A way to determine the optimum detection frequency for microwave sensing

• Published in: Measurement : journal of the International Measurement Confederation Vol. 202; p. 111736
• Main Authors: Wu, Jia-Kang, Gao, Ke, Yue, Wei, Qiang, Tian, Kim, Nam-Young, Song, Wenhao, Gu, Xiao-Feng, von Gratowski, Svetlana, Liang, Jun-Ge
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.10.2022
• Subjects: Frequency optimum; Humidity sensor; Microwave detection; Sensitivity; Humidity sensor; Sensitivity; Microwave detection; Frequency optimum
• ISSN: 0263-2241; 1873-412X
• DOI: 10.1016/j.measurement.2022.111736

•A method is proposed to determine the optimum sensing frequency with the layout unchanged.•The response and recovery speed of the humidity sensor changes as the frequency.•The change of sensitivity (return-loss variation) and sensitivity (frequency shift) is opposite with frequency.•The frequency changes the energy loss, which affects the quality factor. Besides the sensor design, the detection frequency is another vital factor for microwave detection. We present a frequency-adjustable humidity sensor to determine the optimum sensing frequency and study its effect on humidity sensing performances. The designed humidity sensor comprises a microstrip resonator, a varactor diode, and polyimide. Changing the bias voltage from 0 V to 30 V causes the resonant frequency to shift from 2.97 GHz to 3.40 GHz. The overall size of the sensor is 13.5 mm × 8.4 mm, the key size is 0.4 mm, and the maximum quality factor is about 20. The detecting frequency shows a high impact on detection sensitivity and speed: The sensitivity increases from 0.15 MHz/% RH to 0.3 MHz/% RH, 0.0014 dB/% RH to 0.0654 dB/% RH, while the speed is promoted from 15 s to 10 s. The underlying mechanism is discussed by modelling and analyzing the penetration and heating effects.