Near-Field Constrained Design for Self-Tuning UHF-RFID Antennas

Recently introduced self-tuning RFID tags are capable to dynamically modify the input impedance of the embedded microchip transponder in order to compensate the possible impedance mismatch with the antenna, thus making the communication performance rather insensitive to the nearby environment. A general method for the design of this new class of tags is presented with the purpose to master the complex configuration, where the tag is placed at a close distance from the interrogating antenna, and the free-space assumption is not valid. A two-port system is introduced, and the network-oriented reformulation of self-tuning action permits to derive an optimization problem for the minimization of the interrogation power for a wide range of boundary conditions. The method is demonstrated, both numerically and experimentally, through the application of a finger augmentation device, which aimed to achieve a smart interaction with touched objects.