Four-wire Side-connected Balanced Feeder

Author: Edmund A. Laport

FIG. 4.23. Characteristic impedance of two-wire balanced feeder.

This type of balanced feeder (Fig.4.24) has been extensively applied for the transmission of high-power high-frequency energy over the long distances required in large communication stations and in high-frequency broadcast stations where many antennas are located on a single large plot. In some cases the feeder lengths may exceed 1/2 mile. Its relatively low characteristic impedance makes this type less susceptible to the irregularities introduced by insulators and switching arrangements. It has a high power-transmission capacity for the amount of copper used, and its attenuation can be less than that of two-wire feeders.

It is interesting to note that when a square-cross-section feeder of this type is used, its characteristic impedance is equal to that of a pair of two-wire feeders in parallel, each having a spacing equal to the diagonal of the four-wire line. Each diagonal pair is in the neutral plane of the other with no intercoupling. Double power rating is therefore obtained on one set of supports and insulators, and the characteristic impedance is one-half that of one pair.

Figures 4.88 to 4.92 show some of the construction details that have been used, as well as various methods of matching impedances.

The British Broadcasting Corporation has made a systematic study of this type of feeder, and McLean and Bolt31 report that one composed of wires 0.203 inch in diameter, spaced 10 inches and 6 inches, with a characteristic impedance of 320 ohms, has a safe power-transmission capacity of 130 kilowatts carrier with 100 percent amplitude modulation at 21.5 megacycles.

Fig. 4.24

Its attenuation varies from 0.23 decibels per 1,000 feet at 5 megacycles to about 0.53 decibels per 1,000 feet at 21.5 megacycles. Spans of 150 feet are used with conductor tensions of 150 pounds, with a minimum height of 10 feet at mid-span. Tubular steel frames of the type shown in Fig. 4.89 are now used by the British Broadcasting Corporation instead of wood or steel poles. Metallic spacers suspend the lower wires from the upper ones on each side, and both sides are suspended on porcelain insulators.

Figure 4.88 shows another version of this type of feeder, using a small spacing between the wires in parallel on each side but otherwise following the form of construction commonly used for the two-wire balanced line.

The design formulas for the four-wire side-connected balanced line are

When 6 = a,

The attenuation due to copper losses only is

and the approximate total attenuation for typical construction is

There are tabulated in Table 4.1 the characteristic impedances for a four-wire side-connected line of this type when the diagonal spacing