Waveguide Boundaries | XFdtd

The outer edges of a waveguide interface require the application of a boundary condition, similar to the full finite-difference time-domain (FDTD) space. Boundary conditions for waveguides are limited to PEC and PMC, and the appropriate choice depends on the orientation of the electromagnetic fields adjacent to each given boundary and the material properties it intersects. When dielectric and free space materials are coincident with an edge, PEC is preferred for electric fields that are primarily normal to the boundary, and PMC is preferred for those that are tangential. Edges coincident with ground or another conductor are typically set to PEC because the field values are close to zero.

For example, the image shows the typical quasi-TEM E-field distribution for a microstrip along with the four edges of the waveguide interface drawn in green. The E-field along the vertical sides is strongest just above the air-dielectric boundary. At this point, E is mostly tangential to the vertical sides and H (not shown) is mostly normal to them. The vertical boundaries should be set to PMC because it best satisfies this condition. The strongest E along the top horizontal boundary is directly over the microstrip and is mostly normal to the top boundary at this point (H tangential). The upper boundary should be set to PEC because it best satisfies this condition. The lower horizontal boundary is in the conducting ground plane and will have near-zero fields so that boundary type is of little importance. In this case, the boundary would typically be set to PEC.

The Boundaries tab of the new waveguide interface editor provides four drop-down menus corresponding to the four edges of the interface, which are determined by the U, V coordinates under the Geometry tab. Each drop-down menu allows users to select either PEC or PMC.