| Summary: | In this paper, a relative humidity monitoring sensor based on an active resonator with PEDOT:PSS is studied in the microwave regime. The proposed active resonator is patterned on a printed circuit board and excited by electromagnetic field coupling. The active resonator consists of two parts: a passive core resonator and an active circuit. The passive core resonator senses the humidity via the PEDOT:PSS conducting polymer film, which is located in the area of the passive core resonator with the strongest electric field. The active circuit compensates for the loss of the passive core resonator to enhance the quality factor and thus improve the sensitivity of the relative humidity sensor. By combining the passive core resonator with the active circuit, the active resonator exhibits a high quality factor (2887), which is 90 times that of the passive core resonator alone. Moreover, the sensitivity of the proposed humidity sensor is improved when using the active resonator. To verify the performance of the proposed humidity sensor, a commercial sensor is placed alongside the proposed sensor in a well-controlled environment chamber. According to the experimental results, as the relative humidity increases, the transmission coefficient (S<sub>21</sub>) increases, while the resonance frequency decreases. As a result, the sensor exhibits an increase in S<sub>21</sub> of 3.58 dB and a decrease in the resonance frequency of 8.85 MHz when the relative humidity changes from 30% to 85%.
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