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Conclusions
In this thesis, the analysis of a number of electromagnetic aspects involved in modern UHF/microwave RFID systems has been addressed.
A NF focused planar microstrip arrays for RFID portals has been proposed and a prototype operating at 2.4 GHz has been designed and characterized. Array performance has been quantified and compared with respect to conventional far-field focused arrays (equal-phase arrays). When compared to an equal-phase array, the NF focused array is able to radiate a higher power in the near-field region but with a lower power in the far- field region (the latter property being useful to better satisfy the EIRP limits). These advantages are obtained without increasing antenna cost and complexity with respect to a conventional far field focused array.
A detailed performance analysis of NF focused planar arrays has also been addressed. Design curves and performance data have been shown for planar square arrays, as a function of array size, inter-element distance and focal distance. The performance curves have been compared with results presented in earlier studies relevant to continuous aperture antennas, as well as with numerical results obtained from a full-wave analysis of planar microstrip arrays. The above curves can be useful to the antenna designer to determine in advance the trade-off between performance and complexity of NF focused arrays, in comparison with a conventional EPA, without waiting for time-consuming full-wave simulations of large arrays.
Afterward a numerical analysis of the wireless power transfer in NF-UHF RFID systems has been addressed. An impedance matrix approach has been adopted to quantify the effect on power transfer efficiency (PTE) of both the mutual coupling and the loading condition at the tag antenna side, as a function of antenna distance and frequency (860-960 MHz frequency band). NF coupling has been analyzed by full wave numerical simulations with models of real commercial passive tag antennas, so allowing the estimation of the PTE, the far-field boundary and the impedance mismatching loss for practical RFID configurations. A set of polarization alignments between the reader and the tag antennas have also been considered, as well as the “matched-antenna” case
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where two identical commercial tag antennas have been considered at the reader and tag sides. The latter configuration will be further investigated because it looks like more robust with respect to different tag topologies and polarization alignments.
Finally, a synthetic-array phase-based technique for location and tracking of items moving along a conveyor belt and equipped with UHF-RFID tags has been presented (patent pending). It takes advantages of the fact that the tagged items move along a conveyor belt, whose instantaneous speed and path are known a priori. Algorithm performance is shown thorough a numerical model as a function of standardized UHF- RFID electrical parameters. The location algorithm can be implemented in a conventional reader that is based on an I-Q receiver, so the proposed technique does not require any modification of the reader antenna configuration usually adopted in RFID applications and it allows a simultaneous detection and tracking of all tags moving along the conveyor belts in real time.
