Free Space Path Loss (FSPL)
FSPL is the attenuation the signal undergoes due to spherical spreading even when there is no obstacle at all: FSPL(dB) = 32.44 + 20*log10(d_km) + 20*log10(f_MHz). At 100 metres this comes to 65.2 dB for 433 MHz, 71.2 dB for 868 MHz and 80.2 dB for 2.45 GHz. This loss is not an obstacle loss but the thinning out of energy in space.
The transmitter spreads energy over the surface of a sphere; as the radius grows the same energy is distributed over a wider area and the share captured by the antenna decreases. The constant 32.44 in the formula is the conversion coefficient that comes from the kilometre and megahertz units.
- With distance: +6 dB on every doubling. The loss that is 65.2 dB at 100 metres is 71.2 dB at 200 metres.
- With frequency: +6 dB again when the frequency doubles. 868 MHz is 6 dB worse off than 433 MHz at every distance.
- Comparison: the difference between 2.45 GHz and 433 MHz is roughly 15 dB.
The practical consequence is this: the open-area range of a set is only the point where this loss meets the sensitivity limit. On a real site, shadowing, penetration and multipath losses are added on top. This is why a set with 150 metres of open-area range stays well below that inside an enclosed structure, and that is not a fault but expected behaviour.
Context in which this term is used: Wireless remote control installation: pairing, relay, safety