Loading Phantom Group Circuits

Figure 16. Loading coil used on non-phantomed circuits.

Although loading was developed about 1900 and soon was applied to the side circuits, it was not until about 1910 that phantom circuits were successfully loaded.^10,11 One reason for this delay was the difficulty of providing the close balance required in the coil windings to prevent excessive crosstalk. A complete loading system for a phantom group is shown in Fig. 18. The theory is illustrated by the simplified circuit of Fig. 19, where the broken arrows represent instantaneous currents and corresponding fluxes produced by the phantom circuit, and the continuous arrows the currents and resulting fluxes produced by the currents in the side circuits.

Figure 17. Equivalent cores for loading coils. Iron dust core (left); Permalloy dust core (center); molybdenum Permalloy dust core (right). (Courtesy Bell Telephone System.)

TABLE I CONSTANTS OF EXCHAXGE PAPER-INSULATED CABLES, COPPER CONDUCTORS

NOTE: Inductance is 0.001 henry per loop mile for all gauges.

The currents in the side circuits produce adding fluxes, and the maximum inductance is therefore obtained for loading the side circuits. In the phantom loading coil, however, these currents are in opposite directions through identical windings, and thus neither side circuit will produce magnetic flux in the core of the phantom loading coil. Similarly, the phantom-circuit currents pass through the separate side-circuit coils in the same direction, and since these windings are identical, no magnetic flux is produced in the side-circuit coils by the phantom-circuit current. Under these conditions of balance there is no crosstalk due to magnetic coupling.

Figure 18. Standard method of loading phantom and side cable circuits in the Bell System.

TABLE II CHARACTERISTICS OF EXCHANGE CABLE CIRCUITS

⁽¹⁾ Paper-insulated cables. See Table I for primary constants.

⁽²⁾ Numerals indicate total inductance of loading coils in millihenrys, letters indicate load coil spacing as follows: NL, non-loaded; M, 9000 feet; H, 6000 feet; B, 3000 feet.

⁽³⁾ Midsection iterative impedance in cases of loaded circuits. Magnitude in ohms.

Although M-88 and H-135 loading is in service, it is no longer used in new installations.

What is called flutter may occur in loaded circuits simultaneously providing telephone and telegraph facilities.²¹ The relatively low-frequency telegraph currents passing through the windings cause corresponding changes in the effective resistance and the inductance of the coils. These produce variations in the transmitting efficiency of the coils, and thus the talking currents vary and produce a "fluttering" effect.

Figure 19. Broken arrows represent current and flux due to phantom. Full arrows represent currents and flux due to side circuits.