Ultrahigh-Sensitivity Resonant Liquid Sensor Based on 3-D Comb-Shaped Capacitive Structures

In this article, an ultrahigh -sensitivity microwave liquid sensor based on a 3-D comb-shaped capacitive (CSC) structure is developed for detecting the relative permittivity of liquid samples. The utilization of the CSC structure can significantly increase the capacitance associated with the liquid under test (LUT) within the total capacitance, thereby offering a large variation in the capacitance when the liquid sample is loaded. This structure confines the electric field within the grooves of the comb-like configuration and forms a parallelLC-resonator with a spiral inductor. Its exceptional capacitance variation makes it a preferable alternative to planar resonator structures. As the proposed sensor can be manufactured using a standard printed circuit board (PCB) process and does not need liquid containers, such as polydimethylsiloxane substrate, its cost is relatively low. When the liquid sample is loaded, the sensor’s reflection coefficient changes, and the relative permittivity of liquid can be deduced from the variations in resonance frequency and notch depth. A prototype of the proposed sensor is fabricated and tested, and the measured results demonstrate that the sensor has good sensitivity for characterizing liquids with low relative permittivity.