Antenna gain

Antenna gain (G) is the ratio of the radiation intensity in a given direction to the intensity that would be produced if the power accepted by the antenna were radiated isotropically:

G = 4π U(θ,φ) / P_in = η · D

It includes the antenna's losses through the efficiency η. Gain is given in dBi (vs isotropic) or dBd (vs λ/2 dipole; dBi = dBd + 2.15). Example: λ/2 dipole 2.15 dBi; parabolic dish G = η_a(πD/λ)², often 30–50 dBi.