Passive Split Ring Resonator Tag Configuration for RFID-Based Wireless Permittivity Sensing

Wireless electromagnetic detection of non-visible processes is a vital challenge when visual signals are difficult to decipher or blocked, such is the case in pipelines, chemical reactions, or the human body. To guide the design of future microwave resonator sensors, investigative experiments into resonator sensitivity, sensing distance, and RFID array alignment were performed. A horn antenna reader was coupled to a singular flexible ring resonator as well as a flexible ring resonator array with a measured S11 notch frequency of 2.6 - 2.9 GHz. The experimental results closely compared to simulation and the computer model illustrated the detection of an introduced material around the high E-field regions of the coupled ring tag. In the fabricated circuit, liquids in a microfluidic channel are detected by a notch frequency and amplitude variation of 300 MHz and -30 dB, respectively, which can be matched with simulations to determine the materials' permittivity. Other application-based requirements are also investigated such as the sensing range of the antenna-tag structure (measured up to 20 cm), the small size of the sensing tag (9 square cm) and the ability to detect through material barriers.