Compact Circularly Polarized Microstrip Ring Antenna Using Capacitive Coupling Structure for RFID Readers

This paper presents a compact circularly polarized microstrip antenna for radio frequency identification (RFID) readers. The dimensions of the proposed antenna are reduced by etching a cross slot in the radiation patch and loading four grounded coupling patches on the four corners of the radiation patch. An inverted Z-shaped coupling feedline is used to realize circularly polarized operation. The measured & x2212;10 dB $\vert \text{S}_{11}\vert $ bandwidth, 3-dB axial ratio bandwidth and maximum gain are 872 & x2013;1095 MHz, 888 & x2013;933 MHz, and 5.52 dBi, respectively, at 922 MHz frequency. The overall dimension of the proposed antenna is $0.36\,\,\lambda _{\mathrm {g}} \times 0.36\,\,\lambda _{\mathrm {g}} \times 0.05\,\,\lambda _{\mathrm {g}}$ ($\lambda _{\mathrm {g}}$ is the guide wavelength at the center frequency of 915 MHz). The proposed antenna has small size with good overall performance and is suitable for compact RFID devices.