Low-Profile Circularly Polarized Hybrid Antenna for Beam Switching and OAM Mode Switching

A low-profile circularly polarized (CP) hybrid antenna for beam switching and orbital angular momentum (OAM) mode switching is proposed in this article. It hybridizes reconfigurable folded reflectarray (RFRA) and reconfigurable folded transmitarray (RFTA) simultaneously. By employing two pairs of 1-bit reconfigurable unit cells of (0°, 180°) and (90°, 270°), the hybrid antenna achieves 2-bit phase resolution. More importantly, the hybrid structure alleviates the design burden on the bias network, because only half of the unit cells are necessary to be reconfigurable, while the other half are fixed. Meanwhile, the reconfigurable unit cells are dispersed on both of the top and bottom metasurface. To validate the hybrid idea, two prototypes operating at a center frequency of 26 GHz are analyzed and measured, in which the metal stub/open circuit is used to model the “on/off” state of p-i-n diode for simplification. The prototypes can achieve beam switching among four fixed beams and OAM mode switching among four OAM modes. In addition, compared with 1-bit RFRA or RFTA, the proposed prototype for beam switching achieves a much wider 3-dB axial ratio (AR) bandwidth of 27.1% and a higher aperture efficiency of 18.1%. Meanwhile, the other prototype achieves higher order OAM mode switching between <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${l} =-3$ </tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${l} =1$ </tex-math></inline-formula>. The measured OAM purity is 75% and 86% at 26 GHz, respectively, and more than 65% purity is achieved within 24–28 GHz.