CROSSTALK ON OPEN- WIRE LINES 517 



sections and that arising from interactions between the sections tend to 

 combine. In a series of like transposition sections there is a tendency 

 for the crosstalk to increase systematically, sometimes reaching in- 

 tolerable magnitudes. This tendency can be controlled to a degree by 

 introducing transpositions at the junctions between the sections, thus 

 cancelling out some of the major components of the crosstalk. Complete 

 cancellation is impossible because of interaction and propagation effects. 



ABSORPTION 



Since very significant couplings exist by way of tertiary circuits, it is 

 possible for crosstalk to reappear on the disturbing circuit and thus 

 strengthen or attenuate the original signal. This gives rise to the ap- 

 pearance of high attenuation known as absorption peaks in the line 

 loss characteristic at certain critical frequencies. The evaluation of such 

 pair-to-self coupling requires the use of coefficients which differ from 

 those between different pairs and these are given for standard configura- 

 tions. 



STRUCTURAL IRREGULARITIES 



It is impracticable to maintain absolute uniformity in the spacing 

 between wires and in the spacing of transpositions. Thus there are un- 

 avoidable variations in the couplings between pairs from one transposi- 

 tion interval to the next. This in turn reduces the effectiveness of the 

 measures to control the systematic or type unbalance crosstalk and 

 produces what is known as irregularity crosstalk. Since the occurrence 

 of structural irregularities tends to follow a random distribution, it is 

 possible to evaluate it statistically and procedures for doing so are in- 

 cluded. In addition to this direct effect of structural irregularities, there 

 is a component of crosstalk resulting from the combination of systematic 

 and random unbalances. A method is developed for estimating the 

 magnitude of this important component of crosstalk. 



EXAMPLES 



In order to demonstrate how the procedures and data are used in 

 solving practical problems, there is included the development of a 

 transposition system to satisfy certain assumed conditions. This is 

 carried through to the selection of transposition types for one transposi- 

 tion section and the selection of suitable junction transpositions. 



Additional examples of transposition engineering are given in the form 



