Near and Far Field - Regions and Their Cause

Regions and Their Cause

The near-field and far-field of an antenna or other isolated source of electromagnetic radiation are regions around the source. The boundary between the two regions is only vaguely defined, and it depends on the dominant wavelength (λ) emitted by the source.

The far-field is the region in which the field acts as "normal" electromagnetic radiation. The power of this radiation decreases as the square of distance from the antenna, and absorption of the radiation has no effect on the transmitter. Absorption of radiation from the reactive part of the near-field, however, does affect the load on the transmitter. Magnetic induction (for example, in a transformer) can be seen as a very simple model of this type of near-field electromagnetic interaction.

Because each part (electric and magnetic) of the EM field in the far-field region is "produced by" (or associated with) a change in the other part, the ratios of electric to magnetic field strength are fixed and unvarying in the far-field. However, in the near field, the electric and magnetic fields are nearly independent of each other, and each one cannot be calculated from knowing the other (thus, they must be independently measured in the near-field). Depending on the type of source, the near-field will be dominated by either a magnetic component, or an electric component.

Near-fields are dominated by dipole-type electric or magnetic fields. Magnetic near-field components due to changing currents must be of a dipole nature since magnetic "charges" (magnetic monopoles) do not exist. Although electric charges do exist and may create static electric fields, the oscillating electric part of EM near-fields that is created by an electric potential in the radiator always shows a dipole nature, because the source of the electric part of the EM near-field is created from an electrical neutral conductor only in a way that temporarily creates a dipole or multipole. This is because the positive and negative charges in a radiating source have no way to leave it, and are separated from each other by the excitation "signal" (a transmitter or other EM exciting potential) only temporarily. A classic example of this behavior is a radio antenna, which on average over time is electrically neutral, and differs from this state only by temporarily becoming an electrical dipole (or multipole) under the influence of the signal from the transmitter, which separates charges within it for brief periods only. .

In the far-field, the shape of the antenna pattern is independent of distance and the angular field distribution is in essence independent of distance from the source. The far-field is also frequently referred to as the "radiation zone", or "free space". A more precise definition is given by the propagation properties. The radiation zone is important because far-fields in general fall off in amplitude by 1/r. This means that the total energy per unit area at a distance r is proportional to 1/r2. The area of the sphere is proportional to r2, so the total energy passing through the sphere is constant. This means that the far-field energy actually escapes to infinite distance (it radiates). In general, the purpose of antennas is to communicate wirelessly for long distances using far-fields, and this is their main region of operation (however, certain antennas specialized for near-field communication do exist).