How dish gain is calculated
A parabolic reflector's gain grows with its area measured in wavelengths. Real dishes lose some of that ideal gain to feed spillover, surface error and blockage, which the aperture efficiency captures.
- Gain:
G = η·(π·D/λ)², in dBi as10·log₁₀(G) - Half-power beamwidth:
θ ≈ 70·λ/Ddegrees - Far-field distance:
2·D²/λ - Mismatch loss:
Γ = (VSWR−1)/(VSWR+1), loss= −10·log₁₀(1−Γ²)
Worked example: 1.2 m Ku-band dish
At 12 GHz the wavelength is 25 mm, so a 1.2 m dish is 48 wavelengths across. With 65% efficiency the peak gain is about 41.7 dBi and the half-power beamwidth about 1.46°. A feed VSWR of 1.3:1 costs only about 0.07 dB.
Good to know
- Typical efficiency is 55–70% for prime-focus and offset dishes.
- Narrow beams need accurate pointing. Pro estimates pointing loss as 12·(error/beamwidth)² dB and computes G/T.