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High-sensitivity RFID sensing antennas: From sensing mechanism selections to antennas structure designs

Sensitivity is an important parameter to evaluate the performance of RFID sensing antennas. In order to obtain high sensitivity, two different sensing mechanisms are selected for temperature monitoring. One utilizes the electrical properties of sensing materials, and the other utilizes the thermal properties of sensing materials. Water and high density polyethylene-Ba0.3Sr0.7TiO3 (HDPE-BST) are the ones with temperature dependent electrical properties. On the other hand, mercury and Polytetrafluoroethylene (PTFE) are selected for their distinct thermal expansion properties. To magnify the effects of the sensing materials on the antennas characteristics, two different antenna configurations are used. Patch antennas are designed for the materials with electrical properties, and cavity backed slot antennas are designed for easy integration with the sensing materials with thermal properties. All of the four designs are frequency reconfigurable sensing antennas. In this situation, frequency shift per temperatures (Δf/ΔT), realized gain, and realized gain bandwidth are used to evaluate the antenna sensitivity. Finally, measurement results show that 40 MHz/10°C frequency shift with 14 m read range is realized by a capacitively-loaded cavity backed slot sensing antenna using thermal properties of the PTFE material.