752 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1951 



that the highest-capacitance cable pairs in the individual loading sections 

 will not have too much capacitance. When the loading is installed, the 

 capacitances of the various pairs in the individual loading sections are meas- 

 ured, as also are the mutual capacitances of the loading coils and their 

 associated stub cables, and then enough shunt capacitance is added to 

 obtain the desired theoretical total capacitance, per loading section. 



In many installations, especially in underground cables, the theoretically 

 best loading points for the carrier loading coils (after engineering allowances 

 have been made for cable-capacitance deviations) frequently occur at points 

 where it would be unduly expensive to install the loading. In such instances, 

 shortened spacings are used, and the building-out adjustments are increased 

 to correct for the geographical spacing-deficiency along with the cable- 

 capacitance deviations. 



For reasons above mentioned, the individual coil-spacings may vary con- 

 siderably in the same project, and the average coil-spacing may be quite 

 different on different projects involving the same type of loading. The 

 "representative coil-spacings" given in Table XVI are representative job 

 averages. 



As with the voice-frequency loading, the choice of cable conductor-gauge 

 is important in the impedance-matching performance of the C4.1, C4.8, and 

 B15 loaded cables at low voice-frequencies. The optimum resistance rela- 

 tions between the cable conductors and the open-wire conductors are the 

 same as in voice-frequency loading. In the use of the Type J loading as 

 practiced on short cables, this resistance-ratio question is unimportant 

 because such loaded cables are substantially '^ transparent" at voice fre- 

 quencies. 



The loading terminations are unimportant factors in voice-frequency 

 impedance-matching. This follows from the fact that the voice frequencies 

 are low relative to the loading cut-off, for which reason the carrier loaded 

 circuits act as electrically smooth lines in this range. 



Type C Loading: These loading systems were designed for use on cable 

 pairs connected to 12-inch spaced open-wire pairs. The impedances of the 

 open-wire pairs vary substantially with the conductor size and because of 

 this a single cable-loading system would not be satisfactory as regards 

 carrier-frequency impedance-matching for all types of open-wire. The C4.1 

 system is used on cable pairs connected to 165-mil open-wire pairs. The 

 C4.8 is a compromise system for use on cables connected to the less important 

 and less expensive 128-mil and 104-mil open-wire pairs. 



It is of interest that the theoretical coil-spacing for Type C loading is 

 one-sixth of that of the E-spacing described in the discussion of voice- 

 frequency impedance-matching loading. 



