In computational electromagnetics, when modeling structures that include finitely conducting metal bodies that are more than several skin depths deep, one often uses a surface impedance boundary condition at the metal surface instead of trying to rigorously compute the rapidly decaying fields within the metal. This small package MetalSurfaceImpedance implements the approximate formulas from a recent paper: D. N. Grujić, “Simple and Accurate Approximation of Rough Conductor Surface Impedance,” IEEE Trans. Microwave Theory Tech., vol. 70, no. 4, pp. 2053-2059, April 2022, for computing surface impedance and effective conductivity. As reported in the reference, these formulas agree with rigorous solutions of the second order differential equation to a relative accuracy better than 0.001 from 10^{-8} Hz to 10^{12} Hz. The Julia implementation returns a result in about 500 nsec on my machine.
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