Free-Space Path Loss
FSPL between isotropic antennas — the first line of every link budget.
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The engineering
Free-space loss isn't absorption — it's geometry. Power spreads over a sphere, so every doubling of distance or frequency costs 6 dB. The famous constant 32.45 is just unit bookkeeping for MHz and km.
Real links add margin for fading, rain, and obstructions on top of this floor; FSPL is the physics you cannot negotiate with.
Where this math comes from
Harald Friis at Bell Labs wrote the transmission formula in 1946, tidying two decades of radio-link empiricism into one equation relating antenna gains, wavelength, and distance. He also gave us the noise-figure definition — few careers own more of the link budget.
The equation scaled from Holmdel's horn antennas to deep-space links: the same 20·log terms that price out your Wi-Fi also price out Voyager.
- 1901Guglielmo MarconiTransatlantic radio makes path loss an engineering question.
- 1946Harald T. FriisThe Friis transmission formula — FSPL formalized.
See the full timeline of the math behind every calculator →
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