A Three-Dimensional Incident Angle-Insensitive Circular Polarization Frequency Selective Slab With High-Frequency Selectivity

In this letter, a three-dimensional incident angle-insensitive circular polarization frequency selective slab (CP-FSSlab) with high-frequency selectivity is presented. Based on the equivalent medium theory and the analysis of the phase derivatives, the refractive index is identified as the primary equivalent material parameter influencing the phase difference of the transmission coefficient. According to this, by introducing additional three-dimensional structures optimized through a local equivalent circuit model between the upper and bottom layers of a double-layer planar frequency selective surface, control over the change in refractive index with different polarizations is achieved, thus ensuring polarization consistency. The proposed CP-FSSlab is fabricated and measured, the results demonstrate that the CP-FSSlab maintains high-frequency selectivity while exhibiting improved polarization consistency in phase. For the normal incidence, the measured lower and upper roll-off fraction bandwidths are 9.7% and 3.3%, respectively. The maximum measured insertion loss of the ripple in the passband under oblique incidence with an incident angle up to 60° is 1.4 dB. In addition, at the incident angle of 60°, the measured −3 dB magnitude fraction bandwidth and 3 dB axial-ratio fraction bandwidth are 18.8% [(5.3 to 6.4) GHz] and 12.4% [(5.3 to 6.0) GHz], respectively.