An Adjustable Quarter-Wavelength Meandered Dipole Antenna With Slotted Ground for Metallically and Airily Mounted RFID Tag

In this paper, a loop-bowtie strap feed antenna with $\lambda /4$ meandered dipoles and modified T-matching networks is proposed. T-matching networks are employed to guide current, enhancing gain of the antenna. Aiming to improve robustness of the antenna used both on metallic objects and in air, two steps are implemented, respectively. Noticeably, the method is universal to design antimetal tag which also has a good performance in air. Four movable connection stubs and H-slots together raise degrees of freedom to tune global RFID frequency band. Besides, a simple equivalent circuit is presented, giving a convenient design rule and reasonable analysis of the design. Finally, reflection coefficients are measured using two methods, one is using the calibration kit made by us and the other is using port extension. Reading distances in different cases are also measured, showing the maximum distances are 1.8 and 2.7 m on metallic objects and in air, respectively. Measured and simulated results are with a good agreement, verifying the rationality of the design.