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THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1951 



ment work on improved loading for long repeatered circuits, it is opportune 

 to include some detailed information on the previously mentioned improve- 

 ment of phantom loading for the so-called "medium-heavy" loading system, 

 i.e., the combination of new H63 phantom loading with the existing HI 74 

 side circuit loading to constitute the HI 74-63 phantom-group loading system, 

 data for which are included in Table IV. The practical importance of this 

 development was in the substantial reduction it permitted in loading costs. 

 It marked the beginning of commercial use (1923) of phantom loading 

 coils having cores similar in size to those of the associated side circuit coils. 

 The cost savings resulted directly from the coil size-reduction, and from the 

 increased size of loading complements that could be placed in standard-size 



0.001 



3000 



3500 



1000 1500 2000 2500 



FREQUENCY IN CYCLES PER SECOND 



Fig. 7 — Attenuation frequency characteristics of short and long loaded toll cable cir- 

 cuits having a net attenuation loss of 10 db at 1000 cycles per second. 



pots, and from the simplification of pot assembly and cabling arrangements. 

 The choice of phantom coil inductance (63 mh) was such as to make the 

 phantom circuit 1000 cycle attenuation the same as that on the associated 

 side circuits, without increasing the side circuit attenuation above that in 

 HI 74-106 loading. This reduction in the loading inductance substantially 

 improved the phantom circuit's transmission performance-characteristics 

 by virtue of the substantial increase in cut-off frequency and transmission 

 velocity, and made the repeatered phantom circuits electrically suitable for 

 much greater distances than their side circuits. This superiority was seldom 

 utilized, however, because of the practical operating flexibility advantages 

 inherent in the established practices of using the associated side circuits and 

 phantoms interchangeably between the same operating or switching centers. 

 In due course, the sizes of the other toll cable phantom loading coils were 



