Compact broadband circularly-polarised antenna with a backed cavity for UHF RFID applications

A compact broadband circularly-polarised (CP) antenna is designed with a novel method for universal ultra-high-frequency (UHF) radio-frequency identification (RFID) readers. It is composed of a compact ring-shaped patch which aims to decrease the size of the antenna. A quadrature 3 dB coupler placed below the ground plane creates 90° phase differences to generate the CP radiation of the antenna. To improve the performance of the proposed antenna and minimise its size, the authors use a coupling feeding method, an FR4 dielectric slab, and a metal cavity. The coupled feeding is implemented to eliminate the mismatching between the long probe and the ring-shaped patch, and to improve the gain over the operating band. The introduction of an FR4 dielectric slab and a cavity can reduce the antenna size and improve the impedance matching and axial-ratio (AR) bandwidth. The antenna with a backed cavity can increase the front-to-back ratio remarkably and improve the CP performance further. The measured results show that the antenna with a low profile (0.45λ 0 × 0.45λ 0 × 0.06λ 0 at 915 MHz) has the impedance bandwidth of 30.2% (730–990 MHz) and 3 dB AR bandwidth of 24.2% (760–970 MHz). Both the impedance and the AR bandwidth cover the worldwide UHF RFID band.