A Novel Microwave-based Inspection System for Continuously Streaming Materials Using a Cavity Resonator

In this paper, we present a novel ultra-fast material inspection system for continuously streaming dielectric materials, such as paper, plastic films, non-aqueous liquids etc. The material stream is being passed through a cavity resonator whose electrical parameters are altered by the presence of the material, due to its interaction with the electric field inside the resonator. The latter is employed as the frequency-determining component in an amplitude-controlled oscillator circuit. Small changes in the electrical properties of the resonator due to fluctuations in the material stream cause the oscillator circuit to be detuned and damped. Monitoring the frequency of oscillation as well as the gain yields realtime information about the resonator's electrical parameters and, thus, allows assessment of material properties, such as water content, contamination, density etc.