Calculate antenna gain in dBi from aperture size, frequency and efficiency.
Antenna gain measures how effectively an antenna concentrates energy in a direction compared to an isotropic radiator. For aperture antennas (dishes, horns), gain is proportional to the physical aperture area, the square of frequency, and the aperture efficiency factor.
Parabolic antenna gain
G = efficiency * 4*PI*A / wavelength^2
dBi is decibels relative to an isotropic antenna — a theoretical point source that radiates equally in all directions. A 30 dBi antenna concentrates energy 1000x more effectively in its main beam direction than an isotropic radiator would.
Higher frequency means shorter wavelength, so the same physical aperture captures more wavelengths across its diameter, producing a narrower beam and higher gain. This is why large dishes are extremely high-gain at microwave frequencies.