When [[EM.Cube]] generates an FDTD mesh, a large number of geometrical considerations are taken into account. These include the bounding box of each object and its corners, the ends of a line, the apex of a cone or pyramid, or the locations of lumped sources, field probes and sensors, vertices of plane wave or far field boxes, to name a few examples. These points are âlockedâ as fixed grid nodes in the FDTD mesh. [[EM.Cube]] determines these points internally to generate a mesh that best approximates the original structure. As you saw earlier, you can use the FDTD mesh settings to control the shape and resolution of the mesh, for example, around the curved portions of your structure, or on slanted lines or faces, etc. These settings are global and apply to all the objects making up your physical structure.
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[[Image:FDTD81.png|thumb|250px| Advanced Mesh Settings dialog.]]
You can control the global mesh more selectively using the Advanced FDTD Mesh Settings Dialog. To open this dialog, click the '''Advanced '''button at the bottom of the FDTD Mesh Settings dialog. For example, you can control the quality of the gradual grid transitions by setting the value of '''Max Adjacent Cell Size Ratio'''. The default value of this parameter is 1.3, which maintains a smooth grid line spacing scheme with no more than 1:1.3 ratio for adjacent cells. By default, grid lines are enforced at all source and observable locations. You have the option to disable this feature and round up source locations to their closest grid lines. You may also uncheck the box labeled "Adapt mesh resolution to material properties". In that case, the same effective wavelength will be used to determine the mesh resolution inside all materials as well as the free-space regions.
[[Image:FDTD81.png]]
Figure 1: [[FDTD Module]]'s Advanced Mesh Settings Image:FDTD147.png|thumb|200px|Locking the mesh density of a material group in its property dialog.]]Â
In certain cases, you may wish to exert some level of local mesh control. For example, you may want to increase the mesh density at a very particular area of your structure. Or you may want to increase or decrease the mesh resolution inside certain types of materials independent of their permittivity and permeability. [[EM.Cube]] provides two additional mechanisms for local control of the FDTD mesh: locking mesh of object groups and user defined fixed grid points. You can lock the mesh density of each material group. In this case, the areas inside and around the "Locked" material objects are meshed according to the locked mesh density, while the rest of the computational domain is meshed using the project's global mesh density. Locking mesh is done through the property dialog of each material group. At the bottom of the material property dialog, in the "Mesh Properties" section, there is a check box labeled '''Lock Mesh''', which is unchecked by default. If you check this box, it enables the box labeled '''Mesh Density''', which initially has a value equal to the project's global mesh density.
[[Image:FDTD147.png]]
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Figure 2: Locking the mesh density of a material group in its property dialog.
===Adding Fixed Grid Points===
* You can also remove a fix grid point from the FDTD mesh using the '''Delete''' button.
[[{{twimg|Image:FDTD38.png]] [[Image:FDTD39.png]] Figure 1: |Adding a new fixed grid point in [[FDTD Module]]'s Fixed Grid Points Settings dialog. [[Image:FDTD36.png|400px]] [[Image:FDTD37FDTD39.png|400px]]}}
Figure 2{{twimg|Image: FDTD36.png|User defined fixed grid points in an FDTD mesh. (Left) a Regular mesh, (Right) |Image:FDTD37.png|User defined grid points in an Adaptive mesh.}}
==Excitation Sources==