A Circularly Polarized UHF- and Microwave-RFID Reader With a Metasurface-Inspired Superstrate

This letter proposes a circularly polarized RFID reader which covers the European UHF (865MHz), North-American UHF (915MHz), and the microwave (2.45GHz) RFID bands. The reader consists of a coaxially-fed slotted-patch antenna and a metasurface-inspired superstrate placed 5mm above the radiator. The antenna-superstrate arrangement radiates broadside RCP waves within both the UHF bands and broadside LCP waves within the microwave band. The reader has measured gains of 3.95dBic, 3.98dBic, and 5.67dBic at 865MHz, 915MHz, and 2.45GHz, respectively. The main feature of the proposed reader is its wide 3dB axial ratio beamwidths of 134<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sup>, 158<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sup>, and 70<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sup> within the three respective RFID bands. This ensures accurate CP identification for off-boresight tag detection. The antenna has a volume of 0.36 × 0.36 × 0.023<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\lambda _{0}{^{3}}$</tex-math></inline-formula>; where <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\lambda _{0}$</tex-math></inline-formula> = free space wavelength at 865MHz. The compact size enables the reader to be used for both handheld and desktop applications.