RF Power Density
Far-field power density from EIRP and distance — the exposure math.
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The engineering
Spherical spreading again, now with safety consequences. FCC OET-65 controlled-exposure limits run 1–5 mW/cm² depending on frequency; the percentage row references the strictest common limit. This is far-field math — inside an antenna's near field it's measurement territory, not formulas.
The 377 Ω converting S to volts per meter is free space's own impedance.
Where this math comes from
The 4πd² is Johannes Kepler's inverse-square insight applied to Poynting's 1884 energy-flow theorem. RF exposure limits grew from postwar radar medicine — the 10 mW/cm² thermal threshold era — and matured through ANSI C95.1 (1966) into today's FCC OET-65 (1997) and IEEE/ICNIRP frameworks.
The math is three hundred years old; only the limit lines keep moving.
- 1884John Henry PoyntingEnergy flux of the EM field — S itself.
- 1966ANSI C95.1First US RF exposure standard.
- 1997FCC OET-65The compliance-calculation handbook this card mirrors.
See the full timeline of the math behind every calculator →
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