Coupled-PILAs for Miniature On-metal RFID Tag Design

A new structure that consists of two magnetically coupled planar inverted-L antennas (PILAs) is proposed for designing a miniature radio frequency identification (RFID) tag that can be used for on-metal tagging applications. The two PILAs, which act as a radiator and a coupler separately, is magnetically coupled to induce meandered currents on the radiator for achieving miniaturization. As a result, the tag size can be scaled down to $25.00 mm \times 18.75 mm \times 3.1 mm (0.0763\lambda \times 0.0572\lambda \times 0.0094\lambda)$. Despite being electrically very small and low in profile, the proposed tag can be read from beyond 8.3 m (4 W EIRP) on metal at the resonant frequency of 915 MHz. This shows that the induced meandered currents are effective for scaling down the antenna size without affecting the radiation characteristics much. The proposed method is able to introduce lumped inductance and capacitance to antenna input impedance for achieving conjugate match with the chip impedance.