Comparative Analysis of Relative Permittivity Variation in an Innovative Low-Cost Interdigitated Capacitive Sensor According to S-Parameters

In this paper, the development and analysis of a two-port interdigitated capacitive sensor were carried out, using an FR4 substrate and copper electrodes, targeting a low-cost and low-tech solution for sensing in liquids. The sensor response was analyzed through the measurement of its scattering parameters to liquid relative permittivity samples. Thus, capacitance and resistance extracted from the scattering parameters were analyzed, respectively at 10 MHz and 100 MHz, using two methods: input impedance and an electric circuit model which consider reflection and transmission impedances. The results showed that the capacitance exhibits a logarithmic relationship according to the relative permittivity, while the resistance increases with the presence of a liquid on the sensor surface compared to air but decreases hyperbolically as the liquid relative permittivity increases. Furthermore, it was found that the sensor impedance extracted from the electrical circuit model presented variation much higher than those obtained using only the input impedance ($\times 2.9$ and $\times 3.8$ respectively for capacitance and resistance). Therefore, we concluded that the use of transmission impedance measurements might improve accuracy and sensitivity compared to the more classic use of the input impedance for capacitive sensors.