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Distance

Fréquence

MHz

Free Space Path Loss

120.0 dB

FSPL at 10x Distance140.0 dB

Free Space Path Loss vs Distance

Formule

FSPL is the loss in signal strength as a radio wave spreads out over distance, assuming no obstacles.

FSPL (dB) = 20 x log10(d_km) + 20 x log10(f_MHz) + 32.44

This comes from the Friis equation: FSPL = (4 x pi x d / lambda)^2, converted to decibels.

Doubling the distance adds 6 dB of loss

Doubling the frequency adds 6 dB of loss

This is the absolute minimum loss; real-world losses are higher due to obstacles, rain, and atmospheric absorption

2.4 GHz Wi-Fi signal over 10 km in free space.

Why is FSPL so high even in free space?

FSPL accounts for the spreading of energy over the surface of an expanding sphere. It is geometric, not absorption loss.

Does free space path loss depend on power?

No. FSPL is independent of transmit power. It describes attenuation, not the received signal level.

When is the free space model appropriate?

For satellite links, point-to-point microwave, and as a baseline for more complex propagation models.

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