How to Calculate Antenna Gain
Antenna gain relates the physical aperture area to the operating wavelength and antenna efficiency.
G = 10 * log10( eta * 4 * pi * A / lambda^2 )
Where eta is the efficiency (0 to 1), A is the physical aperture area in square meters, and lambda is the wavelength in meters. For a parabolic dish, A = pi * (D/2)^2.