Compact Broadband Dual Coupling-Feed Circularly Polarized RFID Microstrip Tag Antenna Mountable on Metallic Surface

A square-shaped microstrip RFID tag antenna designed for 915 MHz band is proposed. The antenna is capacitively excited by a dual-offset coupling feed network with one of the feed lines a quarter-wavelength longer than the other. By selecting appropriate dimensions for the coupled lines and feed-lines, along with desirable coupling distance between the coupled line and square patch, the input impedance of the proposed tag antenna can be easily controlled, which results in excellent impedance matching. Moreover, by incorporating the dual-offset feed technique with the method of embedding a cross slot of unequal arm lengths into the square patch, the bandwidth of the circularly polarized (CP) operation can be enhanced. The measured 6-dB return-loss bandwidth of the tag antenna is 25 MHz (902-927 MHz), and its corresponding 3-dB axial-ratio (AR) bandwidth is 20 MHz (903-923 MHz). Further experimental results show that the proposed tag antenna can provide slightly better reading range and gain when mounted on a large metallic surface.