Which S parameter is equivalent to forward gain?

Scattering parameters, or S-parameters, describe the electrical behavior of linear electrical networks, such as amplifiers or filters. In a standard two-port network, Port 1 is typically the input and Port 2 is the output. The standard naming convention for S-parameters is S(output)(input). Following this rule, S21 represents the power transferred to Port 2 (the output) from Port 1 (the input). This measurement is the exact definition of forward gain or the forward transmission coefficient. By comparison, S11 measures the signal reflected back to Port 1 (input match or SWR), S22 measures the reflection at Port 2 (output match), and S12 measures reverse isolation, which is the signal leaking backward from the output to the input.