Free Space Path Loss

Formula

Description

Free space path loss (FSPL) quantifies the attenuation of a radio signal propagating through free space with no obstacles. It increases with both distance and frequency: doubling the distance adds 6 dB of loss, and doubling the frequency also adds 6 dB. This is not because higher frequencies are more attenuated, but because a fixed-size receiving antenna captures a smaller fraction of the expanding wavefront at shorter wavelengths. FSPL is the baseline loss used in link budget calculations; real-world losses from obstacles, reflections, and atmospheric absorption are additional.

Variables

• FSPL — Path loss in decibels (dB)
• d — Distance between transmitter and receiver (m)
• f — Signal frequency (Hz)
• c — Speed of light ≈ 3 × 10⁸ m/s

Practical Notes

At 2.4 GHz, FSPL at 1m is about 40 dB, at 10m about 60 dB, and at 100m about 80 dB. Indoor environments add 10-30 dB of additional loss from walls and objects. For 5 GHz WiFi, add about 6 dB more loss compared to 2.4 GHz at the same distance.