212 BELL SYSTEM TECHNICAL JOURNAL 



groups and one span are shown. The two pairs shown are nearly 

 "non-inductive" for direct crosstalk in this span. 



Consideration has been given to applying this principle to a number 

 of pairs in order to reduce the crosstalk coefficients. Since all the 

 crosstalk coefficients could not be made very small, transpositions 

 would be needed. The experience to date indicates that this method 

 does not look attractive because it is not very effective in reducing the 

 indirect crosstalk, the mechanics of transposing are difficult, the 

 variations in sag are likely to be abnormal and the system is compli- 

 cated. 



There remains the simple method of improving the configuration 

 of the wires in a given pole head area by reducing the spacing between 

 the wires of a pair and increasing the spacing between wires of different 

 pairs. 



The crosstalk per mile between pairs is evidently reduced by this 

 procedure since the two wires of a pair are approaching the ideal of 

 being equally distant from every other conductor. The "sag difference 

 crosstalk" is also reduced and higher frequencies may be used for a 

 given crosstalk result. Fig. 27-A and Fig. 27-B indicate a 20-wire 

 line with the wire spacing used in the past and also the configuration 

 commonly used today on lines where heavy carrier development is 

 involved. The spacing between the two wires of a pair has been 

 reduced from 12 inches to 8 inches and the spacing between pairs 

 correspondingly increased. 



It was not possible to reduce the spacing of the pole pairs and for 

 this reason they are unsuited for the higher carrier frequencies and it 

 is sometimes uneconomic to string them. For such cases the crossarm 

 indicated by Fig. 27-C may be considered. The 8-inch spacing of 

 pairs is retained but the distance between pairs is further increased. 

 With this last crossarm, phantom circuits are not superposed on the 

 8-inch pairs since their use results in greater crosstalk between the 

 pairs and restricts the possibilities of multi-channel carrier operation. 



The crossarm with 8-inch pairs and pole pairs may be used on lines 

 where multi-channel carrier operation is not employed. In such 

 cases, the 8-inch pairs may be phantomed. Since the average spacing 

 between the side circuits of such a phantom is not reduced by the 

 8-inch spacing, the crosstalk between the phantom circuits is about 

 the same as with the 12-inch pairs. The crosstalk from a side circuit 

 into a phantom is somewhat reduced because of the reduced spacing 

 of the pairs. For a given pole head area it does not appear practicable 

 to devise a configuration which will result in marked reductions in the 

 susceptibility of both phantoms and side circuits to crosstalk and 



