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Wireless & Mobility · Foundational

Free Space Path Loss (FSPL)

Natural decrease in signal amplitude as an RF wave spreads out over distance.

Definition

Free space path loss is caused by the wavefront expanding over a larger area, not by absorption. It increases with both distance and frequency (for antennas of fixed gain). A common formula is FSPL(dB) = 20log10(d km) + 20log10(f MHz) + 32.44, and the 6 dB rule states that doubling distance adds about 6 dB of loss.

Example

A 2.4 GHz signal loses about 80 dB at 100 m in free space.

Exam tip

FSPL happens even in a vacuum; it is not the same as absorption.

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