near field

The near field is the electromagnetic region immediately surrounding an antenna or radiating element, extending roughly to one half wavelength from the source, though more precisely defined by the radiansphere (the boundary at distance r = λ/2π, where λ is wavelength). In this zone, the electric and magnetic fields are not yet in the simple plane-wave relationship that characterizes far-field radiation. Energy sloshes back and forth between source and surrounding space rather than propagating cleanly outward.

The near field dominates when an antenna is electrically small, meaning its physical size is much smaller than the wavelength it operates at. A small loop antenna at 1 MHz (wavelength 300 meters) exhibits pronounced near-field behavior within tens of meters. By contrast, a large parabolic dish at the same frequency operates mostly in the far field. The transition is gradual, not sharp, and the Fraunhofer distance (the start of reliable far-field behavior) often sits well beyond the radiansphere itself.

Practical consequences and measurement challenges

Near-field behavior creates measurement headaches. Antenna gain, pattern, and efficiency cannot be characterized using conventional far-field methods when the measurement antenna sits in the near field of the device under test. Near-field antenna ranges use planar, cylindrical, or spherical scanning geometries to map the complex field distribution, then apply numerical transforms to infer far-field performance. These measurements are time-consuming and expensive, but essential for compact antennas and tightly integrated RF systems.

Reactive coupling between closely spaced antennas is a near-field effect. Two antennas separated by less than a wavelength can exchange energy through magnetic or electric coupling, degrading isolation and skewing radiation patterns. MIMO systems and phased arrays must account for mutual coupling in the near field, where element spacing (often half a wavelength) puts them squarely in one another's reactive region.

The term arises from the field structure itself: near the source, electric and magnetic field components are not perpendicular to the radial direction and do not decay uniformly with distance. Far from the source, the fields become transverse and fall off as 1/r, like true radiation. The boundary between these regimes is fuzzy, which is why standards invoke the radiansphere as a practical dividing line, though real-world behavior depends on antenna type, frequency, and application precision.