742 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1951 



mentioned. This, however, was not a serious limitation in the non-loaded 

 repeater circuits. After open-wire loading became obsolete, further improve- 

 ments in telephone repeaters and in transmission standards led to progressive 

 improvements and refinements in the loading used on cables in non-loaded 

 lines. 



Beginning around 1920, the rapidly increasing use of open-wire telephone 

 and telegraph carrier systems made it necessary to use in the associated 

 incidental cables improved loading systems that provided good impedance- 

 matching and attenuation-reduction properties over the complete voice 

 and carrier-frequency bands used by the carrier transmission systems. The 

 use of several different carrier telephone systems employing materially 

 different frequency-band widths made it economically desirable in due 

 course to use several different types of loading to provide the necessary 

 transmission bandwidth through the incidental cables. 



The various types of cable loading mentioned above are separately con- 

 sidered under suitable headings in the following pages. The two principal 

 subdivisions of Part V are devoted to voice-frequency loading and to carrier 

 loading, respectively. A third subdivision briefly describes a special type of 

 voice-frequency phantom loading which is used in coordinated phantom- 

 group combinations with side-circuit carrier loading systems that provide 

 10-kc and 30-kc transmission bands. 



The importance of the incidental cable loading described in Part V of this 

 article is due to its substantial, beneficial contributions to the transmission 

 service-performance of the relatively expensive open-wire facilities, rather 

 than from the amount of loading so employed. This is quite small relative 

 to that used in voice-frequency toll cables and exchange cables. 



(V-A) Voice-Frequency Impedance-Matching Loading 



Since the most important early uses of the vacuum-tube repeaters on 

 open-wire facilities were on loaded lines, the first new impedance-matching 

 loading system was developed for this particular use. As noted later, this 

 had an important effect on the loading system subsequently developed for 

 use on cables in non-loaded lines. 



Loading for Cables in Loaded Open-Wire Lines 



The new phantom-group loading for this use was designed to have closely 

 the same values of nominal impedance and theoretical cut-off frequency as 

 those of the loaded lines. The cable coil inductances had to be a little higher 

 than the open-wire coil inductances, in consequence of the smaller amount of 

 distributed inductance in the cable. A standard cable coil-spacing of about 



