Impedance Boundary Condition
The Impedance Boundary Condition
is used at boundaries where the field is known to penetrate only a short distance outside the boundary. This penetration is approximated by a boundary condition to avoid the need to include another domain in the model. Although the equation is identical to the one in the low-reflecting boundary condition, it has a different interpretation. The material properties are for the domain outside the boundary and not inside, as for low-reflecting boundaries. A requirement for this boundary condition to be a valid approximation is that the magnitude of the complex refractive index
where μ1 and ε1 are the material properties of the inner domain, is large; that is, | N | >> 1. Furthermore, the exterior material properties are assumed to be isotropic.
The source electric field Es can be used to specify a source surface current on the boundary.
The Surface Roughness subnode is available from the context menu (right-click the parent node) or from the Physics toolbar, Attributes menu.
Figure 4-5: The impedance boundary condition is used on exterior boundaries representing the surface of a lossy domain. The shaded (lossy) region is not part of the model. The effective induced image currents are of reduced magnitude due to losses. Any current flowing into the boundary is perfectly balanced by induced surface currents as for the perfect electric conductor boundary condition. The tangential electric field is generally small but nonzero at the boundary.
Impedance Boundary Condition
Select a Surface impedance definition — From material properties (the default) or User defined.
For From material properties, select an Electric displacement field model — Relative permittivity (the default), Refractive index, Loss tangent, loss angle, Loss tangent, dissipation factor, Dielectric loss, Drude-Lorentz dispersion model, or Debye dispersion model. See the Wave Equation, Electric node, Electric Displacement Field section, for all settings. However, notice that only isotropic (scalar) material parameters are supported for this boundary condition.
Enter a Surface impedance for User defined (SI unit: Ω). The default is Z0_const that is the predefined COMSOL constant for the wave impedance of free space.
Source Electric Field
To display this section, click the Show More Options button () and select Advanced Physics Options in the Show More Options dialog box.
Enter a Source electric field Es (SI unit: V/m). The default is 0 V/m.
See Skin Depth Calculator to evaluate the skin depth of a homogeneous material.
Impedance boundary conditions can be applied to domains that deselect the physics within those domains and instead apply the boundary condition to exterior boundaries of the domains. Using the domain feature may reduce the number of steps defining selections.
Coaxial to Waveguide Coupling: Application Library path RF_Module/Transmission_Lines_and_Waveguides/coaxial_waveguide_coupling
Computing Q Factors and Resonant Frequencies of Cavity Resonators: Application Library path RF_Module/Verification_Examples/cavity_resonators