The beam divergence of an electromagnetic beam is the increase in beam diameter with distance from the aperture from which the beam emerges in any plane that intersects the beam axis.
Beam divergence is usually used to characterize electromagnetic beams in the optical regime, i.e., cases in which the aperture from which the beam emerges is very large with respect to the wavelength.
Beam divergence usually refers to a beam of circular cross section, but not necessarily so. A beam may, for example, have an elliptical cross section, in which case the orientation of the beam divergence must be specified, e.g., with respect to the major or minor axis of the elliptical cross section.
Beamdivergence: Of an electromagnetic beam, in any plane that intersects the beam axis, the increase in beam diameter with distance from the aperture from which the beam emerges.
Note 1: Beamdivergence is usually used to characterize electromagnetic beams in the optical regime, i.e., cases in which the aperture from which the beam emerges is very large with respect to the wavelength.
A beam may, for example, have an elliptical cross section, in which case the orientation of the beamdivergence must be specified, e.g., with respect to the major or minor axis of the elliptical cross section.
Beamdivergence, and the directivity of an antenna.
For practical beams, where the radiation does not suddenly fall to zero at the cone edges, we may take the beamwidth between -3dB contours as (2 alpha), as alpha is the cone semi-angle.
For a non-conical beam shape we can very roughly approximate the area of the beam footprint on the unit sphere as the product of the beam widths in E-plane and H-plane, or in azimuth and elevation.
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