LOADING FOR TELEPHONE CIKCUITS 22>9 



The practical solution of this fundaiiicntal repeater-line balance 

 problem required (a) the construction of lines having extremely 

 regular impedance characteristics over the frequency band which 

 the repeater is designed to transmit and {b) the development of 

 balancing networks " capable of accurately simulating the scnding- 

 end impedance characteristics of the impro\ed lines throughout this 

 frequency range. On account of the great difficulty of getting a high 

 degree of balance at frequencies near the cut-off frecjuency of the 

 loading, partly due to line irregularity effects and partly due to net- 

 work design complications, it has been found desirable to use electric 

 wave filters ^- in the repeater sets which cut off at a frequency below 

 the cut-off frequency of the loading. This margin of cut-off effects is 

 usually 200 cycles or more, depending upon the repeater design and 

 the type of loading involved. 



The "regular" line referred to in (a) is one which is free from im- 

 pedance irregularities. In the case of loaded lines, the loading coils 

 should have very closely the same inductance values, and the sections 

 of line between loading coils should have closely the same value of 

 capacitance. These uniformity features should be permanent, which 

 requires that the coils should have a high degree of stability in their 

 inductance characteristics; i.e., they should be capable of resisting 

 the magnetizing effects of abnormal service conditions. Some of the 

 older types of coils did not meet this requirement. The satisfactory 

 way in which these fundamental coil requirements are fulfilled in 

 the newer types of coils wall be described in a subsequent section. 



Uniformity in the loading section capacitance values involves uni- 

 formity in cable and line capacitance values as well as precision in 

 the coil spacing. In toll cable loading the maximum deviations from 

 the average spacing are kept below 2 per cent., and the axerage devia- 

 tions are in the order of 0.5 per cent, or less. 



In exceptional cases where physical obstructions are encountered 

 in reducing the spacing deviations to a sufficiently low value, use is 

 made of "building-out condensers" or "building-out stub cables" 

 to normalize the capacitance of loading sections.'^ Abnormally long 

 loading sections can usually be split up into two sections, one or both 

 of which may then be "built out" to the nominal standard capacitance 

 values. 



Trayncontinental Lines — High Stability Loading Coils. The in- 



" R. S. Hoyt "Impedance of Loaded Lines and Design of Simulating and Com- 

 pensating Networks," Bell System Technical Journal, July 1924. 



12 U. S. Patents Nos. 1,227,113, and 1,227,114— G. A. Campbell. 



13 U. S. Patent No. 1,219,760— John Mills and R. S. Hoyt. 



