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BELL SYSTEM TECHNICAL JOURNAL 



noise The "square phantom" indicated by A of Fig. 25 has theo- 

 retical possibiUties but studies of the effect of wire spacing deviations 

 make this arrangement appear impracticable. 



The proposal to reduce the spacing of the wires of a pair from the 

 historic value of 12 inches naturally raised the question of swinging 

 contacts. However, extensive experience with 8-inch spacing has 

 shown no appreciable increase in the number of wire contacts. This 

 applies to lines where ordinarily the span length did not exceed about 

 150 feet. With long span crossings, crossarms were supported from 

 steel strand at intervals of 260 feet or less. 



The effectiveness of the reduction in wire spacing is indicated by 

 the following table. The table shows the measured near-end and 

 far-end crosstalk coefficients for important circuit combinations and 

 for the two-pole head diagrams of Figs. 27-A and 27-B. 



Crosstalk Per Mile Per Kilocycle — 104-Mil Conductors 



General Transposition Design Methods 

 The preceding discussion will indicate that transposition design 

 involves much more than consideration of the locations of the trans- 

 positions. 



In practical design, the first step is to estimate the crosstalk due to 

 unavoidable pole spacing and wire spacing irregularities for the 

 configuration of wires under consideration and for a wide frequency 

 range. This crosstalk represents the best that can be done with an 

 ideal transposition design. It must be kept in mind that great 

 precision is impracticable. The pole spacing of a line may change 

 from time to time due to minor reroutings caused by highway changes, 

 etc. The wire sag differences change with temperature and are 

 affected by sleet. 



If two long circuits are on adjacent or nearby pairs in one repeater 

 section, they should, as far as practicable, be routed over non-adjacent 



