A TWELVE-CHANNEL CARRIER TELEPHONE SYSTEM 129 



number of pairs for type J transmission, structural changes, such as 

 respacing the wires of the pairs concerned to six inches, are necessary 

 in order to reduce the coupling. 



One factor of extreme importance is that of reflected near-end cross- 

 talk. In the application of transposition systems it is usually not 

 possible to reduce the near-end crosstalk to a magnitude approximating 

 the far-end crosstalk. It is the latter with which the carrier systems 

 are chiefly concerned, since similar types of systems on different pairs 

 all transmit the same frequency range in the same direction. If, 

 however, the lines concerned do not have smooth impedance character- 

 istics, i.e., a high degree of freedom from reflection effects, near-end 

 crosstalk may be converted by reflection into far-end crosstalk of 

 sufficient magnitude to be controlling over the true far-end crosstalk. 



This means that lines to be used for several type J systems must be 

 made unusually smooth electrically — impedance variations kept 

 within a few per cent. The achievement of such smoothness consists 

 chiefly in: 



(1) Reducing the electromagnetic and electrostatic couplings to other 



pairs so that there are no large energy interactions, with corre- 

 sponding impedance irregularities. Generally speaking, when 

 the pairs concerned have been transposed for reduced far-end 

 crosstalk up to the maximum frequency transmitted, this 

 condition is also satisfied. 



(2) Minimizing the effect of intermediate and terminal cables. This 



latter problem has caused considerable concern and is responsi- 

 ble for the development of several new techniques in the design 

 and treatment of such cables, where they appear in a long line 

 otherwise consisting chiefly of open wire. 



Cable Treatment 



As a means of overcoming the reflection and attenuation effects of 

 short pieces of terminal or intermediate cable, loading naturally sug- 

 gests itself, as applied in type C systems, where the cable pairs in- 

 volved are commonly equipped with carrier loading coils, spaced at 

 about 700-foot intervals. This compares with the 3000-foot or 6000- 

 foot spacings which are standard for voice-frequency loading. How- 

 ever, loading, pairs in existing cables satisfactorily up to 140,000 

 cycles would mean coils at approximately 200-foot intervals. Because 

 of physical limitations, existing manhole spacings, etc., this is highly 

 impractical. A reasonable solution has, however, been found in the 

 creation of a new form of low-capacitance high-frequency cable, — 

 a disc-insulated unit which has constructional features in common 

 with the coaxial cables and a capacitance of only .025 microfarad per 



