Y. Zeng, Z. N. Chen, X. Qing and J. Jin, "A Directional, Closely Spaced Zero-Phase-Shift-Line Loop Array for UHF Near-Field RFID Reader Antennas," in IEEE Transactions on Antennas and Propagation, vol. 66, no. 10, pp. 5639-5642, Oct. 2018. doi: 10.1109/TAP.2018.2860619
A zero-phase-shift-line (ZPSL) loop array with two closely spaced elements is proposed to achieve a directional distribution for ultrahigh frequency (UHF) near-field radio frequency identification (RFID) applications. The proposed array consists of two coaxially positioned ZPSL loop antenna elements of the same size, with an aperiodic ZPSL loop acting as a driven element and a periodic ZPSL loop functioning as a parasitic element. A directional magnetic field distribution can be achieved when an out-of-phase current along the parasitic loop is induced. A ZPSL loop array prototype with an interelement spacing of 15 mm, or 0.046 of the operating wavelength at 915 MHz, achieves a desired directional magnetic field distribution over a circular interrogation zone with a diameter of 160 mm. As a UHF near-field RFID reader antenna, the proposed array is able to achieve a 100% detection rate of SAG tags up to 120 mm by using an Impinj Speedway reader with an output power of 30 dBm.