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 𝑳 − 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 <1W RF power.

Multiport magnetoinductive (MI) devices with directional filter properties are presented. Design equations are developed and solved using wave analysis and dispersion theory, and it is shown that high-performance directional filters can be realised for use both in MI systems with complex, frequency-dependent impedance and in conventional systems with real impedance. Wave analysis is used to reduce the complexity of circuit equations. High-performance MI structures combining directional and infinite rejection filtering are demonstrated, as well as multiple-passband high-rejection filtering. A new method for improving filtering performance through multipath loss compensation is described. Methods for constructing tuneable devices using toroidal ferrite-cored transformers are proposed and demonstrated, and experimental results for tuneable MI directional filters are shown to agree with theoretical models. Limitations are explored, and power handling sufficient for HF RFID applications is demonstrated, despite the use of ferrite materials.

Difficulties arise in the definition of power flow in transmission-line systems with a complex propagation constant. These were resolved by Kurokawa using quantities known as “power waves,” which contain both voltage and current terms and correctly separate power flow into forward- and backward-traveling components. Similar difficulties must arise for electromagnetic metamaterials since any discrete, periodic structure leads to band-limited propagation, with a complex propagation constant both inside and outside the bands due to loss and cutoff, respectively. Here, discrete power waves are defined for magneto-inductive (MI) systems, metamaterials based on chains of magnetically coupled LC resonators. These waves are shown to satisfy the discrete power conservation equation for MI waves and are used to calculate scattering parameters for multi-port MI devices without the anomalous predictions of conventional methods. The results will allow correct evaluation of internal scattering parameters in MI systems.

High-frequency near-field communication is an inherently short-range technology. However, the total capture volume can be increased with traveling-wave antennas. Here, we report on analysis, design, and measurements of flexible waveguide antennas and discuss their performance for near-field communication and localization. The antennas comprise sections of coaxial transmission lines loaded periodically with fieldgenerating inductive networks. Several topologies were compared to each other theoretically and the best-performing candidate was selected to fabricate antennas between 5 and 48 meters long, each containing 15 read nodes. Waveguiding properties of the antennas were measured and agreement with theory was demonstrated. Afterwards, each antenna was integrated with a custom NFC reader and shown to be capable of near-field communication with and localization of commercial off-the-shelf transponders compliant with ISO 14443 Type A protocol. The transverse detection range was 10 cm with 1 W input RF power. Both one-dimensional and quasi two-dimensional configurations were tested. The proposed antennas are flexible, scalable, have low loss, and could be used for near-field communication, identification, and tracking of distributed and mobile tags.