Ring-Shaped Patch Antenna Embedded With Multiple Inductive Loads for Omnidirectional On-Metal Tag Design

A ring-shaped patch antenna is embedded with inductive loads for designing an on-metal tag antenna. Multiple inductive loading structures such as I-shaped patch, C-shaped arms, inductive stubs, and L-shaped stubs have been tactfully integrated into the radiating patch, all on a single layer without requiring additional footprint, for generating sufficient antenna inductance so that the tag resonance can be tuned down to the UHF RFID passband. The proposed tag is planar, and it has a compact size of 50 mm $\times $ 50 mm $\times 3$ .3 mm. It can achieve good omnidirectional characteristics, maintaining a consistent read range of 11.5 - 12.7 m in the azimuthal plane, due to good impedance matching. The operating frequency of the tag is found to be very stable, and it is not affected much by changes in the backing metal.