Free Space
Understanding Free Space Propagation
Free-space propagation is the simplest model of electromagnetic wave propagation and serves as the baseline for all wireless link budget analysis. Real-world propagation adds losses and effects (multipath, atmospheric absorption, rain, diffraction) on top of the free-space model.
Free-Space Path Loss
FSPL in dB = 32.44 + 20 log10(f_MHz) + 20 log10(d_km). This shows that path loss increases by 6 dB each time distance doubles (20 dB/decade) and 6 dB each time frequency doubles. At 10 GHz over 10 km, FSPL = 32.44 + 80 + 20 = 132.4 dB.
Free Space vs Real World
- Atmosphere: Adds 0.01-15 dB/km depending on frequency and weather.
- Rain: Adds attenuation above 10 GHz. Severe above 30 GHz.
- Multipath: Reflections from ground, buildings create fading and enhancement.
- Diffraction: Partial signals around obstacles.
FSPL (dB) = 32.44 + 20 log10(f_MHz) + 20 log10(d_km)
Or: FSPL = (4 pi d / lambda)^2 (linear)
Example values:
1 GHz, 1 km: FSPL = 32.44 + 60 + 0 = 92.4 dB
10 GHz, 10 km: FSPL = 32.44 + 80 + 20 = 132.4 dB
60 GHz, 1 km: FSPL = 32.44 + 95.6 + 0 = 128 dB
Frequently Asked Questions
What is free-space propagation?
Free-space propagation is the idealized model where electromagnetic waves travel without obstacles, reflections, or atmospheric effects. Signal strength decreases as 1/distance^2. All real-world link budgets start with free-space loss and add additional degrading factors.
Why does FSPL increase with frequency?
FSPL appears to increase with frequency because the equation implicitly assumes isotropic antennas whose effective aperture decreases with wavelength. If antenna physical aperture is held constant, the link budget actually improves at higher frequencies (more gain compensates for seemingly higher FSPL).
Is free-space loss a real physical effect?
Free-space loss is not absorption or dissipation. It is the geometric spreading of energy over a larger sphere as distance increases. The power density decreases as 1/R^2. The total radiated power remains constant; it is just spread thinner.