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A Low-RCS and High-Gain Planar Circularly Polarized Cassegrain Meta-Antenna

Achieving highly-directive radiation with broadband operation, low scattering and thin profile for a circularly polarized (CP) antenna is particularly challenging and yet rarely reported. Here, we propose a strategy of a CP Cassegrain meta-antenna by combining a planar helical antenna, a metasurface main reflector and a metamaterial sub-reflector. The main reflector is designed to achieve focusing for CP waves at 13 GHz. The sub-reflector is chessboard configured chiral metamaterial slab composed of two different types of chiral meta-atoms, aiming to achieve spin- and direction-selective CP transmissions and reflections. The distance between two reflectors is half of focal length, which enables our antenna to be dubbed as a folded reflectarray. The low radar cross section (RCS) is achieved based on scattering cancellation technique by realizing near 180° reflection phase difference between two neighboring chessboard sub-meta-atoms. Thanks to the architecture of the two reflectors, the proposed antenna exhibits high-gain and low-profile simultaneously according to image theory. For verification, a planar CP Cassegrain antenna, excited by a left-handed circularly polarized (LCP) planar helical antenna, is fabricated, numerically and experimentally studied. Numerical results are in good agreement with the experimental ones, showing a peak right-handed circularly polarized (RCP) gain of 26.6 dBi at 12.6 GHz. Furthermore, the backward monostatic RCS of the antenna is dramatically reduced over -10 dB in a broad bandwidth from 8.4 to 15.7 GHz when it is illuminated by an LCP planar wave. Our proposed Cassegrain antenna features simultaneously broadband, high gain, low profile and low RCS, providing a new avenue to low-profile CP reflectarrays with invisibility.