Noninvasive electrical characterization of materials at microwave frequencies using an open-ended coaxial line: test of an improved calibration technique

Dielectric measurements using a probe consisting of a coaxial transmission line with an open-circuit end placed against the sample are discussed. For the 2.99- or 3.6-mm (OD) probes considered, a simple lumped parameter model shows errors above 1 GHz that increase greatly with frequency. An approximate model based on measured probe impedances from 1 to 18 GHz with samples consisting of water, methanol, and dioxane-water mixtures is evaluated. This model is more accurate than the lumped-parameter model and is better suited for calibration of the automatic network analyzer (ANA). The errors introduced in dielectric measurements by the use of approximate models for the probe are discussed. The technique succeeds because of partial cancellation of errors in modeling the probe in ANA-based measurements