Production of Electromagnetic Waves

Maxwell proved mathematically that, when the potential or current at a given point changes, the influence of this change is not felt at surrounding points immediately, but a definite time later, and that the propagation of electromagnetic waves was a possibility, a fact later established experimentally by Hertz.

The two wires forming the antenna of Fig. 1 are connected to an oscillator producing a radio-frequency voltage. The wires are about one-half wavelength long and are in free space. When the upper oscillator terminal is negative and the lower terminal is positive, electrons wall flow up into wire A and up out of wire B. An electric current will flow down at this instant and will produce a magnetic field shown by the solid lines. Because wire A is negative and B is positive, a difference of potential will exist between these two wires, and an electric field will be established as shown by the broken lines. During the next half-cycle the applied radio-frequency voltage will reverse and the directions of the current flow, the potential differences, and the fields also will be reversed.

Figure 1. Electric field (broken lines) and magnetic field around an antenna. The electric vector E and magnetic vector H are in time phase but are 90° apart in space. The direction of travel (propagation) is indicated by T. Electric and magnetic fields exist in planes other than the two planes shown.

If point P of Figs. 1 and 2 is very dose to the antenna, it is said to be in the near zone,⁴ and in this region the induction field predominates. This is the field that is considered in low-frequency work to be directly proportional to the current and voltage. The induction field alternately accepts power from, and returns power to, the electric circuit. The intensity of the induction field in free space varies inversely as the square of the distance from the antenna.^2,3 At point P of Figs. 1 and 2 a radiation field exists in addition to the induction field described. In the near zone close to the antenna, the induction field is much stronger than the radiation field. At a distance from the antenna of approximately one-sixth of a wavelength,³ the induction and radiation fields are of equal strength. Beyond this distance the far zone⁴ exists, and the radiation field is the stronger. The intensity of the radiation field varies inversely as the first power of the distance from the antenna.^2,3 The production and propagation of an electromagnetic radio wave are illustrated in Fig. 2. The antenna A-B produces both induction and radiation fields at point P near the antenna. Beyond point P the induction field is negligible. The radiation field travels outward, and the electric and magnetic components reach point P' a finite time later than they were produced. Similarly, they reach point P" at a later time. The lengths of the electric and magnetic vectors are less at P" because the field intensity decreases inversely with the distance. In Fig. 2 E'\ E" are the electric intensities, and H', H" are the magnetic intensities of the radiation field. At a given point E and H are in time phase but are 90° apart in space. The fields at point P" lag behind those at P', the amount depending on the distance. It is assumed in Fig. 2 that the frequency is 100,000 cycles and that the wave velocity is 186,000 miles per second.

Figure 2. A radio wave in space. The wave consists of electric (E) and magnetic (H) fields that vary sinusoidally. The decrease in intensity occurs because the wave "expands" from point source P. The frequency is 100,000 cycles.

The induction field is considered to be in a quasi-stationary state,³ and the electric and magnetic components have no effect on each other. That is, in a given region there can exist a strong electric field and a weak magnetic field, and vice versa. Each field is in phase with the voltage or current producing it.

The radiation field is in a dynamic state;⁸ a changing magnetic field has the ability to produce an electric field, and a changing electric field has the ability to produce a magnetic field. The radiation field is the portion of the total field about an antenna that, in a sense, cuts itself adrift from the total field produced. As an analogy, long after a stone has settled to the bottom of a pond, water waves travel over the surface of the pond.

(1)	A mathematical solution3 gives the average power radiated by a short isolated wire to be Pa = ω2I2l2/(3c2), where ω2 = (2πf)2, I is the current (maximum value), / is the length of the wire, and c is the velocity of light.