Friis Free-Space Path Loss Calculator

Harald T. Friis, a Danish-American radio engineer at Bell Labs, published the transmission equation bearing his name in a 1946 paper, "A Note on a Simple Transmission Formula," in the Proceedings of the IRE. Friis had been working on propagation studies for transatlantic radiotelephone links since the 1920s — the equation distills decades of empirical work into a single elegant formula.

The Friis equation assumes ideal free-space propagation: no reflections, no atmospheric absorption, polarization-matched antennas, and far-field distances (d > 2D²/λ for antenna aperture D). It's the baseline for every wireless link-budget calculation since 1946.

For real-world propagation, Friis is augmented by additional-loss models: Okumura-Hata (1968) for urban-cellular paths, Longley-Rice for VHF/UHF over rough terrain, and modern ray-tracing for indoor/WiFi coverage. But FSPL remains the reference point — the theoretical best-case path loss against which real losses are measured.

Enter frequency (with SI suffixes) and line-of-sight distance. The tool returns FSPL in dB, plus supplementary values: wavelength, distance in meters, and the d/λ ratio (useful for checking if you're in the near-field where Friis doesn\'t apply, d < 2D²/λ).

The curve visualization shows how loss scales with distance on log axes — every decade (10×) of distance adds 20 dB of loss. Use this output as a loss term in your full link-budget calculation. Everything runs client-side.