Coplanar capacitor probes design for a moisture soil sensor operating at high frequencies

In this work, we study the suitability of a coplanar capacitor (CPC) for a low-cost soil volumetric water content (VWC) sensor in the context of vegetable growing automation. As an operating frequency above 50MHz is desired for better measurement accuracy, its compatibility with a high-frequency oscillating circuit is verified through the extraction of its electrical parameters after electromagnetic simulations, such as capacitance, self-resonance frequency (SRF), and parasitic inductance, while varying the conductive layer dimensions, the length of feeding traces, the number of conductive layers, and the number of vias. Moreover, a 137MHz SRF is measured in an implemented single-layer CPC while buried in wet soil. These results should serve as guidance during the design of the complete acquisition circuit.