Free Space Path Loss: Why Signal Strength Drops With Distance
A radio signal loses power as it travels through space simply because it spreads out over an ever-larger area — Free Space Path Loss (FSPL) quantifies this geometric spreading, independent of any obstacles, weather, or interference. The FSPL formula shows loss increasing with the square of distance and the square of frequency, which is why doubling the distance between transmitter and receiver doesn't just halve signal strength — it reduces it to a quarter, a common surprise when estimating wireless range.
A full link budget calculation chains together transmit power, antenna gains (which focus energy rather than amplify it, a subtle but important distinction), path loss, and any additional cable or connector losses, to arrive at the actual power level the receiver sees. This estimated received power then gets compared against the receiver's sensitivity threshold — the minimum signal level it can reliably detect — to determine whether a link will actually work at a given distance and configuration.
Link budget calculations are the standard first-pass sanity check before deploying any wireless system, since discovering a link doesn't close after installation is a far more expensive mistake than catching it on paper first.