Where Does the Far Field of an Antenna Start? [Stand on Standards]
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The far field of an antenna is generally considered to be the region where the outgoing wavefront is planar and the antenna radiation pattern has a polar variation and is independent of the distance from the antenna. Hence, to generate a local plane wave in the far field, the radial component of the electric field must be negligible compared to the transverse component. Also, the ratio of the electric and the magnetic far fields should equal the intrinsic impedance of the medium. These two requirements-that the radial component of the field should be negligible when compared with the transverse component and the ratio of the electric and the magnetic fields equal the intrinsic impedance of the medium-must hold in all angular directions from the antenna. So to determine the starting distance for the far field, we need to examine the simultaneous satisfaction of these two properties for all theta and phi angular directions, where theta is the angle measured from the z-axis and phi is the angle measured from the x-axis. It is widely stated in the antenna literature that the far field of an antenna operating in free space, where all the aforementioned properties must hold, starts from a distance of 2D2/lambda, where D is the maximum dimension of the antenna and lambda is the operating wavelength. © 2016 IEEE.
Antenna radiation; Electric fields; Far field; Free spaces; Operating wavelength; Plane wave; Radial component; Transverse components; Directional patterns (antenna)