172 BELL SYSTEM TECHNICAL JOURNAL 



gradually. It is apparent that the variations in attenuation with 

 weather increase with the length of the system, and that more frequent 

 adjustments will be required on the longer systems to maintain the 

 overall circuit net loss within given limits. 



The carrier frequency attenuation of cable circuits is much greater 

 per unit length than that for open-wire lines. Hence, the losses 

 introduced by comparatively short sections of cable, either at entrances 

 to offices or at intermediate points, are of considerable importance. 

 In addition to the attenuation of the cable itself, there are large re- 

 flection losses at the junction of the cable and the open wire, due to 

 the difference between the characteristic impedance of the open-wire 

 and that of the non-loaded cable. The carrier terminal and repeater 

 have been designed to have the same impedance as the average of 

 the open-wire line facilities, so the same considerations apply at the 

 junction of the cable and the carrier equipment. As an example of 

 the magnitude of these effects, the insertion loss at 8000 cycles of two 

 miles of non-loaded 16-gauge cable is approximately 6 db, or about 

 the same as that of 60 miles of 104-mil open-wire. 



By applying carrier loading that has been developed for this purpose, 

 the attenuation loss of such a cable can be reduced to about 1 db at 

 8000 cycles. In addition, the reflections at the terminals of the cable 

 will be reduced to satisfactory low values by virtue of the impedance 

 matching properties of the loading. This method of treatment has 

 the important advantage that it improves the transmission charac- 

 teristics in both the voice and carrier ranges. 



In cases where substantial transmission margins exist, it is sometimes 

 practicable to use impedance matching transformer networks at the 

 cable terminals, as a substitute for carrier loading, with economies 

 that are proportional to the length of the cables. At the present 

 stage of development, this so-called transformer treatment is much 

 less satisfactory in the voice-frequency range than in the carrier- 

 frequency range. Certain inherent limitations in simple transformer 

 treatment result from the fact that the ratio of the (non-loaded) 

 cable impedance to the open-wire impedance varies widely over the 

 frequency band to be transmitted, and the transformer impedance 

 ratio that is optimum at carrier frequencies is distinctly disadvan- 

 tageous in the voice-frequency range. The choice of optimum trans- 

 former ratio for the complete transmission band may thus involve 

 different compromises for different sets of conditions and service 

 requirements. 



Where several carrier systems are to be placed on a pole line, cross- 

 talk between systems becomes an important consideration. Where 



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