FSPL
Understanding Free-Space Path Loss
FSPL is not absorption or dissipation of energy; it represents the geometric spreading of electromagnetic waves as they propagate outward from a source. The power density decreases as the inverse square of distance because the same total power is distributed over an ever-larger spherical surface.
FSPL Equation
The Friis free-space path loss equation includes an apparent frequency dependence, but this is actually due to the effective aperture of an isotropic receive antenna decreasing with frequency, not increased absorption.
Link Budget Application
In a link budget, FSPL is the largest loss term: P_rx = EIRP + G_rx - FSPL - L_misc. Every 6 dB increase in FSPL (double the distance) requires either doubling the transmit power, doubling the antenna gain, or halving the data rate to maintain link quality.
= 32.44 + 20 log10(f_MHz) + 20 log10(d_km)
Examples:
1 GHz, 1 km: 92.4 dB
10 GHz, 1 km: 112.4 dB
28 GHz, 100 m: 101.3 dB
77 GHz, 100 m: 110.1 dB
Doubling distance adds 6 dB
Doubling frequency adds 6 dB
Frequently Asked Questions
What is free-space path loss?
FSPL is the reduction in signal power due to electromagnetic wave spreading over distance in free space. It increases with distance squared and frequency squared. It represents geometric spreading, not absorption; no energy is lost, it is just distributed over a larger area.
Why does FSPL increase with frequency?
The apparent frequency dependence in the FSPL formula comes from the decreasing effective aperture of the receive antenna at higher frequencies (for a fixed-gain isotropic reference). If both antennas maintain constant physical aperture, the end-to-end path loss is actually frequency-independent.
How do you calculate FSPL?
FSPL (dB) = 32.44 + 20 log10(f in MHz) + 20 log10(d in km). For example, at 10 GHz and 1 km distance: FSPL = 32.44 + 80 + 0 = 112.4 dB.