Loop-Fed Planar Inverted-L Antennas (PILAs) for Omnidirectional UHF on-Metal Tag Design

A new type of tag antenna, which is made by placing four identical planar inverted-L antennas (PILAs) in rotational symmetry constellation, is proposed for designing a miniature on-metal tag antenna. It shows good omnidirectional characteristics in the azimuth plane. In the design, the directive radiation beams of the respective PILAs are tactfully combined to form a stable omnidirectional pattern. The proposed tag is compact with a small size of 40 mm × 40 mm × 1.6 mm (0.122 λ × 0.122 λ × 0.005 λ). Here, all the PILAs are inductively excited by a circular loop, which also introduces additional reactance for improving the impedance matching. The proposed tag antenna is able to achieve a stable read distance of 5.9 m on metal in the azimuth plane when it is tested using equivalent isotropic radiated power of 4 W. The operating frequency of the tag is stable and it is not affected much by changes in the backing metal.