142 BELL SYSTEM TECHNICAL JOURNAL 



range about 20 db, which is close to the maximum reduction possible 

 with a universal type of balancing unit. The reduction is limited by 

 the fact that two pairs having identical crosstalk couplings in each of 

 two short lengths at different points in the cable will not produce two 

 identical elements of crosstalk current at a circuit terminal because: 



(1) Cable circuits are not perfectly smooth. Reflections, as at junc- 

 tions of reel lengths or at terminals, alter the two crosstalk currents 

 differently, (2) The propagation constants of each circuit vary slightly 

 from reel to reel in a random fashion and therefore the two crosstalk 

 currents are of slightly different phase and magnitude, (3) In any short 

 length the disturbing circuit produces crosstalk currents in inter- 

 mediate circuits, which are propagated along these circuits and cross- 

 talk again into the disturbed circuit at various points, producing an 

 additional crosstalk current at the circuit terminal. At any frequency, 

 this interaction crosstalk current has a random phase and magnitude 

 relation to the crosstalk current for the short length considered by 

 itself, and depends also upon the position in a repeater section of the 

 short length. 



A 20 db crosstalk reduction is not required, considering the number 

 of K systems anticipated in any one cable. Studies were made, there- 

 fore, to determine whether satisfactory results could be obtained with 

 a less expensive type of balancing, as outlined below. 



The effects of frequency and circuit impedance on crosstalk coupling 

 are as follows: (1) Crosstalk in a short length due to capacitance and 

 to inductance coupling increases about directly as the frequency in- 

 creases for circuits whose impedance is independent of frequency. 



(2) Crosstalk due to capacitance coupling varies directly as the im- 

 pedance of the circuits while that due to inductance coupling varies 

 inversely as the impedance. Changing the impedance from about 800 

 ohms for loaded voice circuits to about 135 ohms for non-loaded carrier 

 circuits and changing the frequency from about 4 kc. to 60 kc. increases 

 the crosstalk due to capacitance coupling by a factor of about 2.5 and 

 that due to inductance coupling by a factor of about 90. 



Capacitive coupling in existing cables was reduced by design to as 

 great a degree as practicable, particularly for the most closely asso- 

 ciated circuits, because it is of most importance in the loaded voice fre- 

 quency case. These same design measures also reduce inductive 

 coupling but not to the same extent. Capacitive coupling decreases 

 rapidly with separation due to the shielding effect of copper in inter- 

 vening circuits while inductive coupling is not much affected by inter- 

 vening copper wires. To minimize magnetic coupling it is necessary 

 to use different lengths of twist for the pairs. Existing cables have 

 relatively few lengths of pair twists. 



