Also known as: path loss, propagation loss
Path loss is the attenuation a signal experiences travelling from transmitter to receiver.1 It is dominated by the spreading of energy over distance, plus extra losses from terrain, buildings, and foliage — and it is the single largest term in almost every link budget.
How it works
In empty space, a transmitter radiates power over an expanding sphere. The power crossing a fixed receive aperture falls as the square of distance — the inverse-square law — so free-space loss rises 20 dB for every tenfold increase in range and, for a fixed-size antenna, another 20 dB for every tenfold increase in frequency. This idealised case is free-space path loss, captured by the Friis transmission equation.
Real environments lose far more. The catch-all model writes received power as falling with distance raised to a path-loss exponent n: n ≈ 2 in free space, but 2.7–4 in cluttered urban and indoor settings, and even higher through heavy obstruction. On top of that distance trend sit two extra effects: shadowing, the slow variation as terrain and buildings block the path (often modelled as a log-normal spread of a few dB), and multipath fading, the fast fluctuation from reflected copies of the signal arriving out of phase. A link budget reserves a fade margin to survive these dips.
Not all of the loss is bulk absorption. Diffraction over rooftops and hills, reflection off the ground and buildings, and blockage of the Fresnel zone around the direct line all shape how much energy reaches the receiver. This is why raising an antenna even a few metres — clearing obstacles and opening the Fresnel zone — can buy more than a large increase in transmit power.
In practice
Because path loss so often totals 100 dB or more, it dwarfs the handful of dB available from better cable or a preamp — which is why the biggest wins come from geometry: antenna height, a clear line of sight, and picking a band that suits the range. Lower frequencies generally carry farther for the same power (less free-space loss and better diffraction around obstacles), one reason VHF public-safety systems reach farther per site than UHF.
Path loss also explains the shape of a received-power map: signal strength does not fall off a cliff at some range but decays smoothly with the logarithm of distance, so coverage fades gradually into the noise floor rather than stopping sharply. Predicting coverage means estimating path loss over terrain and comparing the result against receiver sensitivity.
Relevance to SDR
Path loss is why a distant or obstructed system arrives near the noise floor with barely enough SNR to decode, and why antenna height and a clear path (propagation) matter so much for a scanner install. GopherTrunk receives whatever the path delivers; when a known nearby system is un-decodable, path loss (obstruction, low antenna) is usually the first suspect, ahead of anything in the DSP chain.