TWELVE-CHANNEL OPEN-WIRE CARRIER SYSTEM 375 



obtained on a side circuit transposed for operation at frequencies only 

 up to about 10 kilocycles. The absorption at frequencies above this 

 becomes very large. Curve B shows the absorption present on one of 

 the C carrier side circuits on the same line transposed for operation up 

 to 30 kilocycles. Curve C shows how absorption disappears on a non- 

 phantomed pair specially transposed for type J operation. If this 

 pair were measured at much higher frequencies, similar absorption 

 "bumps" would be found, perhaps at frequencies of 200-300 kilocycles 

 or higher. 



Since absorption effects depend on the systematic addition of 

 crosstalk currents along a line, a continuous succession of identical 

 transposition sections tends toward greater absorption while a random 

 succession of different kinds of transposition sections of different 

 lengths will reduce it. The Dallas-Longview J system is operating on 

 an Alternate Arm side circuit, transposed for C carrier operation and 

 without any modifications to adapt it for the higher frequencies. 

 Because of the fortunately irregular succession of different transposition 

 sections found here, it was possible to select, after tests, a pair with no 

 serious absorption. 



Construction Irregularities 



With the new transposition designs, the systematic crosstalk 

 resulting from the transposition arrangements has been reduced in 

 nearly every case so far that the remaining crosstalk is controlled 

 principally by construction irregularities. An important source of 

 irregularity is the difference in sags of the various wires in each span of 

 the line, particularly sag differences between the two wires of each pair. 

 Another potentially important source of irregularity is the variation in 

 the spacings between successive transposition poles. It is relatively 

 easy to make this factor unimportant as compared with sag differences. 



The large amount by which the crosstalk can be reduced by careful 

 methods of construction coupled with the highly developed systematic 

 transposition patterns is illustrated by the fact that between certain 

 pairs the crosstalk in a 75-mile repeater section is reduced to a value 

 which would be produced by a capacitance unbalance between them of 

 less than 2 mmf, which is about the same in magnitude as the capaci- 

 tance between wires of a foot of the open-wire pair. This large 

 crosstalk reduction is in spite of the fact that at 140 kilocycles the 

 phase change along an open-wire circuit is about 7° in a single span, the 

 shortest distance between any two transpositions, and about 28° for 

 the more common four-span interval. 



