200 THE BELL SYSTEM TECHNICAL JOUENAL, JANUARY 1951 



ohm loading, especially at low frequencies. It is an acceptable substitute for 

 B22 loading for transmission of AM broadcasting material, being appreciably 

 better in the frequency range 4 to 8 kc, because of its much higher cut-off 

 frequency, and it is substantially as good at low frequencies. Accordingly, 

 to provide plant flexibility for possible future broadcasting service require- 

 ments, this 800-ohm 15-kc loading is being installed on short-haul program 

 pairs in new toll cables in situations where the anticipated initial needs are 

 for AM broadcast programs. When the loading is installed in the course of 

 the cable installation, the slightly higher cost of the broader-band loading is 

 negligible in comparison with the plant flexibility-advantage above cited. 



The field of use for the 480-ohm 15-kc program loading is in short repeater 

 sections where the transmission requirements permit its use, and where it is 

 desirable to take advantage of its lower cost, relative to the 800-ohm loading. 

 This cost advantage is maximum when the loading has to be applied to 

 cables previously installed. 



The loading coils for the new 15-kc program loading are of the same size 

 as the molybdenum-permalloy core coils developed for B22 loading. Accord- 

 ingly, when installation conditions are favorable, economies can be achieved 

 by placing the coils within the loading splices. The new program loading 

 coils use 60-permeability, compressed, molybdenum-permalloy powder cores. 

 This relatively new grade of molybdenum-permalloy has much lower eddy- 

 current losses than the 125-permeability grade used in voice-frequency 

 loading coils (and in B22 program loading), and is very advantageous in the 

 reduction of attenuation over the upper two-thirds of the 15-kc trans- 

 mission-band. The 60-permeability cores also have superior non-linear dis- 

 tortion-characteristics. 



(14) Control of Crosstalk 



The control of crosstalk between adjacent telephone circuits has been a 

 very important problem from the beginning of the use of loading, especially 

 in loaded cables, and has been most difficult in long loaded, repeatered, 

 quadded toll cables. 



14.1 Cable Unbalances 



The effect of loading in increasing the circuit impedance raises the voltages 

 which act upon the unavoidable cable capacitance-unbalances, and reduces 

 the magnitudes of the line currents which act upon the series resistance 

 (and inductance) unbalances. Consequently, with the introduction of load- 

 ing, it became necessary to improve the design of the cables so as to materi- 

 ally reduce the capacitance unbalances. This was accomplished by using 

 different lengths of twist in adjacent pairs and in adjacent layers, and by 

 using different lengths of lay in adjacent layers. 



