Also known as: beamwidth, half-power beamwidth, HPBW
Beamwidth is the angular width of an antenna’s main lobe — how wide, in degrees, the “beam” of strongest radiation is.1 The standard measure is the half-power beamwidth (HPBW): the angle between the two directions on either side of the peak where the radiated power has fallen to half (−3 dB) of its maximum. Beamwidth is read directly off the radiation pattern and is inversely related to antenna gain — a narrower beam concentrates energy into a smaller solid angle, so it delivers more gain.
How it works
Radiation from an antenna is strongest along the main-lobe axis and falls away to either side. If you sweep a receiver around the antenna and note the two angles where the received power drops to one-half (3 dB below peak), the angle between them is the HPBW. Antennas have two beamwidths that generally differ — one in the azimuth (horizontal) plane and one in the elevation (vertical) plane — because the main lobe is rarely circular in cross-section. A parabolic dish, for instance, might have a 4° azimuth beamwidth and a 6° elevation beamwidth.
Two other measures appear in datasheets. The first-null beamwidth (FNBW) is the wider angle between the nulls that bracket the main lobe; it is always larger than the HPBW. Occasionally a −10 dB beamwidth is quoted for feed antennas illuminating a reflector.
In practice
Beamwidth and gain are two views of the same thing: focusing. A rough and widely used estimate ties the product of the two principal-plane HPBWs to directivity,
G (dBi) ≈ 10 · log₁₀( 41 000 / (θ_az · θ_el) ),
with the beamwidths in degrees. A pencil-beam dish with 2° in each plane predicts roughly 40 dBi; a 60°-by-60° panel predicts about 10 dBi. The constant (near 41 000) folds in typical aperture efficiency and side-lobe losses, so it is an approximation, not an identity — but it captures the essential trade-off: halving the beamwidth in both planes adds about 6 dB of gain. The cost is aiming: a 2° beam must be pointed within a fraction of a degree, while a 60° beam is forgiving.
Relevance to SDR
Beamwidth decides how carefully you must aim a directional scanner antenna. A modest Yagi with a 40°–50° beamwidth adds useful gain toward a distant trunking site while still being easy to point by hand. A narrow parabolic or long-boom array, with a beamwidth of only a few degrees, gives much more gain and rejection of off-axis interference but demands a rotator and accurate bearings. For omnidirectional scanning you deliberately want a wide azimuth beamwidth (effectively 360°) so no bearing is missed, accepting the lower gain that implies. GopherTrunk does not measure or use beamwidth — it is a property of the antenna hardware — but the beamwidth you choose sets the signal-to-noise ratio the decoder ultimately sees.