RF Field Guide · interactive lesson
Free-space path loss
See how much signal level is lost as an unobstructed radio wave spreads through space. Start with a distance or place two endpoints.
Question
How much signal level does distance and frequency remove from an otherwise unobstructed radio path?
Inputs
Answer
Free-space path loss
—Enter valid inputs to preview the normalized path.Preview values update as you edit. Submit to refresh the server-calculated comparison plot. Path loss is not received signal level; a link budget also includes transmit power, gains, losses, and receiver threshold.Working
FSPL (dB) = 32.44 + 20 log₁₀(f MHz) + 20 log₁₀(d km)
- FSPL
- Free-space path loss, in dB
- f
- Carrier frequency, normalized to MHz
- d
- Path distance, normalized to km
- 32.44
- Constant for the MHz-and-km form, in dB
Enter valid inputs to substitute f and d.
- Frequency term
- —
- Distance term
- —
- Result
- —
Visual explanation
The comparison plot will show 2.4, 5, and 6 GHz over the selected positive distance range.
Interpretation
Because distance is inside a 20 log₁₀ term, doubling distance adds 6.02 dB of free-space loss. Doubling frequency does the same. This describes spreading loss only; it does not say whether a receiver can decode the signal.
Limitations
Assumptions and excluded effects
- Assumes
- Unobstructed free space, point endpoints, one carrier frequency, and great-circle ground distance for map endpoints.
- Excludes
- Terrain and Fresnel obstruction, vegetation and buildings, diffraction and reflections, atmospheric and rain loss, antenna patterns and alignment, polarization mismatch, cable losses, interference, noise, and equipment performance.
- Public bounds
- Positive distances through 500 km. The logarithmic graph never samples zero.
- Calculation source
- WISP Planner's free-space path-loss calculation; formula values and plot samples are calculated on the server.
Next step
Put this loss into the link-budget lesson, or check terrain and Fresnel clearance before treating a path as viable.