INDUCTIVE LOADING FOR TELEPHONE FACILITIES 741 



PART V: LOADING FOR INCIDENTAL CABLES 

 IN OPEN- WIRE LINES 



Introduction 



From the earliest days of telephony, when it became necessary to use 

 pieces of cable in long-distance lines to provide toll entrance facilities at toll 

 centers or for other purposes at intermediate points, such cables have had 

 more or less objectionable effects on the. over-all transmission-system per- 

 formance. These impairments resulted from the much greater transmission 

 loss per unit length in the inserted cable, and from reflection effects occurring 

 at the cable junctions with the open-wire — these being due to the large 

 differences between the cable and open-wire impedances. 



Prior to the advent of loading, the losses in incidental cables could be 

 reduced to low unit-length values only by using expensive coarse-gauge 

 cables. Cable loading became available just in time to head off the instal- 

 lation of some very expensive coarse-gauge cables that had been proposed 

 for unusually long entrance facilities in the New York and Boston areas. The 

 use of loading on the open wires greatly increased the economic importance 

 of attenuation reduction in the incidental cables occurring in such Unes. By 

 substantially raising the line impedance, loading also increased the magni- 

 tude of the reflection losses at junctions with non-loaded cables. 



Starfdard "heavy" loading (Table II, page 156) came into general use 

 on long entrance cables in the loaded lines. While this loading did not have 

 a sufficiently high impedance to match that of the loaded line, it was close 

 enough to reduce the junction reflection losses to acceptable values. A special 

 light-weight loading found some use on incidental cables in non-loaded 

 lines. 



When satisfactory types of telephone repeaters became available for ex- 

 tensive use on loaded open wires, the cable junction impedance-irregulari- 

 ties, and other irregularities, had to be reduced to very small values so 

 as to avoid repeater circuit unbalances that would objectionably restrict 

 the repeater gains. These severe requirements put a high premium upon the 

 use of an improved type of cable loading having impedance characteristics 

 that matched closely those of the associated open-wire circuits. This "extra- 

 high" impedance loading also had very satisfactory attenuation properties. 



Subsequently, when the exploitation of the vacuum-tube repeater started 

 on non-loaded open-wire lines it became necessary to use a new, low-im- 

 pedance type of impedance-matching loading on their associated incidental 

 cables. Because of the low impedance, the attenuation reduction was con- 

 siderably less than that provided by the extra-high impedance loading just 



