INDUCTIVE LOADING FOR TELEPHONE FACILITIES 753 



Considerable Type C loading has been used on cables associated with 

 open-wire pairs which have their conductors spaced 8 inches apart. The 

 closer wire-spacing reduced the open-wire impedances below the values for 

 which the carrier loading was originally designed. To obtain better im- 

 pedance-matches when used with these lower-impedance lines, the Type C 

 carrier loading was "modified" to have lower impedances by systematically 

 building-out each loading section to have a higher total loading-section 

 capacitance. This procedure also reduced the loading cut-off by an amount 

 proportional to the impedance reduction, which effect limited the allowable 

 impedance reduction. The "standard" modification of C4.1 loading dropped 

 the nominal impedance to 558 ohms, and the cut-off to 42.5 kc. The ''modifi- 

 cation" of C4.8 loading dropped the nominal impedance to 625 ohms, and 

 the cut-off frequency to 40.5 kc. The standard Type C loading apparatus 

 was used in these installations. 



B15 Loading: The single-channel open-wire carrier system with which 

 this type of incidental cable loading is associated is a short-haul transmission 

 system, principally used on 104-mil open-wire pairs. Since the impedance- 

 matching requirements are much more lenient than those for loaded cables 

 in the multi-channel systems a single weight of loading is sufficient. 



The cable-capacitance deviations tend to be considerably smaller than 

 with Type C loading, because of "random" splicing at a considerably larger 

 number of intermediate cable splices within the individual loading sections. 

 In consequence, the average amount of capacitance building-out is much 

 smaller (on a percentage basis). 



Type J Loading: Because of the higher frequencies involved in the Type J 

 carrier systems the impedance-matching requirements are even more severe 

 than those for the Type C systems. For this reason, a series of three Type J 

 loading systems were made available, as noted in Table XVI. 



To make carrier loading economically feasible for 140-kc transmission it 

 was necessary to develop an entirely new type of low-capacitance cable for 

 use with the loading. The new cable makes use of shielded, "spiral-four" 

 units of 16 ga. conductors. The conductors are supported by means of 

 insulating discs at the corners of a square, and the diagonally opposite 

 conductors are associated as working pairs. The spacing between these wires 

 and between them and the quad shields is such as to obtain a mutual capaci- 

 tance very close to 0.025 mf/mi in the individual carrier pairs. The structural 

 relations between the associated pairs of the individual units are such as to 

 minimize crosstalk coupling. The over-all dimensions of the shielded units 

 are such that not more than 7 or 8 units can be provided in a single cable 

 without using an over-size sheath. In consequence the cable cost per carrier 

 pair is high. 



