The AC/DC interfaces have built-in support for S-parameter calculations. For connecting transmission lines and other lumped feeds, use a lumped port that approximates a connecting transmission lines or a voltage source with a known internal impedance. The lumped port should only be used when the port width is much smaller than the wavelength. In the Electric Currents and Electrostatics interfaces, use the terminal feature with the terminated setting to approximate a connecting transmission line or a voltage source with a known internal impedance. For a terminal the voltage measurement is always with respect to ground so at least one ground feature is also required in the model.