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True-time-delay UWB reflectarray with linear to circular polarization transformation

We present a new technique for designing low-profile circularly-polarized reflectarrays with broadband true-time-delay (TTD) responses and linearly-polarized feeds. The proposed reflectarray exploits the unit cells of ground-plane-backed, anisotropic miniaturized-element frequency selective surfaces as its spatial time-delay units (TDUs). Each TDU is composed of a stack of non-resonant rectangular-shaped capacitive patches. Besides providing the necessary time delay to create beam collimation, each TDU creates a broadband 90° phase difference between the two orthogonal components of the incident field. This results in a wideband, TTD reflectarray that converts a linearly polarized spherical incident field to a circularly-polarized collimated beam. A prototype of such a reflectarray operating at X-band is designed, fabricated, and characterized. Measurement results show consistent radiation characteristics, excellent time domain performance, and an axial ratio lower than 1.5 over at least 40% bandwidth. The offset-fed versions of the prototype were also shown to be capable of providing off-axis, TTD UWB beams within the wide field of view of ±45°.