Sub-wavelength structures (SWS) are periodic diffraction structures. A plane wave with wavelength less than the periodicity produces a zeroth order reflection.

Unlike planar FSS structures, sub-wavelength structures are three dimensional in nature.

The usual method for computer modeling them is to use rigorous coupled-wave theory (R.C.W.T.). This method provides excellent agreement with measured results.

R.C.W.T. considers structures as a dielectric material, however, metals can be modeled by using a complex permittivity to approximate the metal's conductivity.

The drawback to modeling metals with this method is that it requires a large number of waves to be used in the calculation (resulting in extended computation time).