First-order Minkowski fractal circularly polarized slot loop antenna with simple feeding network for UHF RFID reader

Abstract

A circularly polarized (CP) antenna with simple feeding network for ultra-high-frequency (UHF) radio-frequency identification (RFID) reader application is presented in this letter. The proposed antenna consists of a slot loop etched on the ground and a simple feeding network using bended microstrip lines. And the two parts of the antenna are printed on either sides of a thin substrate, thus a low-profile antenna is obtained. The slot loop is meandered based on the first-order Minkowski fractal technique for antenna size reduction. To generate circularly polarized radiation, two branches of the feeding network are designed to be orthorhombic with 90◦ phase difference. The antenna is simulated and practically fabricated with a compact size of 80 × 80 × 1.6mm3. The 10-dB impedance bandwidth and 3-dB axial-ratio (AR) bandwidth are measured to be 50 MHz (0.896–0.946 GHz) and 12 MHz (0.916–0.928 GHz), respectively. The measured peak gain exhibits stable value of 3 dBi over the impedance bandwidth. Furthermore, a wide 3-dB beamwidth of 120◦ is achieved for the proposed antenna. Based on the above, this antenna is well suited for applications in UHF RFID handheld readers.