156 



THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1951 



earlier commercial beginning. ^^^ Three different cable loading standards 

 were adopted, to provide for a range of attenuation-reduction performance. 

 Some data regarding these systems are given in Table II. The transmission 

 data apply to the early types of cable having an average mutual capacitance 

 of about 0.070 mf/mi. 



The theoretical loading cut-off frequencies were approximately 2300 

 cycles (about 7000 loads per second). This initial standard was the result 

 of extensive series of speech transmission tests to determine the minimum 

 cut-off frequency that would be commercially satisfactory with respect to 

 intelligibility. A materially higher cut-off would have increased the loading 

 costs by requiring the loading coils to be more closely spaced. 



Table II 



First Standard Cable Loading Systems 



(Using Coils or Table I) 



Note: The figures given in the columns headed "nominal impedance" and "nominal 

 velocity" apply for the nominal impedances and the nominal velocities of the 

 hypothetical "corresponding smooth Hnes," having the same total inductance 

 and total capacitance. 



The first standard open-wire loading used No. 501 coils at about 8-mile 

 spacing, giving an impedance of about 2100 ohms and a cut-off frequency 

 close to the standard cut-off frequency for cable loading. Under dry-weather 

 insulation conditions (5 megohm-miles or better) the attenuation losses 

 in the 104-mil and 165-mil lines were about 0.031 and 0.014 db/mi, re- 

 spectively. The corresponding losses without loading were 0.075 and 0.033 

 db/mi, respectively. The approximate 8-mile spacing fitted in with the 

 open-wire transposition arrangements and gave a satisfactory attenuation- 

 loss reduction. The earlier attempts to secure a much greater attenuation 

 reduction had involved shorter spacings, ranging down to 2.5 miles, and 

 were unsuccessful. At extended periods of low line-insulation caused by wet 

 weather, these higher-impedance loading arrangements had poor trans- 

 mission, sometimes worse than non-loaded lines. Excessive noise, crosstalk, 

 and reflection losses also were unfavorable factors. 



(•*> N. Y.-Chicago, 165-mil open-wire line, November 1901 ; New York-Newark cable, 

 August, 1902. 



