Friis Transmission Equation

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.

An expression for free‑space line‑of‑sight link power: the received power equals the transmitted power multiplied by the transmit and receive antenna gains and by the square of the wavelength over (4π times the separation distance) squared, i.e. Pr = Pt·Gt·Gr·(λ/(4πR))^2, valid in the far‑field for matched, polarization‑aligned antennas and no obstructions.