f 



INDUCTIVE LOADING FOR TELEPHONE FACILITIES 



453 



ing the design of the new loading systems, the engineering of the exchange 

 cable plant continued for some time on the customary volume-efficiency 

 basis, and the standards of over-all attenuation in the trunks were the same 

 as before, in the use of the older loading systems. The improvement in 

 intelligibiUty previously stressed was directly due to the abihty of the new 

 loading systems to transmit efficiently a band of important high-frequency 

 overtones which were suppressed by the old standard loading systems. The 

 subscriber services directly benefited from the improved transmission 

 quahty. 



Used in the foregoing manner, the new loading systems also yielded large 

 economies in the first costs of new plant by extending the transmission 

 range of the cheaper types of cables. In this respect, the M88 system was 

 by far the most important of the new standards, since it made feasible the 

 use of loaded 22 ga. cable for short trunks, in place of non-loaded 19 ga. 



Table VIII 

 Improved Exchange Area Loading Standards 



Loading 

 Designation 



Loading 



Sp>acing 



(feet) 



Coil 



Inductance 



(mh) 



Approximate Cut-off Frequency* 



High-Capacitance 

 Cables 

 (cycles) 



Low- 

 Capacitance 

 Cables 

 (cycles) 



M88 

 H135 

 HI 75 

 D175 



9000 

 6000 

 6000 

 4500 



88 

 135 

 175 

 175 



2900 



2800 



Not recommended 



2900 



3200 

 3200 

 2800 

 3200 



* These particular figures take 0.083 mf/mi and 0.066 mf/mi as representative values 

 for high-capacitance and low-capacitance cables, respectively. 



cable, and the aggregate length of the short trunks is a large fraction of the 

 total exchange trunk mileage. The special economic importance of 22 ga. 

 cable loading received recognition in the development of much cheaper 

 loading coils which are described later on. 



The more expensive new H-spaced loading was advantageous on the 

 longer cables, and in shorter cables when lower transmission equivalents 

 were necessary. HI 75 loading was important because of its suitability for 

 use on low-capacitance cables. 



The D175 system provided a field for the reuse of 175 mh loading coils 

 that were displaced in the course of the plant rearrangements, previously 

 mentioned. Also, it faciUtated the conversion of old Ml 75 facihties to meet 

 the new cut-off standards, and with decreased attenuation. This conversion 

 procedure involved the introduction of additional 175 mh loading coils, at 

 or near the electrical centers of the old medium loading sections. 



Some typical performance characteristics of the new loading systems on 



