Transmission Lines

Author: J.B. Hoag

In general, the transmitter apparatus cannot be mounted at the same position as the Hertz antenna. The energy is fed from the h.f. oscillator to the antenna by means of a transmission line. This consists of wires so arranged that they radiate as little energy as possible. There are two types, called tuned or resonant, and untuned or non-resonant lines. Examples are shown in Fig. 8 I.

Fig. 8 I. Examples of tuned (left) and untuned transmission lines

A tuned line may conveniently be thought of as the mid-portion of the antenna folded upon itself in such a manner that the currents in adjacent wires are in opposite phase but are of equal magnitude. Then the radiation from one wire nearly cancels that from the other.

In the case of the untuned line, we shall consider two closely-spaced parallel wires, or, equally as well, a wire down the axis of a hollow metal tube. For a moment, the parallel wires or the concentric line shall be assumed to be infinitely long. When an alternator is first connected at one end, a wave travels outward at a velocity somewhat less than that of light. An ammeter in series with the line or a voltmeter across the line at various points would show a gradual decrease from the input value. The current decreases because some of the current is bypassed at each point through the distributed capacity existing between the wires and because a very small amount passes between them by ordinary conduction through the air or other insulator which separates the conductors. The voltage decreases because of the resistance and inductance drops along the conductors. However, the ratio of the voltage to the current at every point is the same as at all other points. This constant is called the characteristic impedance of the line. At radio frequencies, it is practically a pure resistance and hence is independent of the frequency.

A line of finite length, connected at its output end to a resistance load numerically equal to the characteristic impedance, acts just like a line of infinite length. The wave travels down the line and is all absorbed in the load; none is reflected to set up standing waves; the line is non-resonant.