Free-Space Path Loss Calculator
Calculate free-space path loss (FSPL) between two radio stations.
Enter frequency and distance to get signal attenuation in dB.
Free-Space Path Loss
Free-Space Path Loss (FSPL) is the attenuation of a radio signal as it travels through open space, with no obstructions, atmospheric absorption, or ground reflection.
Formula: FSPL (dB) = 20 log₁₀(d) + 20 log₁₀(f) + 92.4
Where d is distance in kilometers and f is frequency in GHz. This gives FSPL in decibels.
Alternative (MHz and km): FSPL = 20 log₁₀(d_km) + 20 log₁₀(f_MHz) + 32.4
Key insights:
- Every time you double the distance, path loss increases by 6 dB (inverse square law)
- Every time you double the frequency, path loss increases by 6 dB
- Higher frequencies need either more transmitter power or higher gain antennas
Typical FSPL examples:
- 2m (146 MHz) over 50 km: ~105 dB
- 70cm (435 MHz) over 50 km: ~116 dB
- WiFi 2.4 GHz over 100 m: ~80 dB
- Satellite (2 GHz) at 36,000 km altitude: ~190 dB
Link budget: Transmitter power (dBm) - FSPL (dB) + antenna gains (dBi) = received signal (dBm). A typical HF transceiver at 100W = +50 dBm. A 3 dBi antenna adds 3 dB. Receiver sensitivity might be -120 dBm.