S-Parameters in Terms of Voltage and Electric Field
Scattering parameters (or S-parameters) are complex-valued, frequency dependent matrices describing the transmission and reflection of electromagnetic waves at different ports of devices like filters, antennas, waveguide transitions, and transmission lines. S-parameters originate from transmission-line theory and are defined in terms of transmitted and reflected voltage waves. All ports are assumed to be connected to matched loads/feeds, that is, there is no reflection directly at a port.
For a device with n ports, the S-parameters are
where S11 is the voltage reflection coefficient at port 1, S21 is the voltage transmission coefficient from port 1 to port 2, and so on. The time average power reflection/transmission coefficients are obtained as | Sij |2.
Now, for high-frequency problems, voltage is not a well-defined entity, and it is necessary to define the scattering parameters in terms of the electric field.
For details on how COMSOL Multiphysics calculates the S-parameters, see S-Parameter Calculations.