Balancing Networks

Impedance variations for an open-wire telephone line composed of 165-mil hard-drawn copper wires spaced one foot apart are shown in Fig. 13. Curve 1 is the resistive component of the input impedance, and curve 2 is the capacitive reactance component of the input impedance, with the distant end properly terminated.¹² Theoretically, these curves should be perfectly smooth, but small irregularities cause minor reflections and introduce the small impedance variations.

Figure 13. Curve 1 is the resistance component, and curve 2 is the capacitive reactance component of the input impedance of a typical open-wire line terminated in its characteristic impedance. Curve 3 is the resistance component, and Curve 4 is the capacitive reactance component of the input impedance of the balancing network of Fig. 14(6). The frequency is in cycles per second.

It is evident that the resistance component is by far the larger of the two, and hence the line could be approximately balanced with a 600-ohm resistor. Accurate balancing, such as in repeater circuits, demands a closer simulation, and this can be accomplished by adding a capacitor in series with the resistor as is done in Fig. 14(a). The network can be made to balance the line more exactly by adding another capacitor and another resistor as in Fig. 14(b). The curves for this final balancing network are numbers 3 and 4 of Fig. 13, and as is evident they closely balance the line impedance over a wide range of frequencies.

The balancing networks just described simulate the impedance of an open-wire line. Balancing networks^{23, 24} are also needed for use in repeaters connected to loaded cables.

Figure 14. Simple balancing networks sometimes used with open-wire lines.