252 BELL SYSTEM TECHNICAL JOURNAL 



flexibility, the new loading systems all have the same coil spacing of 

 6,000 feet. 



The coil spacing being fixed, it necessarily follows that any reduc- 

 tion in coil inductance for the purpose of raising the cut-off frequency 

 will also increase the transmission velocity. The attenuation im- 

 provement obtained by the loading decreases as the velocity is in- 

 creased. High velocity loading is more expensive than low velocity 

 loading, in the sense that more repeaters are required for the same 

 over-all loss. Obviously, although high velocity loading could be 

 used for short haul traffic, it would not be so economical as a low 

 velocity loading. Commercial considerations thus justify a series 

 of loading standards, graded to meet the requirements of the different 

 lengths of circuits. 



At the present time the two phantom group loading standards, 

 H-44-25 and H-174-63, are sufficient to meet the graded requirements 

 of commercial toll cable circuits, when used with suitable combina- 

 tions of conductor sizes and repeaters. Three different general types 

 of repeaters are used, known as the 21, 22, and 44 types.^^ The 21 

 type is used on two-wire circuits requiring only one repeater, under 

 conditions where switched connections involving other repeaters are 

 not involved. The 22 type is used on two-wire circuits requiring one 

 or more repeaters. The 44 t>pe is used on four-wire circuits, where 

 one pair of wires is used for one-way transmission in one direction 

 and the other pair of wires for transmission in the opposite direc- 

 tion. When phantom circuits are worked on a four- wire basis, each 

 one-way transmission path actually uses four wires. 



Table XI lists the combinations of loading, conductor gage, and 

 type of repeater circuit which are used in meeting the wide range of 

 commercial requirements. The position of the facility item in the 

 table indicates the sequence of transmission excellence. Item (i) being 

 the highest grade facility in this respect. In general, the cost of these 

 facilities is in reverse order to the sequence of electrical excellence. 



The exact limits of the field of use of a given type of facility depend 

 upon the magnitude of the permissible over-all transmission loss, 

 and upon the grade of repeater balance obtainable. A discussion of 

 these features would bring in complicated engineering questions 

 beyond the scope of the present paper. So far as loading features are 

 concerned, it is sufficient to state that H-44-25 loading is generally 

 used on circuits of approximately 500 miles or more. On circuits 

 intended for switched business, it is frequently necessary to use this 



^' Gherardi — Jewett, Loc. cit. 



