A Microwave Sensor Based on Grounded Coplanar Waveguide for Solid Material Measurement

In this article, a microwave resonant sensor in the form of a grounded coplanar waveguide (GCPW) transmission line is proposed for sensitive and accurate measurement of the relative permittivity of solid materials. The interdigital capacitor (IDC) is connected in parallel, and its unique interdigital electrode arrangement and double wide gap structure can improve the electric field aggregation ability. The split-ring resonator (SRR) is directly connected to the microstrip line and acts on the IDC in the form of coupling, improving the resonance area. The GCPW transmission line is designed in a bend shape to minimize the design size. Detailed explanations of the design process are provided, including the working principles of the GCPW and parallel IDC (P-IDC). The influence on the sensor transmission characteristics when the signal line is in a bent state is analyzed. The study examines the influence of IDC finger length and width on the transmission zero (TZ) and the effect of the air gap on the sensitivity during measurement are studied. The article further proposes an equivalent circuit model and verified by simulation and experimental results. It simultaneously studied the effect of material under test (MUT) thickness and loss tangent on the sensor. The proposed sensor has an average sensitivity of 11.13% in the relative permittivity range of 1–10.2. The average frequency detection resolution (FDR) and the average error are 317.74 MHz<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$/\Delta \epsilon $ </tex-math></inline-formula> and 0.58%, respectively. The work provides a compact and low-cost, high-sensitivity solution for detecting the permittivity in the microwave planar sensor domain.