提出了一种基于交指电容裂环谐振器的差分微波传感器。通过把交指电容结构嵌入到裂环谐振器的开口处以增强电场束缚效应，并且为了抑制外界环境因素的干扰，制作成差分结构。差分结构采用两个裂环谐振器分别分布于单根微带线两边的结构形式，谐振器与微带线呈磁耦合效应。该传感器结构可以克服传统微波传感器的低灵敏度和易受外界干扰的问题。在实验中，将一个谐振器作为参考单元，另一个谐振器作为测量单元，待检测液体样本放置于测量交指电容处，随着不同浓度的乙醇-水混合溶液的注入，测量单元的谐振频率相较于参考单元会发生明显的频率偏移和陷波幅值的偏差，由此可以拟合出复数介电常数与它们之间的关系以预测不同浓度的乙醇-水混合溶液的复数介电常数值。实验结果表明：所提出传感器的单位体积平均灵敏度可达3.630 8%/μL，比同类型的传感器提高了几倍至几十倍。

In the traditional Split Ring Resonator (SRR) -based microwave microstrip sensor, the microstrip line is coupled with SRR, and the mantic coupling is appeared between microstrip line and SRR. The electrical filed is distributed at the gap of SRR. A high resonant frequency is occurred due to the relatively low equivalent capacitance. And, the traditional SRR-based sensor shows a low sensitivity. In order to improve the detection sensitivity, an Interdigital Capacitance Structure (ICS) is inserted into the opening of the split ring resonator to enhance the density of electric field. From the simulated distribution of electrical field, it can be seen that more electrical field is concentrated at the slot of interdigital capacitance with comparison to traditional SRR-based sensor. The meander slot can increase the interacted path of electrical field, create a higher equivalent capacitance, so more electrical field is focused on the slot of interdigital capacitance. As known, the external environmental factors, such as temperature, humidity, gas pressure, etc, can cause an influence on the measured results and result in a large measurement error. In order to suppress the interferences of external environment, a differential structure is added. With comparison to traditional microstrip sensors, the proposed differential microwave sensor can overcome low sensitivity and effects of environment factors. The designed differential structure adopts the form of two split ring resonators distributed on both sides of a single microstrip line, and the microstrip line is coupled with the two split-ring resonators, and the magnetic couplings distributed on both sides are equal. An equivalent circuit model is adopted to analyze the operating principle of proposed microwave sensor, the element values of equivalent circuit model are calculated, then Keysight ADS is adopted to fine-tune all the element values to match the simulation results. It can be found that there is a good agreement between them. The consistent also reveals the correctness of proposed equivalent circuit model. Besides, mathematical analysis is utilized to explain the phenomenon that the ICS-SRR has a higher confined electrical field than traditional SRR. Then, simulation and experimental results both verify the correctness of theoretical analysis. In the experiment, one resonator serves as the reference unit and the other resonator serves as the measurement unit. The liquid under test is placed at the test resonator. In measurement, the water-ethanol mixtures with water concentrations changing from 0% to 100% with a step of 20% are adopted to establish the mathematical model. As the mixture solutions of water-ethanol with different volume fractions of water are injected, the resonance frequency of the measurement unit will show significant frequency shift and notch amplitude deviation compared to the reference unit. The deviations of resonant frequency and notched magnitude can be used to fit the complex permittivity. Then, in order to demonstrate the correctness of established mathematical model, the water-ethanol mixtures with water concentrations altering from 10% to 90% with a step of 20% are utilized. By substituting the measured resonant frequencies and notched magnitudes for water-ethanol mixtures with water concentrations changing from 10% to 90% into the established mathematical model, the real and imaginary parts of complex permittivity for water-ethanol mixtures with water concentration changing from 10% to 90% can be obtained. The experimental results show that the average sensitivity of per unit volume of the proposed sensor can reach 3.630 8%/μL, which is several times higher than the other designs. In addition, the proposed microwave sensor can also be adopted to measure other binary mixed solutions by establishing the corresponding mathematical models.