Another issue of concern in an FDTD simulation is the numerical stability of the time marching scheme. You can set the mesh grid cell size to any fraction of a wavelength. Normally, you would expect to get better and more accurate results if you increase the mesh resolution. However, the time step is inversely proportional to the maximum grid cell size in order to satisfy the Courant-Friedrichs-Levy (CFL) stability condition. A high resolution mesh requires a smaller time step. Since you need to let the fields in the computational domain fully evolve over time, a smaller time step will require a larger number of time steps to achieve convergence. [[EM.Cube]] automatically chooses a time step that satisfies the CFL condition.
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For more detailed information, see [[Waveform, Bandwidth, Stability]].