Design of novel thin frequency selective surface superstrates for dual-band directivity enhancement

A novel design for a thin frequency selective surface (FSS) type of metamaterial superstrate for dual-band directivity enhancement is presented as an alternative to a conventional FSS superstrate which is thicker. In the proposed new design, two strip-dipole arrays with the same periodicity, but with different alignments are placed above and below a thin dielectric layer to overcome the problem of different directivities realised at the two bands, when two FSS arrays with dissimilar periodicities are used. On the basis of unit-cell simulation, several important parameters that characterise the thin FSS superstrate are investigated, and the procedure for designing such a superstrate is described. Resonant frequencies and quality factors of the unit cell are compared with those of three FSS antenna composites with different quality factors and the optimal quality factors for the dual-band directivity enhancement are identified. Additionally, how to choose the optimal FSS array size for which the superstrate enhances directivity most efficiently is discussed. An antenna composite has been fabricated to validate the proposed approach, and the measured results show good agreement with the simulated ones