174 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1951 



The compressed, unannealed, powdered iron-core loading coils previously- 

 described were found to be satisfactory with respect to inductance stability 

 and other properties, including hysteresis effects for use in the improved 

 loading systems. New smaller inductance values were necessary, however, 

 for the coils used in the longest circuits. It was of course necessary to con- 

 trol the geographical spacing deviations, and the factory deviations in cable 

 capacitance and in the loading coil inductances, so as to obtain a satis- 

 factory degree of "regularity" in the impedance characteristics of the loaded 

 circuits. 



7.2 Transmission Limitations of Existing Loading Systems 



(a) General: As the lengths of loaded cables progressively increased beyond 

 'those involved in the establishment of the first standard loading systems, 



certain transmission effects which were initially unnoticed or comparatively 

 unimportant became very noticeable as objectionable transmission impair- 

 ments. By increasing the electrical lengths of the circuits, the use of repeaters 

 greatly aggravated these impairments and it became very desirable to correct 

 them so far as feasible at their source. Complex problems thus arose in 

 providing better quality of transmission over much greater distances. 



The impairments referred to above were directly related to the band 

 width of frequency transmitted by the line, which is determined by the 

 cut-off frequency, and to the velocity of transmission. They are discussed 

 briefly in the following paragraphs. 



(b) Attenuation-Frequency Distortion: At frequencies above about 70% of 

 the theoretical cut-off frequency, the attenuation increases with rising fre- 

 quency at a continuously accelerating rate, in consequence of the accumu- 

 lation of the effects of internal reflections at the individual loading points. 



At lower frequencies where these so-called 'lumpiness of loading" effects 

 are not of dominating consequence, the attenuation increases with rising 

 frequency are largely due to the energy losses in the loading coils. (Usually 

 the eddy-current losses in the cores are the most important component 

 loss, since they are proportional to the square of the frequency.) It thus 

 happens that the attenuation losses may pile up in long loaded cables in 

 such a way as to substantially suppress the transmission of the higher- 

 frequency components of speech, even when the attenuation losses are tol- 

 erable at lower frequencies. In consequence, the width of the transmission 

 band which is effectively transmitted over a long loaded cable becomes 

 narrower and the quality of transmission progressively deteriorates as the 

 circuit length increases, unless suitable auxiliary equalizing networks and 

 additional repeaters are utilized. The large amount of attenuation-frequency 

 distortion which occurred in the first transcontinental telephone circuits 

 was previously commented upon. 



