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Radio-frequency identification (RFID) technology is a consolidated example of wireless power transfer system in which passive electromagnetic labels called tags are able to harvest electromagnetic energy from the reader antennas, power-up their internal circuitry and provide the automatic identification of objects. Being fully passive, the performance of RFID tags is strongly dependent on the context, so that the selection of the most suitable tag for the specific application becomes a key point. In this work, a cost-effective but accurate system for the over-the-air electromagnetic characterization of assembled UHF RFID tags is firstly presented and then validated through comparison with a consolidated and diffused measurement systems. Moreover, challenging use-cases demonstrating the usefulness of the proposed systems in analyzing the electromagnetic performance of label-type tags also when applied on different material or embedded into concrete structures have been carried out.
The optimal design problem for a wireless power transfer link based on a resonant inductive coupling is addressed in this paper. It is assumed that the magnetic coupling coefficient and the inductor quality factors are known. By employing the conjugate image impedances, the values of the inductances realizing the optimal design with respect to given values of the network input and load impedances are derived. It is demonstrated that there is just one optimal design maximizing both the power delivered to the load and the power transfer efficiency of the link. The four possible schemes corresponding to the use of a parallel or a series arrangement for the two coupled resonators (Parallel-Parallel, Series-Series, Parallel-Series, and Series-Parallel) are considered and discussed. Closed form analytical formulas are derived and validated by circuital simulations.