A Stand-Alone Moisture Content Sensor Based on a Loaded Self-Oscillating Antenna

In this work, a portable moisture-content sensor is proposed, based on a self-oscillating microwave antenna loaded by the material under test, for the present case a tree trunk in different hydration states. The operating principle is based on the relationship between the oscillator steady-state regime and its dispersive load, represented by a loaded patch antenna. First, a HEMT-based microwave oscillator with a purely resistive load is designed in the 2.4 GHz range; at the same time, a resonant patch-type antenna is optimized at the oscillator frequency. Then the patch near-field behavior is characterized, in terms of input impedance, both in free space and loaded by the wood in various controlled hydration conditions, over a band spanning the fundamental oscillator frequency and its higher harmonics. The oscillator is then analyzed using the dispersive antenna equivalent impedances, measured at various loading conditions. The self-oscillating antenna behavior is monitored for sensor read-out by a novel approach based on the oscillator's steady-state dc conditions only. By spanning the dc gate bias tuning range of the oscillator and by monitoring the corresponding driven dc drain current, the moisture content is retrieved. Simulations and measurements show that this relationship is deterministically related to the oscillator steady-state regime variations with the dispersive loading conditions. The proposed solution allows the implementation of a portable sensor with no need for bulky instrumentations for RF spectra monitoring, which are usually adopted for resonator-based microwave sensors.