Coaxial microwave resonant sensor design for monitoring ionic concentration in aqueous solutions

Nitrate efflux from agricultural lands mixes with surface streams and adversely affects both human health as well as aquatic life. Currently, there is a lack of low-cost, effective, real-time systems for monitoring ion concentration. In this work, a microwave resonant sensor is designed using an open-ended coaxial transmission line, which can be evanescently perturbed by a liquid sample, and a suitable coupling structure which allows transmission measurements. The sensor is developed to have high sensitivity at agriculturally relevant concentrations, low manufacturing costs, and small dimensions to be potentially field deployable. Finite Element Analysis simulations are carried out using ANSYS HFSS, employing complex permittivity data of aqueous solution samples with varying concentrations of nitrate, sulfate, and chloride ions. Appropriate functions are determined that model the correlations between resonant frequency and ion concentration, and discussion on the feasibility of the sensor for field deployment is presented.