Impedance of Coil-Loaded Cables

As is evident from Fig, 13, the characteristic impedance of a loaded cable circuit depends upon the type of termination used. The mid-section termination is shown in Fig. 14, and, as indicated, the first full loading coil is located at one-half the length of a loading section from the end of the line. This is accordingly equivalent to a π low-pass filter, and the characteristic impedance of a loaded cable with midsection terminations is similar to that of the π filter section (Fig. 28, page 170).

Figure 12. Velocity-frequency characteristics of loaded and non-loaded 19-gauge cable pairs. (Reference 11.)

For the midcoil, or midload, termination of Fig. 15, the two coils placed at the two ends of the circuit have one-half the inductance of the regular coils. This arrangement is similar to the T low-pass filter, and thus the characteristic impedance is similar to that of such a filter. Combinations of fractional loading sections and fractional coils are frequently used, especially at the junction of open-wire lines and loaded cables, to reduce the impedance mismatch. This is similar to the use of m-type filter sections.

Figure 13. Impedance-frequency characteristics of loaded and non-loaded 19-gauge cable pairs. (Reference 11.)

Tables I, II, and III give the characteristics of cables. Of particular interest are the cable circuits for program transmission¹⁶ listed last in Table III. Data for both long toll cables and local or exchange cables are included. These latter are usually unloaded, except in the case of the trunks connecting the various central offices, and particularly long local circuits.