Two-wire Balanced Feeder

Author: Edmund A. Laport

This is the commonest type of feeder (Fig. 4.20) for balanced operation and is used for a great variety of applications for high and very high frequencies where open-wire lines are desired. It is simple to construct and is relatively inexpensive. When used for high-power transmission, certain precautions are necessary with regard to potential gradients and details of construction. The conductor size can be increased as much as desired to increase the power-handling capacity, but beyond a point it is more economical to obtain the same effect with two or more conductors in parallel on each side of the circuit, as shown in some succeeding cases.

Fig. 4.20

Open-wire lines of this type have generally a characteristic impedance from 500 to 625 ohms in the form most widely used, and 600 ohms is perhaps the value most often chosen. Because of the high characteristic impedance, care is necessary to use insulators of very low capacitance in order to obtain an essentially smooth line. If insulator capacitance causes irregularities in the line constants, their effect can be neutralized by arranging the wires for greater spacing close to the insulators in order to maintain a constant L/C ratio past points of support.

In all balanced feeder systems, great care must be taken to ensure that the total length of wire is exactly the same for both sides of the circuit between transmitter and load. When passing through switching devices, transpositions, or bends or turns in the line and entering or leaving the equipment and buildings, the conductor arrangement must always be such as to provide for this essential requirement. When this cannot be done directly, it can be done by means of wire loops inserted in the short side at some convenient point. In making a corner in the line, the arrangement shown in Fig. 4.21 can be used. Another method is to turn the plane of the line 90 degrees before making the turn and bringing it back to horizontal for the next straightaway run (Fig. 4.22). For this type of line,

but when h ^ a,

A range of values of characteristic impedance as a function of the ratio of wire spacing to wire radius is shown in Fig. 4.23, which includes the case of small spacings where the proximity effect and the nonuniform peripheral charge distribution on the conductors come into play. In this figure, the spacing is that between the centers of the conductors.

When correctly balanced to ground, the radiation from a feeder of this type with 12-inch spacing is virtually negligible up to 25 megacycles.

Frequently it is necessary to route several such feeders in parallel, as is done with telephone lines. The amount of cross talk that will occur depends upon the spacing between circuits and upon the cross section of any one circuit, the length of the run in parallel, and the selectivity of the load circuit. For a given amount of cross talk between feeders, the circuits can be placed nearer each other if they are arranged to have a common neutral plane (that is, arranged with one circuit above the other) rather than with all the lines in the same plane. The latter, however, is often more convenient, even though it is necessary to use greater spacing between circuits.

FIG. 4.21. Method of making a corner in a two-wire balanced line to maintain equal wire lengths.

To minimize cross talk, various transposition techniques may be applied as in ordinary telephony.

It is difficult to place any specific limitations on tolerable cross coupling between circuits. One might believe that two parallel feeders en route to two different antennas are quite isolated if the power induced into the adjacent circuit is 40 decibels below that in the main circuit.

FIG. 4.22. Plan view of a two-wire balanced feeder, including a bend where feeder is turned 90 degrees to maintain equal-length wires.

Yet if the antenna to which this cross talk is delivered has no selectivity to discriminate against this signal and radiates it with full antenna gain in some undesired direction, its effect can be undesirable and cause interference out of proportion to the actual power level. This sort of cross talk has not been studied much in the past, but if the most effective use of a crowded frequency spectrum is to be achieved in the future, far greater care will be needed to reduce cross talk between feeder and radiating systems. For the same reasons, directive antennas will require far greater suppression of radiations in undesired directions. Undoubtedly many existing feeder arrangements, if critically studied, would be found to have intolerable cross-talk ratios because of close proximity.