HF RFID Tag Location Using Magneto-Inductive Waves

Location of passive RFID tags in the HF regime presents significant problems, because of the absence of radiating fields at the low frequencies involved. Here we present a solution for one-dimensional localization based on magneto-inductive (MI) waves. Passive tags are interrogated using a travelling wave antenna based on a MI waveguide, a magnetically coupled array of \mathbf {L}-\mathbf {C} resonators supporting travelling waves. Load modulation signals generated by the tag during its unique identifier response are coupled into the waveguide and travel to either end with low group velocity. Signal timings are measured by cross-correlation, and the tag position is estimated to the nearest resonant loop from the difference in their arrival times. Correlation detection is demonstrated using a system model, and theoretical predictions are confirmed using an experimental system containing eleven transformer-coupled resonators operating at 13.56 MHz frequency. Accurate localization is obtained up to the tag reading limit using < 1\text{W} RF power.