Which S parameter represents input port return loss or reflection coefficient (equivalent to VSWR)?

Scattering parameters, or S parameters, describe the electrical behavior of linear electrical networks. In a 2-port network, the S parameters are labeled S11, S12, S21, and S22, where the first number indicates the port where the signal is measured, and the second number indicates the port where the signal is injected. S11 represents the signal reflecting back to port 1 when a signal is injected into port 1. Therefore, it directly corresponds to the input port reflection coefficient, return loss, or VSWR. Distractors like S21 represent the power transferred from port 1 to port 2 (forward gain or insertion loss), while S22 is the output port reflection coefficient.