Calculate gain of an antenna with losses using simple dish antenna calculator online.
Note that the gain is proportional to the aperture area and inversely proportional to the square of the wavelength. For example, if the frequency is doubled, (half the wavelength), the aperture could be decreased four times to maintain the same gain.
Formula:
G = (4ηπA) / λ²
Where,
η - Aperture Efficiency,
A - Physical aperture area,
λ - Wavelength,
G - Gain of an antenna with losses