INDUCTIVE LOADING FOR TELEPHONE FACILITIES 755 



lar advantage assumed critical importance in later years when it became 

 necessary to work to stringent over-all limits of non-linear distortion in the 

 long-haul carrier facilities. As an example of the importance of controlling 

 non-linear distortion, it became necessary during the late 1920's to mount 

 the carrier loading coils in individual, shielding containers in order to prevent 

 the small leakage fields of the toroidal air-core coils from penetrating nearby 

 magnetic parts of the loading coil cases, thereby causing objectionable inter- 

 channel modulation interference. 



The satisfactory control of non-linear distortion has made it necessary to 

 continue the use of non-magnetic cores in the carrier loading coils, notwith- 

 standing the large improvements that have been made in magnetic core- 

 materials during the last three decades. These improvements would make it 

 possible to use much smaller coils without objectionably degrading the 

 steady-state transmission performance. However, the hysteresis character- 

 istics of the best available magnetic materials are such that if these materials 

 should be employed it would be necessary to use coils larger and more ex- 

 pensive than the non-magnetic core coils, in order to meet present-day se- 

 vere limits on allowable intermodulation interference in the Type C tele- 

 phone systems. 



Types C and B Loading 



Ftdl Coils: These loading systems use the same general types of full- 

 weight loading coils and terminal loading units, except as regards their 

 electrical parameters. The over-all dimensions of the full- weight coils are 

 about 6f inches in diameter and 2\ inches axial height. The shielding con- 

 tainer has an over-all diameter of about 7J inches and an axial height of 

 3j inches. 



Terminal Loading Units: The terminal loading units which provide the 

 compensated loading terminations, previously mentioned, include a 0.82 

 fractional-weight series loading coil. This is shunted on the open-wire side 

 (or office side) by a two-element network consisting of a condenser in series 

 with an inductance coil, and located between the two haK-windings of the 

 coil. The complete terminal network may be regarded as an extension of 

 half-coil termination. The portion beyond the half -coil point in the series 

 (fractional) coil functions as an impedance corrective-network to produce 

 the approximate ** corresponding smooth line" impedance, previously de- 

 scribed. The correct electrical proportioning of the elements of this corrective 

 network is very important. The series loading coil is much smaller than the 

 regular full-weight loading coil. Its size and those of the other network- 

 elements are such as to allow the assembly of the complete terminal loading 

 unit in the same size of shielding container as that i^sed for the full- weight 



