The basic mechanism of electromagnetic radiation is discussed in Note 10. For a charge to radiate electromagentic waves in free space, it must be either accelerated or decelerated. Charges undergoing oscillatory motion are continuously accelerated and decelerated and if the velocity of charges is nonrelativistic, electromagentic waves are radiated at the oscillation frequency. Radiation from antennas can be analyzed by superposing elementary radiation fields emitted by individual electrons as shown in Note 10.

Electric field lines of static dipole.

    Animation shows the electric field lines due to an electric dipole oscillating vertically at the origin. Near the dipole, the field lines are essentially those of a static dipole leaving a positive charge and ending up at a negative charge. However, at a distance of the order of half wavelength (lambda = 2Pi is assumed here) or greater, the field lines are completely detached from the dipole. This detachment characterizes radiation fields which propagate freely (without being attached to charges) in free space at the speed c.

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