Electrically Large Dual-Loop Antenna for UHF Near-Field RFID Reader

An electrically large dual-loop antenna is proposed for ultra high frequency (UHF) near-field radio frequency identification (RFID) readers. The proposed antenna is composed of a main loop and a parasitic loop wherein the loops are constructed using segmented lines with distributed capacitors. The parasitic loop enhances and uniforms the magnetic field distribution in the central portion of the larger main loop so that the perimeter of interrogation zone of the dual-loop antenna can be extended up to three operating wavelengths. The measurement shows that a dual-loop antenna prototype printed onto a piece of FR4 printed circuit board (PCB) achieves good impedance matching over the frequency range of 845-928 MHz and produces strong and uniform magnetic field distribution with an interrogation zone of 250 mm x 250 mm. A parametric study is carried out to provide the guidelines for the antenna design.