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propagationbasicITU-R P.525

Free Space Path Loss (FSPL)

#path loss#FSPL#propagation#friis#free space#attenuation

Free Space Path Loss (FSPL)

The Free Space Path Loss formula calculates the attenuation of radio signals as they propagate through free space. This fundamental formula assumes no obstacles, reflections, or atmospheric effects - representing the theoretical minimum path loss.

FSPL=20log⁡10(d)+20log⁡10(f)+20log⁡10(4πc)FSPL = 20\log_{10}(d) + 20\log_{10}(f) + 20\log_{10}\left(\frac{4\pi}{c}\right)

Alternative Forms

Simplified (dB)
FSPLdB=32.45+20log⁡10(fMHz)+20log⁡10(dkm)FSPL_{dB} = 32.45 + 20\log_{10}(f_{MHz}) + 20\log_{10}(d_{km})
Practical form with frequency in MHz and distance in km
Linear Form
FSPL=(4πdfc)2FSPL = \left(\frac{4\pi d f}{c}\right)^2
Path loss as a power ratio (not in dB)

Input Variables

km

Distance between transmitter and receiver(Range: 0.001 – 100000 km)

MHz

Operating frequency of the signal(Range: 1 – 300000 MHz)

Results

Free Space Path Loss (FSPL)
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📖 Example: Wi-Fi Signal at 10 meters

Calculate path loss for a 2.4 GHz Wi-Fi signal at 10 meters

Related Resources

Path Loss CalculatorFrequency and Wavelength Relationships
Reference: ITU-R P.525

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