farfeltantakelser

Far-field assumptions are a set of simplifying conditions used in electromagnetics, acoustics, and related fields to analyze radiating sources when the observation point is far from the source. They are most often applied to antennas, waveguides, and aperture systems to enable tractable math and clear physical interpretation.

A central criterion is the Fraunhofer (or far-field) distance, typically given as Rff = 2D^2/λ, where D is the largest dimension of the radiator and λ is the wavelength. Observations at distances greater than Rff are generally treated as lying in the far field. In this region, the radiated field can be approximated as a spherical wave with wavefronts that are nearly planar over the aperture, and the angular field distribution becomes essentially independent of distance. The field strength falls roughly as 1/r, and reactive near-field components are negligible.

Common consequences of far-field assumptions include the use of plane-wave or far-field radiation patterns, a direct relationship between the aperture distribution and the observed radiation pattern, and simplified expressions for antenna gain and directivity. These assumptions facilitate antenna design, verification, and measurements, as well as analysis in wireless communications and radar.

Limitations exist: the far field is an approximation. For large or complex radiators, or when accurate field values are needed close to the source, the 1/r behavior and planar-wave approximations introduce errors. In such cases near-field (Fresnel region) analysis or full-wave simulations are more appropriate. Far-field assumptions remain a foundational tool, with clear boundaries between valid use and when more exact modeling is required.