524 BELL SYSTEM TECHNICAL JOURNAL 



Inductance and Spacing of Loading Coils 



Theoretically the inductance and spacing of loading coils might be 

 varied for each circuit length in order to obtain the most economical 

 design. Studies which have been made, however, indicate that, 

 taking into account the advantage of flexibility, it is desirable to use 

 only two types of spacings of loading coils and only two general types 

 of loading units. In the Bell System the toll loading coil spacings are 

 3000 feet (915 meters) and 6000 feet (1830 meters), loading with 

 these spacings being designated B and H, respectively. For two-wire 

 circuits, 88-millihenry loading coils for the sides and 50-millihenry 

 coils for the phantoms are used with both spacings, giving loadings 

 designated as B-88-50 and H-88-50. The choice between these is 

 dictated by the repeater spacing. If less than about 45 miles (72 

 kilometers), H-88-50 loading is used; if greater, B-88-50 is used. 

 Two-wire H- 172-63 loading was used in the past for two-wire circuits 

 but this has now been given up for new work in favor of the wider 

 frequency band B and H-88-50 systems. 



For four-wire circuits, as stated above, the standard loading is 

 H-44-25 meaning, of course, 6000-foot (1830-meter) spacing of coils 

 with 44-millihenry coils on the sides and 25-millihenry coils on the 

 phantoms. 



Important Transmission Characteristics of Loaded Cable Conductors 



The characteristics of loaded cable circuits depend principally upon 

 the electrical constants of the cable conductors, the inductance of the 

 loading coils and their spacing. Some of the more important trans- 

 mission characteristics of the loaded cable systems employing cables 

 and loading coils of the type just described are given in Table I. 



Spacing of Repeaters and Automatic Transmission Regulators 



Repeaters are spaced as close to 50 miles (80 kilometers) apart as 

 practicable. In the past variations upward from this to about 60 

 miles (100 kilometers) were allowed but it is now believed best to 

 avoid such long spacings. Where the location of cities or other 

 geographical situations make it desirable, spacings less than 50 miles 

 (80 kilometers) are used. 



Automatic transmission regulators are provided for circuits in aerial 

 cables in excess of 50 to 100 miles (80 to 160 kilometers) in length 

 and are preferably placed at every second repeater station. The 

 devices are such, however, that satisfactory results may be obtained 

 if regulators are as far apart as about 150 miles (250 kilometers), 

 while under certain conditions circuit flexibility considerations call 

 for regulators at adjacent repeater stations. With cable entirely 



