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BELL SYSTEM TECHNICAL JOURNAL 



sponds to a length D. The transposition at the start of the second 

 length (at point B of Fig. 14) reverses the sign of the term for the 

 second length and also the signs for the following lengths until another 

 transposition is reached which makes the next sign plus, etc. 



A practical open-wire line is divided into a series of "transposition 

 sections" of eight miles or less. In each section the crosstalk between 

 any two circuits is approximately balanced out by means of trans- 

 positions. A main purpose of this division ipto sections is to provide 

 suitable points for circuits to drop off the line. A circuit on the line 

 for a part of a section may have more crosstalk to a through circuit 

 than if the parallel extended for the whole section since coupling in 



U D -i- D 



Fig. 14 — -Near-end crosstalk in length 2D between circuits a and b with circuit a 



transposed in the middle. 



the last part of the section may tend to subtract from the coupling in 

 the first part. The ends of sections are, therefore, the most suitable 

 points for circuits to leave or enter the line. Ideally, the sections in a 

 line should all be alike as regards length and transposition arrange- 

 ments since this makes it practicable to so design the transpositions 

 that residual crosstalk in one section tends to cancel that in another 

 section. Practically, the sections vary in length and, therefore, in the 

 transposition arrangements because the ends of some of the sections 

 must fall at particular "points of discontinuity" determined by 

 branching circuits and by requirements for balance against induction 

 from power circuits. 



In designing the transposition sections, type unbalances are com- 

 puted for the section lengths of eight miles or less. For such lengths, 

 the general method of computing type unbalances may be simplified. 

 The general method involves the vector propagation constant 7. For 

 a length as short as a single transposition section, attenuation can, 

 ordinarily, be neglected. Therefore, in the type unbalance formulas 

 7 can be replaced by 7/3 which greatly simplifies the computations. 



