Sensitivity Optimization in Single-Frequency Planar Microwave Sensors for Solid and Liquid Characterization and Microfluidics

This article reviews some recent strategies for sensitivity optimization in planar microwave sensors operating at a single frequency, namely, phase-variation sensors and magnitude-variation sensors. In most cases, both sensor types (fed by a single-tone harmonic signal) consist of a transmission line-based structure, typically (although not necessarily) loaded or coupled with a resonant element, and can operate either in reflection or in transmission. Hence, sensitivity optimization requires that either the phase (in phase-variation sensors) or the magnitude (in magnitude-variation sensors) of the reflection or transmission coefficient exhibits a strong variation with changes in the input variable (or measurand), typically, the permittivity of the so-called material under test (MUT) or any other variable related to it (humidity, temperature, proximity, and so on). It is shown in this article that the key aspect for sensitivity enhancement in phase-variation sensors is to achieve a high slope in the phase response at the operating frequency. Similarly, a high slope in the magnitude response at the operating frequency contributes to boost the sensitivity in magnitude-variation sensors. Nevertheless, there are other strategies to optimize the sensitivity in magnitude-variation sensors that will also be discussed (e.g., disrupting the symmetry in balanced structures, such as couplers). Several prototype examples and potential applications are reported to illustrate the high potential of these sensors.