You may occasionally want to use EM.Cube's FDTD simulator to model planar structures. Although EM.Cube provides the more computationally efficient [[Planar Module]] for this very purpose, there are many cases when an FDTD simulation might prove advantageous over a 2.5-D MoM simulation. Examples include examining the transient response of a planar structure, very wideband simulations, planar structures involving complex materials or 3D geometries embedded inside the substrate layers, to name a few.
A planar substrate usually consists of one or more dielectric layers, possibly with a PEC ground plane at its bottom. Unlike EM.Cube's [[Planar Module]], where the substrate layers are defined implicitly in the "Stack-up Settings" dialog, in the finite-domain [[FDTD Module]], you need to draw each dielectric layer separately and then stack them up manually. The substrate of a planar layered structure extends laterally to infinity. In other words, the ±X and ±Y boundary walls must, in effect, retreat to infinity. This can be accomplished in the [[FDTD Module]] by setting up the CPML layers in a particular way. For this purpose, the lateral CPML layers need to move in and touch the sides of the dielectric layer stack-up. <font color="red">'''<u>In other words, to model a laterally infinite dielectric substrate, <font color="#a52a2a"> you must assign a PML boundary condition to the four lateral sides of the domain box and</font> set the lateral domain offset values along the ±X and ±Y directions all equal to zero. If the planar structure ends in an infinite <font color="#a52a2a">dielectric </font>half-space from the bottom, <font color="#a52a2a">you must assign a PML boundary condition to the bottom side of the domain box and set </font>the -Z offset equal to zero. Similarly, if the planar structure ends in an infinite PEC ground plane from the bottom, you must assign a PEC boundary condition to the bottom side of the domain box and set</u>'''<font color="#a52a2a"><font color="#a52a2a"><u> '''the -Z offset equal to zero.'''</u></font></font><font color="#222222"> </font></font>In the latter case, the presence of the metal plane at the bottom of the physical structure is implied although you will not see it in the project workspace. The CPML layers on the sides and at the bottom of the computational domain will absorb all the incident waves propagating in the free space or inside the substrate layers and thus emulate infinite extents. This leaves only the +Z offset with a nonzero value. The top CPML layer is moved back and placed above the finite parts of the structure.
<font color="#a52a2a"><u>'''In other words, to model a laterally infinite dielectric substrate, you must assign a PML boundary condition to the four lateral sides of the domain box and set the lateral domain offset values along the ±X and ±Y directions all equal to zero. If the planar structure ends in an infinite dielectric half-space from the bottom, you must assign a PML boundary condition to the bottom side of the domain box and set the -Z offset equal to zero. Similarly, if the planar structure ends in an infinite PEC ground plane from the bottom, you must assign a PEC boundary condition to the bottom side of the domain box and set the -Z offset equal to zero.In the latter case, the presence of the metal plane at the bottom of the physical structure is implied although you will not see it in the project workspace. The CPML layers on the sides and at the bottom of the computational domain will absorb all the incident waves propagating in the free space or inside the substrate layers and thus emulate infinite extents. This leaves only the +Z offset with a nonzero value. The top CPML layer is moved back and placed above the finite parts of the structure. {{Note|The current release of EM.Cube's [[FDTD Module]] does not support anisotropic or dispersive layers of laterally infinite extents. In other words, You can only define anisotropic and dispersive material objects of finite size that do not touch the CPML boundaries.'''</u></font><br />}}
[[Image:FDTD24.png]]