586 BELL SYSTEM TECHNICAL JOURNAL 



loaded cable. The present standard types of carrier cable loading 

 systems '" provide for the use of loading coils spaced at intervals of 

 approximately 930 feet. When loaded, the cable circuits have a 

 characteristic impedance closely approximating the open-wire im- 

 pedance over the frequency range used in carrier transmission. This 

 same carrier loading also greatly improves the characteristics of the 

 voice circuit. The high-frequency attenuation is reduced to approxi- 

 mately one half the non-loaded condition. A special type of cable 

 loading is also available for use in improving the transmission charac- 

 teristics of office cable and wiring and very short intermediate and 

 entrance cable. 



External Interference. The carrier channels are unusually free from 

 noise due to extraneous induced currents. However, this is the result 

 of attention to this factor in the design of the apparatus and in laying 

 out the installations rather than anything inherent in the high- 

 frequency feature as such. Our experience has indicated that it is 

 possible, if care is not taken, to have interference from the following 

 external sources: 



a. Harmonics of power frequencies. 



h. Irregular frequencies produced by abnormal power line actions, 

 such as arcing insulators, charging lightning arresters of certain 

 types, electric railways, series street lighting, etc. 



c. Power line carrier systems. 



d. Powerful transoceanic radio transmitters. 



e. Lightning and other atmospheric disturbances. 



In the matter of harmonics of the power line frequencies, the source 

 of their generation normally limits them to very low magnitudes in 

 the high-frequency range which has been employed for carrier systems 

 on telephone lines. In this respect the carrier systems are, in general, 

 affected to a lesser extent than the normal telephone circuits in the 

 voice range. In the latter case, the power circuit harmonics frequently 

 present serious interference problems because the harmonics in the 

 power circuits are substantially greater at the lower frequencies. 



Under particular conditions, however, such as, for example, in 

 connection with a series street lighting system operated with individual 

 series transformers or auto-transformers, where a burned-out lamp 

 causes the saturation of the transformer magnetic circuit, induced 

 harmonics of considerable magnitude, up to 30,000 cycles and over, 

 have been measured in the carrier telephone circuits. Under the same 



^Thomas Shaw and Wm. Fondiller, "Development and Application of Loading 

 for Telephone Circuits," Bell System Tech. JL, April 1926, pp. 221-281. 



