A compact RFID tag antenna using miniaturized patch structure for metallic object applications

A compact RFID tag antenna suitable for metallic object applications is proposed. The antenna structure is based on a quarter-wavelength patch with one end open and the other short-circuited. Narrowing the vertical shorting wall can adjust the input impedance of the antenna while exciting the same resonant mode. The patch length can be further reduced by applying a pair of vertical thin strips or small inductance at the upper ends of the resonating edges. Besides, a section of high-impedance line is connected in series with the patch to increase the input inductance. A prototype design of dimensions 60 mm × 30 mm is designed for conjugate matched to the Alien H3 RFID tag chip at 915 MHz. It is fabricated on a 1.6-mm FR4 substrate, assembled with the Alien chip, and mounted onto a 150 mm × 150 mm metallic sheet. The prototype is tested to have a maximum read range of 70-cm, comparable to the 200-cm read range of a commercially available RFID Alien tag measured under the same setup but without the metallic sheet.