A Real-Time and Highly Sensitive Differential Microwave Sensor for Liquid Characterization

Microwave sensors used for measuring the electrical properties of chemical liquids are vulnerable to ambient conditions. Differential mechanisms with two sensing elements are suggested to minimize these effects. Existing differential sensors suffer from lower sensitivity and/or large sizes caused by two sensing elements. This paper presents a novel differential microwave sensing concept with a single sensing element, which minimizes undesired environmental effects. The proposed single-element microwave sensor configuration provides higher sensitivity and smaller size. This new sensor topology benefits from the integration of a narrow bandpass filter with modified Split Ring Resonators (SRRs). The frequency response of the sensor includes two different transmission zeros (TZ) in the transmission coefficient. The TZ generated from the magnetic-dipole behavior of SRRs in the rejection band, varies with a dielectric constant of material under test (MUT), whereas the TZ associated with direct input/output coupling of the bandpass filter remains constant for different MUTs. Hence, the differential operation can be accomplished using the two different TZs with a single sensing section. The synthesis approach of the proposed architecture is described in addition to parametric analysis for investigating the effect of filter and SRRs dimensions on both TZs. The measurements on the prototype sensor show 3.44% normalized sensitivity, which is 36% higher than the most sensitive sensor for liquid characterization. The proposed sensor could be used in real-time, highly sensitive chemical liquid analysis applications, with robustness against environmental factors.