12.3.8 Near And Far Fields
The electromagnetic field surrounding an antenna may be divided into three regions: the reactive near field, the radiating near field (Fresnel region), and the far field (Fraunhofer region). These regions differ in field structure, phase relationship, and spatial decay.
The reactive near field exists very close to the antenna, and is strongly influenced by the antenna’s geometry and is important in impedance matching and antenna coupling. In this region, the electric and magnetic fields are not in a fixed ratio, there is significant reactive (stored) energy, the field components do not behave as plane waves, and power does not propagate steadily away from the antenna but oscillates between the antenna and the surrounding space.
Beyond the reactive region lies the radiating near field, where radiation dominates but the angular field distribution still depends on distance from the antenna. The wavefront is not yet fully planar, and phase variations across the aperture remain significant.
The far field region begins approximately at a
where D is the maximum dimension of the antenna and λ is the wavelength.
In the far field the wavefront is locally planar, the electric and magnetic fields are perpendicular to each other and to the direction of propagation, the ratio E/H approaches the intrinsic impedance of free space (≈ 377 Ω), the field strength decreases proportionally to 1/r, and the radiation pattern becomes independent of distance.
Most antenna parameters such as gain, directivity, and radiation pattern are defined in the far field.
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