Types of Transmission Lines

Author: J.B. Hoag

One of the main problems in the design of a transmission line is that there shall be as little loss of energy as possible, either by radiation, or by heating in resistances or in neighboring conductors or dielectrics. There are several types of transmission lines whose losses are small: (1) the open-wire line consisting of two parallel wires; (2) the twisted-pair line of two insulated wires twisted together; (3) the concentric cable or coaxial line, where a central wire is mounted along the axis of a metal tube, (a) with insulating spacers every so often, (b) with continuous rubber insulation along the line (usually used with a flexible outer metal mesh); and (4) a single-wire feeder, where radiation is kept low by keeping the current in it small, the ground serving as the return wire.

Fig. 35 B. A transposed open-wire transmission line

In order that the losses shall be small, transmission lines have their go and return wires very close to each other in comparison with the wave-length. Then the magnetic and electric fields of one wire cancel or nearly cancel those of the other and radiation does not occur in appreciable amount.

For the open-wire line a spacing of 2 to 6 inches is used, the smaller values at the higher frequencies.¹ In order that the line shall be electrically symmetrical with respect to its surroundings, a transposition system is sometimes used. This is illustrated in Fig. 35 B. The method proves more useful on longer than on shorter lines, i.e., when the line has a length greater than one or two wave-lengths.

1	The open-wire line cannot be spaced so closely if considerable power is to be transmitted. One must either use larger spacing, with larger wire to keep down the impedance, or use an enclosed line under (gas, usually nitrogen) pressure to avoid flashing and brushing.