Three techniques for characterizing materials using microwaves

We present an MTM microwave sensor that, by using three different techniques, can characterize solid, liquid or gaseous samples regardless of whether the permittivities of the samples are high or very low. Materials exhibit a wide range of dielectric responses depending on their composition and structure, with some having a high permittivity, while others exhibit low permittivity associated with weak polarization effects. Similarly, dielectric losses can vary significantly, with certain materials showing high loss tangents and others behaving as low-loss media. Furthermore, the dielectric properties of many materials are strongly influenced by external factors such as temperature, humidity, pressure, and aging, leading to significant temporal and environmental variability. Other materials maintain relatively stable dielectric behavior under similar conditions. Microwave characterization techniques provide a non-invasive and reliable approach for extracting dielectric parameters. In resonant-based methods, the interaction between the electromagnetic field and the material under test produces measurable changes in the system response, such as shifts in resonant frequency, variations in insertion and return losses, and changes in resonance bandwidth or quality factor. These variations can be exploited to infer material properties, including permittivity, loss characteristics, and frequency-dependent behavior, enabling accurate and repeatable measurements. This work presents measurements of: 1) humidity variations in sliced potatoes, inferred from shifts in a single resonant frequency of a resonant structure; 2) salt and chlorine in water, evaluated using insertion loss from a frequency-comb response; and 3) sulfur hexafluoride concentration within a cell, determined from the quality factor of a single resonance. In the first case, potatoes exhibit moderate permittivity values that vary significantly with humidity. In the second case, water presents a high permittivity, and small contaminant concentrations do not significantly affect this parameter. Finally, SF₆ exhibits very low permittivity, resulting in imperceptible variations in the real part of the effective permittivity of the samples.