High quality factor microwave sensor for ethanol solution concentration detection

When using microwave sensors to detect the concentration of a solution, two important objectives to consider are resolution and sensitivity. This paper presents a bent-wire based microwave sensor that combined an active feedback loop to increase the quality factor from 47 to 295. The external capacitance changes the resonant frequency to study the influence on the detection sensitivity. The resonant frequency of the active microwave sensor gradually increased from 2.62GHz to 3.10GHz with capacitance of $24\mathrm{pF},2.4\mathrm{pF}$, and 1pF respectively. The resonant frequency without external capacitor is 2.26 GHz. Active microwave sensors at four resonant frequencies were used for sensing ethanol solutions from 0 to $ 100\%$ concentration. When characterized by frequency shift, the sensitivity increased from $ 0.17\mathrm{MHz}/\%$ to $ 0.34\mathrm{MHz}/\%$ with increasing frequency, and when characterized by magnitude change, the sensitivity peaked at $ 0.14\mathrm{~dB}/\%$ at the detection frequency of 2.62GHz, in agreement with the theoretical analysis. This result shows that active microwave sensors, combined with the frequency correction effect of chip capacitors, can effectively improve sensing sensitivity.