Friis Transmission Equation

Engineering & Applied Technologies Dictionary
A far‑field radio‑frequency link relation for ideal free‑space line‑of‑sight between matched antennas that expresses received power Pr in terms of transmitted power Pt, antenna gains Gt and Gr, wavelength λ and separation R (commonly Pr = Pt Gt Gr (λ/(4πR))^2 under matched polarization and far‑field assumptions), neglecting multipath, obstructions and near‑field coupling.