S-parameters (scattering parameters)

S-parameters describe an RF component by what it does to waves rather than to voltages and currents. Imagine shining a flashlight at a pane of glass: some light passes through, some bounces back. S-parameters bottle exactly that behaviour for electrical signals: S11 is how much of the wave sent into port 1 reflects back, and S21 is how much arrives at port 2. They are the natural language of microwave engineering because, unlike voltage and current, travelling waves are easy to measure at gigahertz frequencies.

For a two-port like an amplifier or filter, the four numbers S11, S21, S12, S22 (each a magnitude and phase, usually in decibels) fully capture its small-signal behaviour. S21 in dB is the forward gain or loss; S11 in dB is the input match (return loss); S12 tells you how much leaks backward — its reverse isolation. A vector network analyzer (VNA) measures all of them by sweeping frequency and comparing incident and reflected waves.