Chipless Orientation Independent RFID Tags Using Square Slot Ring Structures

RFID is rapidly transforming identification technologies, but reducing tag cost is very essential for replacing conventional barcodes. To address this, researchers are advancing chipless RFID solutions, including reconfigurable square slot ring resonator-based tags. This work presents and compares four configurations of square 4-bit tag designs. The proposed tags are simulated on FR-4 substrate () with height 0.8 mm and can be mounted on paper, plastic, or implanted in ID cards and bank notes, enabling tracking of up to 16 products. Proposed chipless RFID tag is designed and simulated using CST Microwave Studio 2018. The tag is excited by a plane wave, and the backscattered signal is received by an RCS probe at 100 mm away from the tag to receive the backscattered signal at the far-field region. The proposed tag is symmetric, fully passive, planar, and operate in the multi frequency domain, enabling orientation-independent reading by RFID interrogators. Simulation results demonstrate RF barcode performance across the Ultra-Wideband range (3.1-10.6 GHz), including tag responses under different angular excitations. The proposed concept has been validated by exciting the proposed tag from four different angles – 0°, 30°, 45°, and 60°. In all cases, the results remain nearly identical, demonstrating angular stability. Furthermore, the tags are reconfigurable, and their physical size can be maintained while encoding either a higher or lower number of bits. Owing to these features, the proposed tags are well suited for low-cost tracking applications such as banknotes, postal cards, and ID cards.