In telecommunication, the term beamwidth has the following meanings:
1. In the radio regime, of an antenna pattern, the angle between the half-power (3-dB) points of the main lobe, when referenced to the peak effective radiated power[?] of the main lobe.
Note: Beamwidth is usually expressed in degrees. It is usually expressed for the horizontal plane, but may also be expressed for the vertical plane.
2. For the optical regime, see beam divergence.
Source: from Federal Standard 1037C and from MIL-STD-188
