross- 

 talk tests already discussed may be used to compute this effect. How- 

 ever, in order to demonstrate the effectiveness of transpositions, far-end 

 crosstalk tests were made in a 24,000-foot length with and without a 

 transposition in one of the coaxials at the center and with various inter- 

 action crosstalk paths suppressed. The results are given in Figs. 12 

 to 14 and are discussed below. 



To suppress entirely the interaction crosstalk between the trans- 

 posed sections all tertiary circuits were shorted at the transposition 

 point. In these measurements the tertiaries were also shorted at each 

 end of the line in an effort to have both ends of each half of the line 

 terminated as nearly alike as possible. The test results are given in 

 Fig. 12. ^ 



For this condition the crosstalk measured in each half of the line is 

 also shown. Curve AB represents the far-end crosstalk in one line 

 section and A'B' that in the other section. Curve {AB + A'B') 

 gives the results when the two sections are combined with no transposi- 

 tion. Curve {AB — A'B') gives the results when a transformer is 

 inserted in one coaxial at the center. (A similar set of curves are given 

 iov BA, B'A', etc.) 



Note that AB and A'B' coincide very closely in magnitude. When 

 combined with no transposition the crosstalk in two sections is nearly 

 6 db higher over the entire frequency range than in either individual 



