A Compact Dual-Polarized Chipless RFID Tag by Using Nested Concentric Square Loops

Abstract

This letter presents a compact dual-polarized chipless tag. The chipless tag consists of nested concentric coplanar square loops, and adjacent loops are connected by two stub lines at the center of the opposite sides, respectively. There are two critical factors in achieving small size. One is due to the integration of multiple square loops in a nested structure, sharing the same sides. The structure is designed to obtain the following function relationship between the loop number $n$ and resonant mode number $N: N\,= \,2(n- 1)$ . The other critical factor for minimization is that it is the coupling of adjacent loops that responds to resonances. The gap size only influences the coupling strength, and shifts resonance frequency a little, and does not influence the sharp-edged resonance in spite of small gap. This is one highlight. The other highlight is that four variations can be independently adjusted on the basis of the proposed reference tag, and a lot of different encoding identifications can be obtained. In addition, the proposed tag also has the following advantages. The proposed tag is magnetic, and the radar cross section (RCS) magnitude is insensitive to the medium where it is pasted. High RCS magnitude is realized at the condition of minimization. The connected coplanar square loop resonators are reused in both vertical and horizontal polarizations to double the encoding capacity within a fixed bandwidth. The RCS curve keeps unchanged until the orientation angle rotates up to 35°. An 18-bit tag is developed by the proposed nested loops, and ${\text{19bits/cm}}^{2}$ data density is obtained.