Near Field / Far Field
In the region around an antenna extending roughly out to λ/2π, the field is not yet a propagating wave but a reactive cloud of energy tied to the antenna; this region is called the near field. At 433 MHz this boundary is about 11 cm, at 2.45 GHz about 2 cm. A conductive object entering this volume directly detunes the antenna.
In the far field the antenna is a radiator and the objects around it behave only as obstacles along the path. In the near field, however, the object becomes part of the antenna; it changes its impedance, resonant frequency and pattern.
- Consequence 1: leaning the antenna against a metal enclosure or wrapping it with a metal bracket reduces range, because the tuning shifts.
- Consequence 2: on handheld remotes, holding the antenna in your palm has the same effect; human tissue behaves as a conductor.
- Consequence 3: antenna measurements are only meaningful in the far field; a measurement made in the near field will not match the catalogue value.
The installation rule is simple: leave at least a few centimetres of clearance around the antenna, preferably on the order of one tenth of a wavelength. In AXI kits the receiver antenna can be brought outside the enclosure with a cable gland, so providing this clearance is possible; leaving the antenna inside the enclosure produces a two-layer loss, from near-field detuning and from shielding.
Context where this term is used: Wireless remote control installation: pairing, relay, safety