INDUCTIVE LOADING FOR TELEPHONE FACILITIES 747 



Attenuation Data: The relatively low cable impedances which are necessary 

 for good impedance-matching limit the attenuation-loss reduction to smaller 

 values than those obtained with the higher impedance loading systems used 

 on toll cable facilities. The theoretical 1000-cycle attenuation values for 

 H31-18 loading (on a db/mi basis) are 0.56, 0.30, and 0.16, respectively, for 

 19 ga., 16 ga., and 13 ga. cables. The phantom circuit attenuation is nearly 

 20% lower, being about 0.47, 0.24, and 0.13 db/mi. 



The attenuation losses in the other loading systems of Table XV are 

 close to those for H3 1-1 8 loading. This follows from the fact that their im- 

 pedances are nearly the same in magnitude. 



Lffw-Frequency Impedance Matching 



Before ending the discussion of transmission system characteristics, it is 

 important to note that the attainment of optimum impedance-matches at 

 low voice-frequencies, with the types of loading under discussion, involves 

 the use of the so-called ''optimum" cable conductor sizes. This follows from 

 the fact that at these low frequencies the circuit resistances are important 

 factors in determining the open wire and cable impedances. The optimum 

 conductor combinations are 13 ga. cable for use in association with 165-mil 

 open-wire lines, 16 ga. cable with 128-mil lines, and 19 ga. cable with 104-mil 

 lines. Allowing for the loading coil resistances, these combinations of cable 

 and open-wire conductor-sizes closely conform to the fundamental theoretical 

 requirement that the unit-length ratio of series resistance (ohms) to shunt 

 capacitance (farads) to total Hnear inductance (henrys) in the loaded cables 

 should be close to the corresponding linear ratio in the associated non-loaded 

 lines. 



Loading Coils and Cases for Incidental Cables 



In their general design features, excepting inductance and effective 

 resistance, the voice-frequency loading coils for incidental cables corre- 

 sponded with those currently used in toll cable circuits. When the toll 

 cable coils were redesigned to take advantage of new core-materials, or in 

 other important features, the entrance cable coils were included in the 

 general redesign work. 



The first loading units developed for H3 1-1 8 loading were coded in the "P" 

 series. The code designation P4 applied to the "full-weight" loading unit. 

 The fractional-weight loading units developed for use in the "fractional 

 section" and the "fractional coil" loading termination were coded P5 and 

 P6, respectively. These numerical code components have been retained in 

 the code designations of all standard replacement designs, namely the PB, 

 M, SM, and MF series of loading units. 



The potting practices used with the entrance cable coils were generally 



