A Microwave Sensor Based on Grounded Coplanar Waveguide for Solid Material Measurement
Permittivity, which describes how a material responds to an electric field, is a crucial parameter in evaluating the electromagnetic (EM) properties of materials. Currently, due to their simplicity and low design complexity, microwave resonant sensors for permittivity detection primarily employ microstrip lines and coplanar waveguide (CPW) structures.
The grounded coplanar waveguide (GCPW) introduces a complete bottom ground plane to the traditional CPW design. In this structure, the top coplanar ground lines are connected to the bottom ground plane through vias, forming a dual-ground structure that improves signal isolation. Compared to microstrip lines and traditional CPW designs, the GCPW demonstrates superior high-frequency performance, enhanced anti-interference capability, and increased sensitivity, highlighting its significant application potential.
This study proposes a microwave resonant sensor based on GCPW to enhance sensitivity and accuracy in solid material permittivity measurement. The sensor employs a P-IDC as the primary resonant region and GCPW as the main body.
Interdigital capacitors (IDCs), which utilize interleaved "finger" electrodes, are widely used in circuits for applications such as dielectric sensing, radio frequency coupling, and direct current (DC) blocking. Compared to conventional IDCs, the P-IDC enhances total capacitance and achieves a more uniform electric field distribution by connecting multiple interdigital structures in parallel. Due to its flexible tunability, a split-ring resonator (SRR) structure serves as both the GCPW signal line connector and a coupling element for the P-IDC.
The transmission characteristics of the Grounded Coplanar Waveguide were analyzed. The study specifically examined how bending the transmission line affects the sensor's performance. Furthermore, the impact of the IDC finger dimensions, both length and width, on the transmission zero (TZ) was systematically examined. Finally, the study assessed the impact of air gaps on sensitivity when the material under test (MUT) was placed above the resonant region.
Researchers developed a comprehensive equivalent circuit model to elucidate the sensor’s working principle. To establish the mathematical relationship between permittivity and TZ, researchers employed a fourth-order polynomial curve for fitting, and they rigorously verified its validity through simulations and experiments.
The study fabricated a physical prototype of the sensor to validate the feasibility of the proposed design. During measurements, the thickness of the selected MUT was set to 0.762 mm, as this value facilitates sample preparation and ensures stable performance.
Seven materials with distinct relative permittivity were tested, all of which were cut to identical dimensions to eliminate the influence of MUT size variations on experimental results. The results demonstrate that the proposed sensor achieves an average sensitivity of 11.13% over a relative permittivity range of 1–10.2, with an average error of 0.58%, indicating a statistically significant fitting performance.
This work presents a compact, low-cost, and high-sensitivity detection solution using microwave planar sensors for permittivity characterization. The proposed design demonstrated significant application potential in high-frequency communication systems, radar and satellite communications, high-density integrated circuits, and other fields.