Submillimeter Coaxial Probes for Dielectric Spectroscopy of Liquids and Biological Materials

In this study, 0.2-mm radius open-ended coaxial probes were constructed and simulated, to measure reference materials and biological media from 1 to 40 GHz. A novel full-wave 3-D model using CST Microwave's frequency-domain solver is demonstrated and the CST example model is provided online to accompany this paper. With this model, design parameters such as the minimum allowable flange radius, short circuit conductivity, and mechanical tolerances were investigated. In terms of measurements, a short-circuit across a tiny aperture was found to be unrepeatable; hence, a calibration technique using only reference liquids is demonstrated. At increasingly smaller radial dimensions, the aperture fields are absorbed by a proportionally smaller volume of material, resulting in microwave hyperthermia of the material. A 3-D and 1-D steady-state heat simulation is utilized to approximate the induced temperature change. Finally, a Monte Carlo technique is used to quantify the errors associated with the aperture models, calibration, and probe imperfections.