A New Microwave Humidity Sensor with Near-Field Self-Injection-Locked Technology

In this paper, a new microwave humidity sensor is proposed with the near-field self-injection-locked (NFSIL) technology. This sensor is composed of a humidity sensing oscillator and a frequency demodulator. The sensing oscillator is designed at 7.67 GHz using a substrate integrated waveguide (SIW) cavity with embedded air holes. It is used to generate an electric-field distribution in the air holes and in the near-field region. According to the self-injection-locked (SIL) theory, as the moist air is distributed in the sensing region, the oscillation frequency of the SIW oscillator varies corresponding to different relative humidity (RH) levels, namely a frequency-modulated signal produced by this sensing oscillator. A delay-line frequency discriminator is then treated as a demodulator to obtain the voltage outputs, which are related to their corresponding RH. The proposed humidity sensor has been experimentally demonstrated to have an effective capability for humidity detection over a RH range of 50%-90%. To the best of our knowledge, the proposed sensor is the first microwave sensor to detect RH without using a vector network analyzer.