A Compact Strip Dipole Coupled Split-Ring Resonator Antenna for RFID Tags

A compact strip dipole coupled split-ring resonator (SRR) antenna is proposed for radio-frequency identification (RFID) tag applications. The antenna has a simple and uniplanar structure and can be designed to conjugate match to the tag chip. Design details are presented and a tag chip of impedance (33 - j112) Omega at 915 MHz is used to verify the design. The simulated and measured input impedances of the prototype antenna are (34 + j116) Omega and (38 + j94) Omega, respectively, which are very close to the conjugate of the chip impedance. Simulated and measured radiation patterns are in good agreement and are nearly omnidirectional, especially in the H-plane. The minimum power transmitted by the reader and reflected back by the tag with detectable signals at the reader is also measured at two distances of about 1.5 lambda(0) and 3 lambda(0) using a commercially available reader card. The results are satisfactory.