Microwave Resonator-Based Interferometric Sensing Setup for Detection of Aqueous Solutions

Dielectric characterization and material differentiation are important in many research fields, such as life sciences. Although material sensing has been investigated for years, detecting small variations in material electrical properties remains a challenge. To overcome this, a microwave resonator-based interferometric sensing setup is proposed and experimentally validated to detect low-concentration aqueous solutions. A broadband coupler operating from 6.5 to 13 GHz is used to divide two equal signals with a 180° phase difference in the reference and measurement lines, creating complete out-of-phase interference. A phase shifter and an attenuator in the reference line are used to cancel magnitude and phase mismatches between the two lines. For sensing, a complementary split-ring resonator (CSRR) operating around 11 GHz is used, incorporating a microfluidic channel along its gaps. Frequency responses from ethanol–water (EtOH) mixtures, ranging from pure water to 5% ethanol in 1% steps, are measured. This setup provides two metrics of amplitude variations and frequency shifts at two frequency points, enabling accurate and straightforward permittivity extraction. Experimental performance agrees with the mathematical model. The maximum sensitivity is 3.82% for frequency shifts and 45.82 dB/<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Delta \epsilon ''$</tex-math> </inline-formula> for amplitude, which shows improvement in the detection of low concentrations of EtOH mixtures (1%–5% concentrations). Moreover, the sensitivity for the proposed sensing setup has improved 74.9 times compared to the standalone CSRR sensor. Finally, the complex permittivity values of the samples are extracted with good agreement using a mathematical model.