A New Stand-Alone Microwave Instrument for Measuring the Complex Permittivity of Materials at Microwave Frequencies

This article reports the development of a stand-alone and portable instrument designed to measure the complex permittivity of dielectric materials at microwave frequencies. The equipment consists of an in-house single-port vectorial reflectometer and a resonant coaxial bireentrant microwave cavity where the material under test (MUT) is placed inside a Pyrex vial, making the device appropriate for measuring liquids, semisolids, powders, and granular materials. The relationship between the dielectric properties of involved materials and the cavity resonance has been determined by numerical methods based on mode-matching and circuit analyses. In order to increase the measurement range, so that low to high loss materials can be characterized in the same cavity, the effect of the coupling network is deembedded from the resonance measurements. The performance of the newly devised instrument is evaluated by error/uncertainty analysis and comparative studies with other well-established instruments and methods. Errors lower than 2% in the dielectric constant and 5% in the loss factor are found. This simple, portable, affordable, and robust device could help nonspecialized personnel to accurately measure the dielectric properties of materials used in a wide range of microwave applications.