  A low-profile, modified Complementary Frequency Selective Surface (CFSS) with
dual band-pass characteristic is presented. This technique adds independent
control of the operation bands, which was a limitation from previous FSSs
design based on CFSS concept. The FSS structure utilizes resonant elements and
interaction between the layers to deliverer the first operating bandpass
response at a low frequency. It is demonstrated that the first order response
can achieved miniaturized periodic dimensions of about $\lambda_l/15$ and
ultra-thin overall thickness compressed down to $\lambda_l/200$, where
$\lambda_l$ is the free space wavelength at the lowest band of operation. An
equivalent circuit model (ECM) is presented to explain the working principles
of the proposed FSS. This investigation is further supported by parametric
studies based on full wave simulation showing how this simple approach allows
decoupling of the two bands of operation. A prototype, simultaneously operating
at S and C bands is fabricated and tested in a free space environment for
validation purpose.
