free-space path loss

Free-space path loss is the unavoidable thinning-out of a radio signal simply because it spreads over an ever-larger sphere as it travels — even with nothing in the way. Picture spray-painting a balloon: as the balloon inflates, the same paint must cover a much bigger surface, so the colour thins. Radio power thins the same way, falling as the square of distance, which is why a signal that's strong across a room is feeble across a city.

The formula is famously surprising: loss grows not only with distance but with frequency, so higher-frequency bands (like 5G mm-wave at 28 GHz) fade far faster than old 700 MHz coverage signals — one reason mm-wave needs many close-spaced cells. Crucially this loss is geometric spreading, not absorption; the energy isn't destroyed, just diluted, which is why a high-gain antenna or array can claw much of it back by re-concentrating the beam.