Periodic Resonant Antennas

A simple antenna was shown in Fig. 1. From the electric-circuit standpoint such an antenna is far more than two wires, as Fig. 11 indicates. If the antenna is driven with a voltage of frequency such that the total antenna length is approximately one-half wavelength, the current (I) and voltage (E) distribution along the antenna will be as

Figure 11. An antenna is approximately equivalent electrically to the circuit shown.

shown in Fig. 12(a). This is similar to the stationary (standing) wave on a transmission line (Chapter 6) but differs in one important respect. When a theoretical lossless line is considered, the current and voltage distribution drop to zero at certain points. But, even if an antenna were made of lossless wire, when it was .driven, it would radiate energy into space. Therefore, the current and voltage distribution would not be zero at certain points as for a lossless line, except that the current would be zero at the extreme end of each wire. This means that the input impedance of an antenna must always have a finite resistance component.

Confusion exists as to what is represented by the curves of Fig. 12 (a). The dotted current curve I represents the effective values of current that would be indicated by radio-frequency ammeters connected at various points in the antenna wires. These effective current values

(and also voltage values) are plotted with magnitude of current along the horizontal or x axis, marked "center line" in Fig. 12 (a). The solid voltage curve E represents the effective values of voltage that would be indicated by (imaginary) voltmeters connected between the various points on the antenna and a conducting plane passing through the center of the antenna and at right angles to it (along the center line).

Figure 12. The current I and voltage E will be distributed on the antennas approximately as shown. Because the antennas radiate power, the curves do not reach zero (except at the ends), and the input impedance always has a finite resistance component.

From the discussion given in Chapter 6, it follows that the input impedance of the antenna of Fig. 12 (a) is a low value of pure resistance (low voltage, high current) and that the input impedance of Fig. 12(b) is a high value of pure resistance (high voltage, low current). If the driving frequency is such that the antenna is neither a half wave nor a full wave in length, then the input impedance will be composed of both resistance and reactance. The velocity of propagation along an antenna is not exactly the propagation in free space. The actual antenna length should be about 0.95 times the theoretical length. Also, the distribution of current and voltage is not exactly sinusoidal. Because the input impedance is a low value of pure resistance (like a series resonant circuit) at certain frequencies and a high value of pure resistance (like a parallel resonant circuit) at other frequencies, these antennas are often called resonant antennas.