Embeddable Soil Moisture Content Sensor Based on Open-End Microwave Coaxial Cable Resonator

In this article, we propose and demonstrate a novel corrosion-resistant, embeddable open-end coaxial cable soil moisture sensor. This microwave resonator is constructed using two reflectors along the coaxial line. The first reflector is a metal post at the signal input end, short-circuiting the inner conductor to the outer conductor. The second reflector comprises a welded metal plate parallel to the open end of the coaxial line, maintaining a fixed gap. A moisture-sensitive polyvinyl alcohol (PVA) film is inserted into this gap. The resonance frequency of the open-end coaxial cable resonator is highly dependent on the fringe capacitance, which varies with soil moisture levels. As such, tracking resonance frequency changes allows for correlation with soil moisture fluctuations. We provide a detailed discussion of the embeddable open-end microwave coaxial cable resonator (EOE-MCCR) mathematical model and a proof of concept for soil moisture measurement. The demonstration experiments investigate soil moisture content ranging from 4% to 24%. The prototype device exhibits a soil moisture measurement sensitivity of 0.76 MHz/% for soil moisture between 4% and 10% and 1.44 MHz/% for soil moisture between 10% and 24%. The soil moisture sensor presented here is robust, easy to manufacture, chemically resistant, low-cost, and suitable for long-term applications and potential industrial uses. This innovative sensor is ideal for sensing applications in harsh environments, advancing the field of chemical trace sensing.