[[Image:FDTD94.png|thumb|250px|...and its Yee mesh.]]
Since FDTD is a finite domain numerical technique, the computational domain of the problem must be truncated. At the boundaries of the computational domain, proper boundary conditions must be enforced. In a shielded structure, all objects are enclosed within a perfect electric (or magnetic) conductor box. In an open boundary problem like an antenna, some kind of absorbing boundary conditions such as a perfectly matched layer (PML) must be used to emulate the free space. The computational domain must be discretized using an appropriate meshing scheme. EM.Tempo uses a non-uniform, variable, staircase (pixelated) Yee mesh with a mesh density absorbing boundaries should act such that you can customize. A fixed-cell mesh generator is also available, where you can set constant cell dimensions along the three principal axes for the entire computational domainfield propagates through them without any back reflection. The variable mesh density is specified in terms of FDTD simulation time depends directly on the effective wavelength inside material media. As a result, the mesh resolution and average mesh cell size differ in regions that are filled with different types of material. [[EM.Cube]]'s non-uniform mesher generates more cells in the areas that are occupied by dielectric materials, fewer cells in the free space regions computational domain and no cells inside (impenetrable) PEC regions. [[FDTD Module]]'s default "adaptive" mesh generator also refines on how close you can place the mesh around curved segments of lines, surface or solids PML walls to produce a far more accurate representation of your geometrythe enclosed objects. The example below illustrates a dielectric ellipsoid and a 3D view of its Yee mesh:
The FDTD simulation time depends directly on the size of the computational domain. For free space radiation or scattering problems, the computational domain must be extended to infinity, which means an infinite number of cells in the computational domain. The solution to this problem is Click here to truncate the domain by a set of artificial boundaries at a certain distance from the objects in the computational domainlearn more about EM. The absorbing boundaries should be such that the field propagates through them without any back reflectionTempo's [[Perfectly Matched Layer Termination]].
Learn more about The FDTD computational domain must be discretized using an appropriate meshing scheme. EM.Tempouses a non-uniform, variable, staircase (pixelated) Yee mesh with a mesh density that you can customize. A fixed-cell mesh generator is also available, where you can set constant cell dimensions along the three principal axes for the entire computational domain. The variable mesh density is specified in terms of the effective wavelength inside material media. As a result, the mesh resolution and average mesh cell size differ in regions that are filled with different types of material. [[EM.Cube]]'s non-uniform mesher generates more cells in the areas that are occupied by dielectric materials, fewer cells in the free space regions and no cells inside (impenetrable) PEC regions. [[Perfectly Matched Layer TerminationFDTD Module]]'s default "adaptive" mesh generator also refines the mesh around curved segments of lines, surface or solids to produce a far more accurate representation of your geometry. The example on the right illustrates a metal ellipsoid and a 3D view of its Yee mesh.
The FDTD method provides a wideband simulation of your physical structure. In order to produce sufficient spectral information, an appropriate wideband temporal waveform is needed to excite the physical structure. The choice of the waveform, its bandwidth and time delay are important for the convergence behavior of the FDTD time marching loop. By default, EM.Tempo uses a modulated Gaussian waveform with optimal [[parameters]]: t = 0.966/Δf and t<sub>0</sub> = 4.5t, where Δf is the specified bandwidth of the simulation. The time delay t<sub>0</sub> is chosen so that the temporal waveform has an almost zero value at t = 0.
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.
For more detailed information, see [[Waveform, Bandwidth, Stability]].
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==Defining The Physical Structure==