Free Space Path Loss
Calculator
Compute signal attenuation over distance at a given frequency using the Friis transmission equation. The foundation of every RF link budget analysis.
Input Parameters
FSPL at Common Frequencies and Distances
| Frequency | 100 m | 1 km | 10 km | 100 km | GEO (35,786 km) |
|---|---|---|---|---|---|
| 900 MHz | 71.5 dB | 91.5 dB | 111.5 dB | 131.5 dB | 182.6 dB |
| 2.4 GHz | 80.0 dB | 100.0 dB | 120.0 dB | 140.0 dB | 191.1 dB |
| 5.8 GHz | 87.7 dB | 107.7 dB | 127.7 dB | 147.7 dB | 198.8 dB |
| 12 GHz | 94.0 dB | 114.0 dB | 134.0 dB | 154.0 dB | 205.1 dB |
| 28 GHz | 101.4 dB | 121.4 dB | 141.4 dB | 161.4 dB | 212.5 dB |
| 39 GHz | 104.3 dB | 124.3 dB | 144.3 dB | 164.3 dB | 215.4 dB |
| 60 GHz | 108.0 dB | 128.0 dB | 148.0 dB | 168.0 dB | 219.1 dB |
| 77 GHz | 110.2 dB | 130.2 dB | 150.2 dB | 170.2 dB | 221.3 dB |
How Free Space Path Loss Works
Free space path loss describes how an electromagnetic wave's power density decreases as it spreads outward from the source. It is not absorption or dissipation; it is a geometric spreading effect. As the wave propagates, its energy is distributed over an increasingly large surface area, reducing the power captured by a fixed-aperture receiving antenna.
The Formula
Expanded: FSPL = 20×log₁₀(d) + 20×log₁₀(f) + 20×log₁₀(4π/c)
Simplified with km and GHz:
FSPL (dB) = 20×log₁₀(d_km) + 20×log₁₀(f_GHz) + 92.45
Simplified with miles and MHz:
FSPL (dB) = 20×log₁₀(d_mi) + 20×log₁₀(f_MHz) + 36.6
Key Rules of Thumb
- Double the distance = +6 dB loss. Going from 1 km to 2 km adds 6.02 dB.
- Double the frequency = +6 dB loss. Moving from 14 GHz to 28 GHz adds 6.02 dB.
- 10× the distance = +20 dB loss. Going from 1 km to 10 km adds 20 dB.
Beyond Free Space: Real-World Losses
In practice, your link will experience additional losses beyond FSPL. These must be added to the path loss for accurate link budget analysis:
- Atmospheric absorption: Significant at 22 GHz (water vapor), 60 GHz (oxygen), and 183 GHz. Can add 0.1 to 15+ dB/km depending on frequency.
- Rain fade: Major factor above 10 GHz. At Ka-band (28 GHz) in heavy rain, attenuation can reach 10+ dB/km.
- Foliage loss: Trees and vegetation can add 10-20 dB or more at mmWave frequencies.
- Building penetration: Exterior walls add 10-40 dB depending on material and frequency.
Frequently Asked Questions
What is free space path loss?
FSPL is the attenuation of signal power between two isotropic antennas in free space with no obstacles or atmospheric effects. It increases with both frequency and distance, following an inverse-square law. It represents the minimum possible loss for any radio link.
Does doubling the distance double the path loss?
No. Doubling the distance adds 6 dB to the path loss. This is because path loss follows an inverse-square law: 20 × log₁₀(2) = 6.02 dB. Similarly, multiplying distance by 10 adds 20 dB.
Why does higher frequency have more path loss?
It is not that higher frequencies "attenuate more" in free space. The increased FSPL is because a fixed-size receiving antenna captures a smaller fraction of the transmitted energy at higher frequencies (smaller effective aperture). If you increase antenna gain proportionally with frequency (keeping physical aperture constant), the link budget remains the same.
How do I use FSPL in a link budget?
FSPL is the propagation loss component: Received Power = TX Power + TX Antenna Gain - TX Cable Loss - FSPL - Additional Losses + RX Antenna Gain - RX Cable Loss. If Received Power exceeds the receiver sensitivity by the required margin, the link closes. Use our Link Budget Calculator for complete analysis.