Compact Folded Crossed-Dipole for On-Metal Polarization Diversity UHF Tag

For the first time, the folded crossed-dipole structure is explored for designing a compact metal tag (40 mm × 40 mm × 1.6 mm) that can be read in all boresight directions. The tag structure consists of a pair of folded crossed-dipoles which are orthogonally displaced for achieving polarization diversity, where the angular reading range has been extended by removing the null points. In the design, a parasitic metal ring is loaded beneath the radiator for tuning down the tag resonant frequency effectively. Good impedance matching with high power transmission is achievable by employing additional tuning mechanisms such as slotting the radiating dipole arms and adding rectangular patches beneath the slots. Design schemes for optimizing the impedance matching between the antenna and the chip have also been thoroughly discussed. Conjugate impedance match is found to be easily achievable by tuning the design parameters. To understand the impedance characteristics, an equivalent circuit has been derived for the proposed tag antenna. When tested using a linearly polarized reader antenna with a transmitting power of 4 W EIRP, the tag antenna is detectable from 5.6 m to 7.7 m in the boresight and it can be read in all directions.