This paper presents the design and implementation of an oscillator-based humidity sensor. The designed oscillator consists of a SIW cavity resonator connected to a negative resistance oscillator. The sensing principle is based on an oscillation frequency shift caused by permittivity change of the humid air. Oscillation frequency of the oscillator is 3.917GHz and phase noise at 100 KHz offset from carrier is less than -94 dBc/Hz. Sensor\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\'s output shows linear characteristics in the range of 10%-85% relative humidity. The oscillator-based sensor can be used in radio frequency applications like IOT where the sensor needs to be wireless, low cost and with a simple manufacturing process.

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