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A millimeter-wave (mm-wave) high-gain antenna with bidirectional radiation is proposed in this article. It is the first attempt to combine a folded transmitarray (FTA) and a Fabry-Perot cavity (FPC) antenna to achieve bidirectional beams. By using this method, bidirectional radiation can be achieved while keeping a relative low profile when compared to traditional TA antennas. The proposed antenna consists of a dual-polarized (DP) feeding source, a single-layered partially reflective surface (PRS), and a bottom metasurface. By adding metal grids on the bottom surface of the PRS, it can partially reflect the x-polarized electromagnetic (EM) wave and fully reflect the y-polarized EM wave simultaneously. The bottom metasurface can also reflect x-polarized EM wave and transmit y-polarized EM wave. Hence, for the DP feeding source, when the x-polarized wave is generated, forward radiation is realized by FPC effect. When the y-polarized wave is generated, the backward radiation characteristic is achieved due to the FTA function. For validation, a prototype with a center frequency of 35 GHz is designed, fabricated, and measured. The proposed antenna can achieve 18 and 21 dBi in the bidirectional directions, respectively. The proposed design is a candidate for mm-wave long-distance wireless systems that require low profile and high communication flexibility.

A novel linear-to-circular polarizer with a bandwidth of 88% is proposed in this article. It is the first attempt to propose a phase difference compensation method for circularly polarized (CP) bandwidth improvement. Based on this, a new integrated structure combining frequency-selective surface (FSS) and dielectric grating structures, named FSS inserted dielectric polarizer (FIDP), is proposed. By combining the monotonically increasing phase difference of the dielectric grating structures, with the monotonically decreasing phase difference of the FSS, the proposed polarizer achieves a 90° phase difference between two orthogonal $E$ -field components within a wide frequency range. At the same time, the magnitudes of the components are not reduced when the two $E$ -field components pass through the polarizer. Hence, CP conversion over a wide frequency range is achieved. For validation, three feeding horns operating in adjacent bands are designed as the feeds for the polarizer. The experimental results show that the proposed polarizer yields an operating bandwidth of 88%, ranging from 6.8 to 17.5 GHz. The thickness of the polarizer is $1.34~\lambda _{0}$ . The performance demonstrates that the proposed structure is a good candidate for X-band applications that require wideband CP wave radiation. It is fabricated by 3-D printing technology and PCB processes, which are low-cost and mature.