In this paper, a miniature folded dipole is proposed for designing a metal-mountable UHF RFID tag. The proposed tag antenna is low in profile and it has a compact size of 40 mm × 40 mm ×3.1 mm (0.12λ × 0.12λ × 0.009λ). Folding the dipole arms into a two-fold rotational symmetrical style can miniaturize the tag footprint for achieving high compactness. It has been found that the capacitive coupling mechanism between the rotational symmetrical radiating arms is effective in enhancing the vertical radiation, which subsequently improves the achievable read distance in the boresight direction. Also, an incorporated circular loop can provide additional inductance for achieving good impedance matching with the RFID chip. For the proposed tag antenna, a full ground plane is inserted underneath the radiator for isolating it from the backing metal, making the tag tolerant to the metallic platform. The proposed tag antenna is able to achieve a maximum read distance of 7 m at 4 W EIRP when it is tested on metal.
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