Characterization of Low-Loss Liquids With a Double-Concave Fabry–Perot Open Resonator in the 20–50-GHz Range

Advancements in the automation and numerical modeling of a Fabry–Perot open resonator (FPOR) have made it a very convenient technological platform for various applications related to the microwave characterization of materials, including dielectric sheets and conductors. In this article, it is proposed how to use this kind of resonator in the characterization of low-loss liquids, which has recently become of high interest, especially coolants due to their increasing role in the so-called full immersion liquid cooling of big data centers. First, a new rigorous electromagnetic (EM) model of the FPOR loaded with a multilayer structure is proposed and validated. Next, a dedicated liquid container, which allows well controlling the thickness of the liquid under test (LUT), is presented. Eventually, several experiments are performed at room temperature with a few types of liquids, such as fluoroinert, canola oil, and engine oil, and compared against alternative resonant measurement methods to validate the proposed measurement concept and show its unique properties.