  In this work, we introduce and study parallel-plate-waveguide structures that
are formed by two penetrable metasurfaces having arbitrary sheet impedances. We
investigate the guided modes which can propagate in such structures and derive
the corresponding dispersion relations. Different scenarios including series-
and parallel-resonant impedance sheets are considered. The obtained theoretical
formulas are applied to predict the dispersion properties for different
separations between the two symmetric or asymmetric metasurfaces.
