Microwave Sensing Aqueous Solutions Based on Ring-Assisted Mach–Zehnder Interferometer

In this work, we present a novel tunable and ring-assisted Mach–Zehnder interferometer (RAMZI) to realize the highly sensitive and contactless microwave measurement of liquid samples. The designed RAMZI consists of a split ring (SR) and a feedback line for the destructive interference at the resonant frequency of odd or even mode of the SR after appropriately tuning the signal balance. An equivalent circuit model is built to study and explain its working principle. The achieved quality factor is up to 3.1
$\times \,\,10^{5}$
with water loaded. The minimum ethanol concentration level is measured to be 0.005 mole fraction. With the empirical relation between the sensor resonance and the sample permittivity, the complex permittivity of a liquid can be extracted.