Miniature Coplanar-Fed Folded Patch for Metal Mountable UHF RFID Tag

A folded patch, which is centrally fed by a coplanar feedline, is proposed for designing a miniature UHF tag antenna for mounting on metal surfaces. The tag is designed using a flexible polyethylene terephthalate substrate with a very compact size of 25 mm × 40 mm × 3 mm (0.0767λ × 0.123λ × 0.0092λ). Conjugate match is attainable between the antenna and the microchip and it is able to achieve a maximum read distance of 10.7 m at the effective isotropic radiated power of 4 W. In the design, two notches have been incorporated with an inductive stub for tuning of the tag's resonance over a larger frequency range. Also, a simple equivalent circuit has been derived to characterize the impedance of the tag antenna. The resonant frequency of the proposed tag antenna is found to be insensitive to various sizes of the backing metal object, which is a very desirable feature for tag design. The proposed tag can achieve a read distance of greater than 10 m when placed on metal. It is able to be read beyond 5 m when it is placed on dielectrics with permittivity in the range of 1-5.8.