Localization of Compact Circularly Polarized RFID Tag Using ToA Technique

Abstract

A compact, flexible crossed-dipole circularly polarized antenna using commercially available paper substrate is presented which caters North American frequency band. The crossed-dipoles have meandered lines for reduction of size as well as increased inductivity in the antenna. Dipoles have asymmetric T-shaped rectangular endings to provide the required compactness. Two semicircles are induced between the orthogonal dipoles and meandering matching structure to accomplish circular polarization excitation. Good impedance matching with the chip is achieved through a modified meander line matching structure. The proposed design dimensions are 32 x 32 x 0.4 mm(3). Systematic analysis revealed the results comprising circular polarization 3dB-AR bandwidth of 12 MHz (908-920 MHz) and power transmission coefficient bandwidth of 36 MHz (900-936 MHz). Time delay between interrogating signal and backscattered signal is measured and relative distance is calculated. Linear Least Square (LLS) method is applied to approximate the position of tag in interrogation area. The proposed tag is placed at known locations and its position is measured to analyze accuracy of the method by simulating the positioning algorithm code in MATLAB. Six valid tag positions 0.5-2 m read range and 0 degrees-150 degrees angular resolution has been investigated.