Miniaturization and circularly polarized RFID tag/reader antenna for UHF/SHF band use

Abstract

In this work, two antenna designs are proposed. First, we propose a tag antenna which implemented on a square Rogers RO4003C substrate with a compact size of 35 × 35 × 0.508 mm 3 . Four triangle shapes are slightly curved that used to enhance the axial ratio and improve circular polarization (CP). The meander lines remained unchanged, but the end circles are replaced with squares to reduce simulation time. Square delay elements with slight curves are also introduced, while the T-match structure was retained for better impedance matching. These modifications aim to adjust the antenna to the UHF band and enhance circular polarization. At the resonant frequency of 915 MHz, the antenna achieves a low reflection coefficient of −31.12 dB. Additionally, a high gain of 1.62 dBic supports a large reading range of 8.84 m in broadside bidirectional radiation, making it highly suitable for long-range RFID applications in random mobility scenarios. The second part contains a reader antenna which is printed on the RO4003C substrate with dimensions of 104 × 104 × 1.524 mm 3 . Two pairs of double-sided dipoles are printed on the top and bottom of the substrate and fed by a 50 Ω coaxial cable. The reader an- tenna operates using the sequential phase mechanism with phase shifts of 0◦ , 90◦ , 180◦ , and 270◦ to achieve circular polarization. The dipoles are connected through a concentric ring delay-line feed structure, enabling circular polarization across both frequency bands. The longer and shorter dipoles enable circular polarization in the UHF/SHF bands, pro- viding gains of 1.73 dBi and 5.7 dBi, respectively. At the resonant frequencies of 915 MHz (UHF) and 2.4 GHz (SHF), the antenna achieves low reflection coefficients of −30.87 dB and −45 dB, indicating excellent impedance matching. These features make it well-suited for universal RFID systems operating in the UHF and SHF bands