Circularly Polarized UHF RFID Tag Antenna for Wireless Sensing of Complex Permittivity of Liquids

This paper presents a compact circularly polarized UHF RFID tag antenna based on complementary split ring resonator (CSRR). Its operating frequency varies with the injected liquid inside the proposed antenna, which is fabricated by 3D printing technology. Based on this feature, the complex relative permittivity of the liquid under test can be extracted wirelessly at a distance from the reader antenna by measuring the read range. The real and imaginary parts of the complex permittivity of the liquid are then extracted from the frequency with the maximum read range and two neighboring frequency points, at which a read range related parameter is 90% of its maximum value. The circular polarization of the proposed antenna is realized by a pair of CSRRs for achieving a relatively large read range and miniaturizing the antenna size. According to the measurement results, the complex relative permittivity of liquids with real part between 1 and 81 can be extracted from a reader with the distance up to 1.97 m.