Demystifying Free-Space Path Loss
FSPL is the floor of every link budget and the first sanity check on any RF safety analysis. Here's the equation, the assumptions baked into it, and the cases where it lies.
The equation
FSPL (dB) = 20·log10(d) + 20·log10(f) + 32.44Where d is in km and f is in MHz. The 32.44 constant absorbs the 4π and the unit conversions.
What it assumes
Free space. No ground reflection, no atmospheric absorption, no diffraction, no multipath.
Why RF safety still uses it
Because it's conservative for safety analysis: the real path loss in any cluttered environment is higher, so the safety distance computed under FSPL assumptions is always a worst-case bound. The FSPL calculator on the tools page returns both the loss and the implied safe distance.
More from the archive
OET-65 in 60 Seconds: How the FCC MPE Limits Actually Work
A field-engineer's plain-English summary of the General Public and Occupational MPE limits, why they're frequency-dependent, and how time-averaging changes the answer.
2026.04.22EIRP vs ERP: What That 2.15 dB Actually Means
EIRP and ERP are the same number with two different reference antennas. Here's why the 2.15 dB conversion exists and when each one is the right metric to cite.
2026.04.20FCC vs ISED Canada: Cross-Border RF Safety in Practice
The General Public limits look similar — but the time-averaging windows, occupational definitions, and exclusion-zone conventions differ. A quick map for cross-border programs.