Parabolic Antenna Beamwidth & Gain Calculator
Estimate the half-power beamwidth and the gain of a parabolic dish antenna from its diameter and operating frequency. Enter the dish size in metres, the frequency in GHz and an optional aperture efficiency to get the beamwidth in degrees and the gain in dBi.
Enter Values
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How to use this calculator
- Enter the dish diameter D in metres.
- Enter the operating frequency f in GHz.
- Optionally adjust the aperture efficiency (default 0.55; typical dishes are 0.5–0.7) and read the beamwidth, gain and wavelength.
How it works
The wavelength is λ = 0.3 / f (f in GHz, λ in metres). The half-power beamwidth is approximated as 70·λ / D degrees, so bigger dishes and higher frequencies give narrower beams. The gain is 10·log₁₀(efficiency · (π·D / λ)²) dBi, where efficiency captures real-world losses such as illumination taper, spillover and surface error.
Worked example
Worked example. A 1.2 m dish at 6 GHz with 0.55 efficiency: λ = 0.3 / 6 = 0.05 m, beamwidth = 70 × 0.05 / 1.2 = 2.9167°, and gain = 10·log₁₀(0.55 × (π × 1.2 / 0.05)²) = 10·log₁₀(0.55 × 75.398²) ≈ 34.95 dBi.
Common mistakes
- Entering frequency in Hz or MHz instead of GHz — the λ = 0.3 / f shortcut expects GHz.
- Entering the diameter in centimetres or feet; D must be in metres.
- Using an efficiency of 1.0 — real dishes lose to spillover, taper and surface accuracy, so 0.5–0.7 is realistic.
Frequently asked questions
Why does the gain depend on aperture efficiency?
The (π·D/λ)² term is the gain of a perfect, uniformly-illuminated aperture. Real feeds under- or over-illuminate the dish, spill energy past the edge and sit on an imperfect surface, so the actual gain is the ideal value scaled by an efficiency of typically 0.5–0.7.
How accurate is the 70·λ/D beamwidth formula?
It is a widely-used rule of thumb for the −3 dB beamwidth of a circular aperture; constants from about 65 to 70 appear in the literature depending on the illumination taper. Treat it as a design-stage estimate — the true pattern comes from the specific feed and dish geometry.
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