Miniature anti-metal UHF RFID tag antenna loaded with embedded open stubs for enhanced bandwidth.

A miniature tag antenna, which has a geometrical dimension of 20 mm × 20 mm × 3.32 mm (0.061λ × 0.061λ × 0.010λ, where λ is the free-space wavelength at 915 MHz) has been proposed for metal-mountable UHF RFID applications. A single-layer folded patch antenna incorporating two open stubs is designed to provide high inductance for frequency tuning, resulting in an enhanced operational bandwidth of 855-942 MHz defined using read range R ≥ 4 m criterion, where the tag maintains a minimum read range of 4 m. The width of the stubs can be adjusted to fine-tune the tag resonant frequency. These two stubs are symmetrically positioned to generate a broadside radiation pattern above the metal surface, ensuring optimal performance. The proposed tag antenna can be easily constructed using flexible foam and polyimide substrates. The read range of the proposed tag antenna was characterized experimentally by placing it on a 20 cm × 20 cm metal plate. With 4 W effective isotropic radiated power (EIRP), the tag antenna can achieve a maximum measured read range of approximately 7.9 m at boresight. It should be noted that minor differences between simulated and measured results may arise due to practical implementation factors, including the use of a packaged RFID chip during fabrication compared with the bare-die impedance model used in simulations. A thorough analysis has been conducted to comprehend the resonance characteristics, and an equivalent circuit model has been developed for designing and optimizing the tag antenna. It is important to highlight that the resonant frequency of the proposed tag remains stable and slightly impacted by the presence of a metallic object behind it.