Single-layer circularly polarized patch antenna for RFID reader application

Radio frequency identification (RFID) system in the ultra-high frequency (UHF) band has gained much interest in several service industries, purchasing and distribution logistics, manufacturing companies and goods flow system [1]. The RFID system generally consists of the reader and the tag, and the UHF RFID system operates at the bands of North America (902–928 MHz), Taiwan (920–928 MHz) and Europe (865–867 MHz). The RFID reader antenna is one of the important components in RFID system and has been designed with CP operation. Circularly polarized antennas can reduce the loss caused by the multi-path effects between the reader and the tag antenna. A CP antenna with a low profile, small size, and light weight is required in portable RFID reader. A typical technique for producing circular polarization is to excite two orthogonal linearly polarized modes with a 90° phase difference. Single-fed circularly polarized annular-ring, square and circular patch antennas with perturbation elements are reported [2–4]. Using perturbation cuts or strips to suitably differentiate the two orthogonal modes at resonant frequency, the antenna can easily radiate CP wave. However, these antennas provide small impedance bandwidth and narrow axial ratio (AR) bandwidth. To enhance an AR bandwidth, a single-fed slot-coupled microstrip antenna for circular polarization operation is preferred [5–7]. The CP antenna is achieved by using an inclined nonlinear coupling slot or unequal arm of crosss-lot. However, a common slot coupling patch antenna in its basic structure consists of two substrates separated by a ground plane. Two substrates increase the volume of the antenna and the complexity of fabrication. Another drawback is that a multi-layered substrate with the coupling slot on the ground plane can result in coupled surface-wave modes, which will lead to distorted radiation patterns.