Dual-band transmission-type circular polariser based on frequency selective surfaces

A design of dual-band transmission-type linear-to-circular polarisation converter based on frequency selective surfaces (FSSs) has been presented in this study. The proposed converter is implemented by cascading a two-dimensional periodic array of split-ring resonators bisected by metal strips and an array of rectangular patches surrounded by rectangular microstrip rings. The structure composed of metal layers and dielectric layers is designed to behave differently for field components of the two orthogonal polarisations and transmit a circularly polarised wave once illuminated by a linearly polarised plane wave within two frequency bands. Using the equivalent circuit models of the FSSs, the operating principle of the converter is presented and discussed in detail. Also, as an illustrating example, a prototype of the proposed polarisation converter operating in two frequency bands of 6.4-8.8 GHz and 12.1-13.9 GHz is simulated, fabricated and experimentally characterised. The measurement results demonstrate that the dual-band polarisation converter operates <3 dB axial ratio in a field view of ±25° with fractional bandwidths of 31.6 and 13.8%.