14-07-2012, 03:39 PM
Production of Electromagnetic Waves
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It is sometimes assumed that, when the voltage and current in a circuit produce electric and magnetic fields, all the energy entering the circuit is stored in these fields or is dissipated in heat. This is a reasonable assumption at low frequencies, but at any frequency some energy is radiated in electromagnetic waves, and the tendency for such radiation to occur varies as the square of the frequency.(1)
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.
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.