Radiative Quadrature Hybrid Coupler for Near-Zone UHF RFID Applications

Abstract

This paper presents a radiative quadrature hybrid coupler designed using a non-conventional substrate material for ultra-high frequency (UHF) radio-frequency identification (RFID) applications. The coupler generates radiation above the microstrip-lines for near-zone UHF RFID tag detection. The coupler is designed to be wideband by intentionally mismatching the port impedances. The coupler’s return loss is −15 dB while its isolation bandwidth is 155 MHz, which more than meets the UHF RFID requirements. The coupler is insusceptible to performance variations due to the presence of proximity assets. The coupler can read tags up to 1.3 m at 1 W RFID reader power. The surface magnetic field distribution is uniform with no null zones, making it suitable for liquid and metal asset tracking. The coupler yields equal power split with 90° phase difference, which also makes it practical for driving external beamforming antennas. Several of this coupler can be cascaded to form a distributed antenna system for various smart applications.