Half-power beamwidth (HPBW)

An antenna’s sensitivity falls off away from the direction it’s pointed. The half-power beamwidth is the full angle across the main lobe between the two directions where the response has dropped by 3 dB, to half the peak. It’s also called the FWHM of the beam.

For a dish, beamwidth is inversely proportional to diameter in wavelengths. The constant in front depends on how the feed illuminates the surface. A feed that tapers to about −10 dB at the rim, which is what you want for good efficiency and low spillover, gives roughly 70 λ/D degrees. A perfectly uniform illumination would give about 58 λ/D, at the cost of more spillover. Our dish and horn calculator uses 70 λ/D.

Numbers at 1420 MHz

λ = 0.2111 m, so:

• 1.0 m dish: about 14.8°
• 1.8 m dish: about 8.2°
• 3.0 m dish: about 4.9°
• 4.5 m dish: about 3.3°
• a hand-built horn: typically 20 to 25°

For scale, the full Moon is half a degree across. Even a large amateur dish at 21 cm has a beam many times bigger than that.

Why it matters

What you’re averaging. The spectrum you record is a beam-weighted average of everything inside the beam. A 20° horn pointed at the Galactic plane averages over a wide stretch of longitudes, so spiral-arm features blend; a 3 m dish separates them. That’s covered in LAB-012.

Beam filling. If the source is larger than the beam, line strength doesn’t depend on dish size; if it’s smaller, it does. See brightness temperature.

Drift time. In a drift scan, the beamwidth divided by the sky’s drift rate is how long you get on each patch: about 33 minutes for an 8.2° beam on the equator.

Pointing tolerance. Being a tenth of a beamwidth off costs little. Being half a beamwidth off costs you half the signal, which is easy to do with a 3.3° beam and a home-built mount.