A Switch circuit component definition is an active component that allows a change in the geometry's configuration during a calculation. It is defined by specifying an initial state—either open or closed—as well as the time at which the transition begins and the duration of time that the action will last.

## Background

Switches are updated during a finite-difference time-domain (FDTD) simulation by modifying the instantaneous electric field

\begin{equation} E\big|^n = \chi\big|^nE\big|^n \end{equation}where $\chi\big|^n$ represents the state of a switch at timestep, $n$. The electric field at the switch location is first calculated as though the switch were not present. That value is then multiplied by zero or one for a closed or open switch, respectively. Because the initial calculation omits the switch state, the switch's function depends on the material that exists at the mesh location. Given that $E=0$ for perfect electric conductors, a closed switch acts as a wire.

During a transition, a Gaussian function is applied in order to smooth the transition from either zero to one (as a closed switch opens), or one to zero (as an open switch closes). The transition should take place over 60 timesteps in order to reduce the amount of high-frequency transient signals being introduced into the simulation space.

Switches should be used when only time-domain results are of interest. This is because switches violate the assumptions of linear system theory, making it incorrect to apply either a Fourier or discrete Fourier transform to the time-domain results. A simulation containing a switch produces invalid frequency-domain results—including impedances, S-parameters, and steady-state far-zone fields—even if the time-domain results eventually decay to zero. The time-domain results are unaffected, and therefore are valid.