Coupled complementary metasurfaces (CCMTS)
exhibit a passband whose frequency is several times lower than that of the individual metasurface (MTS) passband frequency. In this paper we explain this phenomenon and propose a simple and accurate equivalent circuit for CCMTS comprised of slots and their Babinet complement, dipoles. An equivalent circuit is extracted from a coupled EFIE-MFIE equation using a synthetic
basis function. The same procedure can be conveniently applied to any CCMTS. The model allows one to estimate the large downshift of resonant frequency and the bandwidth utilizing a simple formula. When used in a subresonant regime, the unit cell may have a dimension of a tenth of a free space wavelength with a moderate value of permittivity between the complementary layers
