A Low-Profile Broadband Circularly Polarized Slot Antenna Loaded With Metasurface for UHF-RFID Readers

A low-profile broadband antenna with high-gain circularly polarized (CP) radiation is proposed for ultra-high frequency (UHF) radio frequency identification (RFID) readers. The design is a sequentially rotated 2 × 2 CP slot array. Each slot element utilizes two 45°-rotated stubs to introduce proper structural asymmetry, and consequently, achieve CP radiation. The array is fed by a sequential-phase feeding network for improving CP radiation in terms of gain and 3-dB axial ratio (AR) bandwidth. For a broadside radiation, the slot array is backed by a ground plane (GND). A metasurface, which is composed of 6 × 6 periodic metallic plates, is inserted between the array and GND to allow a low profile with high efficiency. The final design with overall size of 210 × 210 × 32.4 mm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> (0.6λ × 0.6λ × 0.09λ at 0.86 GHz) has been fabricated and measured. The measurements result in a 10-dB return-loss bandwidth of 0.81 − 0.96 GHz, a 3-dB AR bandwidth of 0.86 − 0.93 GHz, and broadside right-hand CP radiation with the peak gain of 8.7 dBic.