Microwave Techniques for Characterizing Low-Loss Solid Dielectrics: Measurement Physics, Resolution Limits, and Validation

Low-loss solid dielectrics are used in microwave substrates, radomes, resonators, filters, antennas, high-speed packages, and millimeter-wave front ends. This review examines experimentally demonstrated methods for characterizing solid dielectrics from approximately 1 to 170 GHz. The range is evidence-driven: the lower bound reflects the microwave examples covered here, while 170 GHz is the upper limit of the second edition of International Electrotechnical Commission (IEC) 63185. Each method is compared by its measured observable, field model, material under test (MUT) shape, quantitative loss-resolution evidence, and validation route. The review covers guided transmission/reflection, free-space and Gaussian-beam systems, cavity perturbation, dielectric and whispering-gallery resonators, split-post, split-cylinder, and balanced circular-disk resonators, open Fabry–Perot resonators, and open-ended aperture probes. Source-checked examples for ceramics, polymers, package dielectrics, and radome fabrics distinguish direct measurements from approximate conversions. Where a source reports a measured-versus-reference deviation or an uncertainty, that evidence is summarized separately; where it does not, the absence is stated. Broadband propagation methods and high-Q resonant methods answer different measurement questions, and credible low-loss reporting requires an uncertainty for the loss-sensitive observable, a documented loss budget, repeated sample reinsertion, and an explicit resolution limit.