Antenna Field Regions
Antenna radiation is made up of several different regions. The transitions between these regions are not distinct and changes between them are gradual.
The reactive near-field region is the region close to the antenna. In the reactive near-field region, the energy decays very rapidly with distance. The electric fields do not exist alone by themselves in free space in this region, rather, they begin and end their loop on the antenna itself, and are 90° out of phase with the magnetic fields. The influence of the electric and magnetic fields on each other serve to contain the energy in this region.
It is called the reactive near field as it behaves similarly to a reactive tuned circuit, with the energy being stored in capacitive and inductive reactance. Objects in this area will directly affect the near-field, altering its properties. This is due to field energy being lost — transferred to electrons in the nearby object. The point where the reactive near-field region transitions into the radiating near-field region is considered to be λ/2π.
In the radiating near-field region, The electric and magnetic fields begin to become radiative, and may exist as closed loops, independent of the antenna. The average energy density remains fairly constant at different distances from the antenna, although there are localized energy fluctuations due to the electric and magnetic field lines having very tight radii in this zone. Even small angular movements will show these energy fluctuations.
The radiating near-field region extends from the reactive region boundary out to a distance defined as, 2D2/λ with D being the largest dimension of the antenna aperture, and λ (lambda) being the wavelength. When using electrically small antennas, less than 1λ, the radiating near-field region may not even exist. In this case, the transition between the reactive/radiating fields is considered to be λ/2π.
Metal objects in the near field regions should be avoided. For example, A monopole antenna parallel to a metal surface will produce image currents in the metal. These currents will be of opposite polarity, and will generate an opposite polarity field, canceling out the radiated currents from the antenna and impacting performance in the far-field. That is, unless you actually wanted a reflector… in which case you’d make sure that the re-radiated fields from the metal object arrive in-phase with the original radiated field, strengthening it via constructive interference.
Beyond this distance is the radiating far-field region where the angular distribution of the energy does not vary with distance, and the power level decays according to the inverse square law with distance. The electric and magnetic fields are in-phase and orthogonal to each other (in the case of linear polarization). It is in this region which radio communications are designed to operate.