Near-field zone
The region r ≪ λ surrounding an oscillating source where the field resembles a time-varying instantaneous quasi-static field (amplitude ∝ 1/r³ for a dipole) that stores and returns energy rather than radiating it. Also called the induction zone or reactive zone.
Definition
The near-field zone is the region surrounding an oscillating source at distances much smaller than a wavelength (r ≪ λ, or equivalently k·r ≪ 1 where k = 2π/λ). In this region the electromagnetic field is dominated by the quasi-static components: for an oscillating electric dipole p(t) = p₀ cos(ωt), the near-field electric field has the same angular structure as an electrostatic dipole (the familiar 3cos²θ − 1 radial pattern and the transverse sinθ dipole lines), but breathing in time as p(t) flips sign. Its amplitude falls as 1/r³ — cube the distance and you have killed the field.
Energy in the near zone sloshes: on one quarter-cycle it flows outward from the source, on the next it flows back in. The time-averaged Poynting flux through a sphere in this region is zero (to leading order in kr). This is the same energy-storage behaviour that characterises capacitor plates or inductor coils — fields that store energy reactively without transporting it away. Antenna engineers call this the reactive near field, and a small capacitively or inductively coupled probe placed in this zone will draw energy from the source via direct mutual coupling rather than by collecting radiation. The transition radius where near-field and far-field amplitudes cross is r = λ/(2π) = c/ω; outside this surface the 1/r² crossover to the 1/r radiative component takes over, and beyond about 2D²/λ (for a source of largest dimension D) one is firmly in the far-field zone where energy flows outward irreversibly.