LOADING FOR TBLIiPllOKE CIRCl'llS 265 



cabling. Of these, the cable and associated loading coils are the major 

 sources of unbalances. 



The phantom-to-side and side-to-side cross-talk unbalances in the 

 cable quads are reduced to small values by exercisng great care both 

 in the various manufacturing processes and in the selection of raw 

 materials. When the cable is installed in the field, a large improve- 

 ment in cross-talk conditions is secured by splicing adjacent lengths 

 of cable together in such a way that the unbalances in one length of 

 cable substantially neutralize the unbalances contributed by the 

 adjacent length of cable. Usually, three such "capacity-unbalance 

 test" splices are made at symmetrical points in each loading section 

 and as a result the average over-all capacity unbalance in a loading 

 section is reduced to about one-tenth of the magnitude which would 

 hold if these test splices were not made. 



In the design of the standard phantom circuit and side circuit loading 

 coils, special care was taken to make them substantially free from 

 inherent unbalances. Also in the manufacture of the coils, great care 

 is exercised to realize the benefits of the inherent symmetry of the 

 designs. In the early days before telephone repeaters came into 

 general use on loaded lines, satisfactory results from the standpoint 

 of self inductance and mutual inductance unbalances were obtained 

 by adjusting the different windings to the nearest turn; i.e., a condi- 

 tion of balance where either adding or subtracting one turn to one of 

 the line windings would increase the cross-talk rather than reduce it. 



Later when repeaters came into general use, it was found neces- 

 sary to obtain much more refined adjustments. Further improve- 

 ments have been worked out in manufacturing methods and processes 

 which allow a greater degree of symmetry. As a result of these various 

 improvements, the phantom-to-side cross-talk unbalances in the 

 loading coils have been reduced approximately 75 per cent, or more 

 below the values obtained before repeaters came into general use on 

 small gage toll cable. The coil cross-talk unbalances are now nearly 

 as low as the cross-talk unbalances in the associated cable sections 

 after the completion of the capacity unbalance test splicing. 



The loading coils used in the very long circuits having H-44-25 

 loading obviously are more important from the standpoint of cross- 

 talk limitations than the coils used in the shorter circuits having 

 H-174-63 loading, and somewhat greater care is required in their 

 manufacture. These coils are adjusted and tested in a factory test 

 circuit which at the cross-talk test frequency simulates the service 

 impedance conditions. In the phantom-to-side cross-talk test, the 

 disturbing test current is superposed on the phantom circuit, and 



