264 HP.LI^ SYSTEM TECHNICAL JOURNAL 



Till" air core sick> circuit coils lia\e a small leakage inductance 

 which must he allowed for in determining the phantom coil induct- 

 ance. For this reason the phantom coil inductance is lower than in 

 the H-2S-1() system (Table XII.) In order to avoid impedance ir- 

 regularity in the carrier circuits at the phantom loading points, it is 

 necessary that the combination carrier-phantom loading units should 

 haye closely the same total inductance and shunt capacitance as the 

 ordinary carrier loading coils. This requires the use of a different 

 type of carrier loading coil at the phantom loading point from that 

 at the non-phantom loading points, haying a lower inductance and 

 capacitance corresponding to the leakage inductance and shunt 

 capacitance of the associated phantom coil. Other refinements of 

 design are inyolyed in these combination loading units.^^ 



IV. Cross-Talk 



One of the greatest practical difficulties which has been encountered 

 in extending the commercial range of long distance telephone service 

 is that of keeping at a tolerably low value, the speech overhearing 

 effects known as cross-talk, which occur between adjacent telephone 

 transmission circuits whenever there is an appreciable amount of 

 electromagnetic or electrostatic coupling between them. 



From the early days of telephony great care has been exercised in 

 plant design and construction work to avoid circuit and apparatus 

 unbalances, but as is to be expected from the nature of the problem, 

 it is practically impossible to obtain and maintain absolutely perfect 

 balance. In short telephone circuits, there is no particular difijculty. 

 in keeping the unbalance effects small enough so that the over-all 

 cross-talk is not serious. As the length of the line increases, however, 

 there are more and more opportunities for unbalances in the lines and 

 in the associated apparatus in the lines and offices. In repeatered 

 lines, moreover, the repeaters amplify the cross-talk as well as the 

 speech transmission. Thus we have the cumulative effects of cross- 

 talk from successive sections in the long repeatered lines. From the 

 service standpoint, moreover, it is necessary that the cross-talk in the 

 very long lines should be within the limits set for the shorter lines. 



The problem of keeping cross-talk low between a phantom circuit 

 and its associated side circuits, and between the two associated side 

 circuits of a phantom group, is by far the most dit^cult phase of the 

 general cross-talk problem in long repeatered cables. It is present in 

 the cables, the loading coils, the terminating apparatus and the office 



«U. S. Patents Nos. 1,501,959, Martin and Shaw; 1,501,926, Shaw. 



