# Surface Impedance (Ohm)

This option specifies a user defined complex surface impedance Zs.

Warning: The impedance boundary condition for the MoM (also then for MLFMM and so forth) has certain limitations regarding the range of validity. Feko will apply the user defined surface impedance as is without regard for the specific configuration (frequency, radius of curvature of the structure and so forth).

For surfaces the unit of Zs is $\Omega$ . For wire structures, the value used by Feko is in units of $\frac{\Omega }{\text{m}}$ and results from the surface impedance expression by dividing it by $\pi \varrho$ where $\varrho$ represents the wire radius.

The properties for the surface impedance are as follows:

Real part
The real part of the surface impedance Zs in $\Omega$ (for triangles) or in $\frac{\Omega }{\text{m}}$ for wires.
Imaginary part
The imaginary part of the surface impedance Zs in $\Omega$ (for triangles) or in $\frac{\Omega }{\text{m}}$ for wires.