The near and far field distinguish source-dependent electromagnetic field regions. Close to a localized source, reactive fields and induction terms contribute; sufficiently far away, the leading transverse fields decrease as the inverse distance and carry outgoing power. The radiation zone describes this far-field limit.
In the radiation zone of a localized source, the leading electromagnetic fields decay as , are transverse to the observation direction, and satisfy . Terms decaying faster than do not contribute to the limiting radiated power.
An isotropic source of average power has intensityat distance . For monochromatic vacuum radiation, its electric-field amplitude is .
Multipole radiation expands radiation from localized time-dependent sources into electric and magnetic multipoles. Electric dipole radiation is the leading electric term, while magnetic dipole radiation and higher electric multipoles contribute when the leading term vanishes or higher precision is needed.
For a localized source whose size is small compared with its radiation wavelength, the radiation fields of its electric dipole moment areTheir total instantaneous power is
For a source of characteristic size and angular frequency , the electric-dipole approximation requiresso the source is much smaller than its radiation wavelength. Observing the pure radiation field additionally requires the radiation zone, and .
If is the angle between the observation direction and , thenIntegrating over solid angle uses and gives the total electric dipole radiation power.
A charge undergoing simple harmonic motion of angular frequency and slowly varying amplitude radiates the averaged powerIts mechanical energy therefore has exponential decay rate and half-life
This is the magnetic-dipole term of the multipole radiation expansion.
If a rigid body's inclined magnetic dipole rotates at angular speed , its radiated power is . Conservation of rotational energy then gives
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Near field and far field are terms commonly used in various fields, including physics, engineering, and telecommunications, to describe regions in relation to a source of waves, such as electromagnetic waves, sound waves, or other types of waves. ### Near Field The near field refers to the region close to the source of the wave where the behavior of the field is not specified by simple wave equations. In this zone, the wave typically does not propagate in the same way as it does in the far field.