High-Q sensor for permittivity detection based on spiral resonator

A high-quality factor microwave planar sensor with microfluidic channels for permittivity detection is proposed in this paper. The sensor is based on microstrip technology, and uses spiral subwavelength resonators to couple with the sample flowing in the microfluidic channel upon the sensor. Then the frequency shift and notch depth of the transmission response are related to the difference in complex permittivities between samples through a mathematic model, which will be used to extract the complex permittivities of samples afterwards. The sensor is designed with dual transmission lines to achieve better robustness and real-time measurement. After the simulation of samples in microfluidic channels, it turns out that the sensor is capable to distinguish samples whose permittivity has a difference of more than 2.56%. The quality factor of the sensor is 218 with empty load. It is validated to effectively distinguish mixture of water and air as well as water and ethanol with different fraction, which can be as low as 5%. The extracted permittivity is compared with theoretical value, which shows a good consistency.