T-matching networks for the efficient matching of practical RFID tags

In this paper, we study T-matching networks that are commonly used for the efficient matching of UHF tags. In order to analyze relationship between T-matching geometry and antenna impedance, we interpret the transformer circuit as an equivalent circuit depending on the design parameters of the circuit. From the parametric study of the T-matching, we derive an optimized topology which is able to overcome electrically change of the geometry resulted from material loading. Next, based on the previous study, we design two types of tag antennas for practical item-level RFID applications that require the discrimination different objects. We calculate theoretically the minimum operating power of tags based on the computed antenna characteristics, antenna impedance, directivity, efficiency, regarding various dielectric constants. To verify the performance of the developed tags in practical conditions, we measure the minimum required power to activate tags on multiple objects in conjunction with commercial measurement system. As a result, the designed tags present almost consistent readability on materials from low to high dielectric constant. Thus, the results clearly verify that the appropriate T-matching geometry provides consistent readability in practical UHF RFID tags.