#### 5.3.3.1. Materials

OM2p and OM3p solve the eigenproblem that results from either real or complex (lossy) materials. See “Simulator-Specific Usage” for the materials available to OM2p and OM3p. Materials may be scalar or anisotropic. If a model only has lossless boundary conditions and materials, then the resulting eigenproblem is real, that is, the finite-element matrix and eigenpairs contain only real numbers. If any loss is present in the model, or periodic boundary conditions are applied, the problem will be complex. Real problems are generally solved much more rapidly than complex problems, and they require less memory for a given mesh.

If a simulated geometry contains bulk PEC, the fields inside the PEC must be exactly zero. To improve simulation efficiency, therefore, Analyst applies PEC boundary conditions to the borders of any PEC regions, and the interior of the PEC is not simulated. When the geometry contains metals that are not PEC, the field amplitude inside the metal decreases rapidly with distance into the metal, with a characteristic decay-length given by the skin depth, which depends on material properties and frequency. In most simulations, the behavior of the field inside the metal is not of interest, and the rapid change in field amplitude within a skin depth of the metal surface requires a very fine mesh to resolve. The default behavior of OM2p and OM3p is to create a new impedance boundary condition, and use it to treat the metal boundaries. As with PEC, the metal interior is not simulated.

The new impedance boundary condition is based on the metal's surface impedance at the shift frequency, where the surface impedance calculation accounts for conductivity, material thickness compared to the skin depth, and surface roughness, if present. The surface impedance calculation loses accuracy when the shift frequency is very different from the mode frequency. If your simulation requires the highest accuracy in loss calculations, adjust the shift frequency to be very close to the mode frequency.

Replacing good conductors with the appropriate impedance boundaries is an excellent approximation, and in general it is well justified by the improvement in performance. If, however, your application requires that metals be modeled as they are defined in the system, you may override this behavior by setting Solve Inside Conductors to Always in the simulation properties dialog box. You can also select “Solving Inside Conductors” on a per-material basis.