Complex dielectric parameter measurement by coaxial resonator and ISM band radio module

In many industrial processes, there is required measuring system to determine the moisture content or homogeneity of materials. Such areas are the paper, foil, textile processing and also the building industry. The manufacturers of such systems demand cheap and possibly high precision sensors, which can measure the moisture content in a nondestructive way. The acquired measurement data can be used not only for process control but also quality assurance. To meet these requirements, an open ended coaxial resonator and radio module based measuring system is introduced. As opposed to VNA based sensors, our system measures the scalar scattering parameter. The resonance frequency and the quality factor of the resonator cavity are considered to evaluate the complex permittivity of the measured material which depends on the moisture content of the sample. The coaxial resonator has been analyzed, and designed using HFSS 13. To determine the complex permittivity values, the 3D finite element model was adjusted by using known parameter materials (such as Teflon, FR4), and a database was created by sweeping with the real and the imaginary part of dielectric constant. To decrease the measuring time, adaptive frequency sampling was applied. After measuring different materials the results show that the equipment is able to measure the complex permittivity with high resolution. The simple and cheap measuring system makes this sensor system particularly interesting for moisture monitoring applications.