Upper Bounds on the Bandwidth of Electrically Small Single-Resonant UHF-RFID Tags

In this communication, the upper limits on the bandwidth of single-resonant UHF-radio frequency identification (RFID) tags as a function of the tag size are investigated, with and without forcing perfect matching between the antenna and the application-specific integrated circuit. By means of a circuit network analysis, it is found that bandwidth upper bounds of small tags are significantly higher in comparison with considering conjugate matching. Particularly, it is shown that the half-power bandwidth is √2 times (approximately 41%) higher, requiring a proper relaxation of the matching level at resonance. It is also shown that bandwidth of small real tags with perfect matching, which is typically far from its upper bound, can also be enhanced approximately the same factor at the expense of a small reduction (13.4%) in the peak read range. A practical example is provided, where two small split-ring resonator-based tags of the same size (k
₀
a = 0.31) are designed. It demonstrates that such improvement on the tag bandwidth can be approximately obtained by simply changing the chip position, without the need of an external matching network. The improved tag was fabricated and measured, as a proof of concept. The results obtained from the proposed analysis allow RFID designers to determine how well a tag performs, compared to theoretical bandwidth limits.