Novel High-Sensitivity Differential Microwave Sensor for Permittivity Determination of Ethanol–Water Solutions

A novel differential-mode planar microwave sensor is proposed to measure the complex permittivity of liquid samples. This sensor combines a curved ring resonator (CRR) and an annular microstrip transmission line (AMTL) structure to enhance field strength and sensitivity. The sensor features a four-port design with two independent sensing structures: one dedicated to the reference sample and the other for the sample, allowing environmental impacts to be eliminated through comparative analysis. When the test object is introduced into the sensing area, the symmetric structure is disrupted, resulting in a resonant frequency difference between <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$S_{21}$ </tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$S_{43}$ </tex-math></inline-formula>. The sensor dimensions are <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$42\times 32\times 0.815$ </tex-math></inline-formula> mm, and its unloaded resonant frequency is 2.58 GHz. The sample container is made of acrylonitrile-butadiene-styrene (ABS) plastic and strategically positioned in an area of high electric field strength. The complex permittivity of the test samples is extracted using a backpropagation neural network (BP-NN). To verify the robustness of the method, a sensor prototype was fabricated and tested. The results show that the average sensitivity of the method is 1.184%, and the measured complex dielectric constant of the liquid sample is consistent with literature values, with a maximum error of less than 3.5%. Experimental results demonstrate that the sensor can distinguish different concentrations of ethanol solution. In addition, the proposed sensor offers advantages such as portability, low cost, and high sensitivity.