Near and Far Field

Near And Far Field

The near field (or near-field) and far field (or far-field) and the transition zone are regions of time varying electromagnetic field around any object that serves as a source for the field. The different terms for these regions describe the way characteristics of an EM field change with distance from the charges and currents in the object that are the sources of the changing electromagnetic (EM) field. The more distant parts of the far-field are identified with classical electromagnetic radiation.

The basic reason an EM field changes in character with distance from its source, is that Maxwell's equations prescribe different behaviors for each of the two source-terms of electric fields and also the two source-terms for magnetic fields. Electric fields produced by charge distributions have a different character than those produced by changing magnetic fields. Similarly, Maxwell's equations show a differing behavior for the magnetic fields produced by electric currents, versus magnetic fields produced by changing electric fields. For these reasons, in the region very close to currents and charge-separations, the EM field is dominated by electric and magnetic components produced directly by currents and charge-separations, and these effects together produce the EM "near field." However, at distances far from charge-separations and currents, the EM field becomes dominated by the electric and magnetic fields indirectly produced by the change in the other type of field, and thus the EM field is no longer affected (or much affected) by the charges and currents at the EM source. This more distant part of the EM field is the "radiative" field or "far-field," and it is the familiar type of electromagnetic radiation seen in "free space," far from any EM field sources (origins).

The far-field thus includes radio waves and microwaves several wavelengths from most types of antennas, as well as all the shorter-wave EM radiation in the electromagnetic spectrum (infrared, light, UV, X-rays, etc.). The latter types of EM radiation in normal experience show far-field behavior almost exclusively, due to their shorter wavelength that gives them a "far-field" character at all but extremely short distances from their sources. For example, visible light shows far-field behavior at all distances larger than one micrometer from its source. The wavelengths of visible light is in the range 0.4 to 0.7 micrometers, so the near field for light is in the expected range.

In practical mathematical terms, the dominance of far-field behavior with sufficient distance from the source appears because both currents and the oscillating charge-distributions in antennas (and other radiators) produce dipole type field behavior. While these dipole near-field intensities may be very powerful near the source, they decay very rapidly with distance in comparison to EM radiation (the far-field). Radiative far-field intensity decays more slowly with distance, following the inverse square law for total EM power that is typical of all electromagnetic radiation. For this reason, the far-field component of the EM field wins out in intensity with increasing distance. Thus, for objects such as transmitting antennas, electrical or electronic equipment, dielectric materials, or where radiation is scattering from an object, the non-radiative 'near field' components of electromagnetic fields dominate the EM field close to the object, while electromagnetic radiation or 'far field' behaviors dominate at greater distances. The near-field does not suddenly end where the far-field begins—rather, there is a transition zone between these types where both types of EM field-effects may be significant.