Linear-to-circular polarizers operating from roughly 17 to 65 GHz, and angles of incidence up to 60° are reported. These polarizers convert incident, linearly polarized radiation into circular polarization upon transmission. First, previous designs inspired by the optics community using cascaded waveplates are scaled down to mm-wave frequencies. The naturally occurring anisotropic crystals that the optics community employed are replaced here with metamaterials. The range of incidence angles is improved by utilizing biaxial, artificial dielectrics whose permittivity in the $x$, $y$ and $z$ directions are all engineered. Next, an ultra-wideband linear-to-circular polarizer consisting of cascaded sheet impedances is reported. The cascaded sheet impedance polarizer utilizes a combination of meanderline and metallic patch geometries. The principal axes of each patterned metallic sheet are oriented at an optimized angle, which increases the design degrees of freedom and performance. This polarizer has the advantages of being thinner and easier to fabricate than the polarizer utilizing cascaded waveplates, but is more difficult to design. Both polarizers rely heavily on genetic algorithm optimization in the design process to realize multiple octaves of bandwidths and robust performance at wide angles of incidence. The polarizers are fabricated with commercial printed-circuit-boards, and then experimentally characterized.
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