Multifrequency Microwave Liquid Identification Sensor Based on Interdigital Split Ring Resonators

In this article, a novel multifrequency microwave sensor comprising four interdigital split ring resonators (SRRs) coupled to a microstrip feed line is proposed for measuring types and volume fractions of liquids. The interdigital SRR is based on a traditional SRR with its split replaced by an interdigital capacitor (IDC) structure. The IDC in the SRR, serving as the sensing region, has strong electric fields, highly responsive to variations in the dielectric properties of the sample. To mitigate the influence of the sample on the microstrip feed line, a ground plane is inserted between the feed line and the sensing region. Different samples exhibit distinct resonant frequencies and transmission magnitudes of the sensor, which are used to invert their complex permittivity through a mathematical relationship model. Subsequently, liquid types and concentrations can be identified based on a pre-established knowledge database. Significantly, the four-resonance frequency measurement enables to determine the dielectric dispersion of the liquids over the 2–6-GHz range, enhancing identification accuracy through the nearest neighbor search principle. Experiments conducted with various liquid types and ethanol solutions of varying concentrations demonstrate the superior identification capability of the sensor. The sensor’s compact size, simple structure, and high identification accuracy highlight its potential for on-site, real-time, noninvasive detection applications.