Computer-Based Permittivity Measurements and Analyses of Microwave Power Absorption Instabilities

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Abstract
Sample volumes of a pure Debye dielectric (ethanol) inside a resonant cavity were heated by microwave power (MWP) while measuring at the same time the average dielectric constant, ε’a, and dielectric losses, ε”a, by an active frequency technique (AFT). Similar liquid samples of ethanol were also measured for ε’ and ε” by time-domain reflectometry techniques (TDR); in this second case the sample temperature was increased by means of a hot water bath, in the case of the AFT experiments, cavity resonance instabilities were observed at a given MWP level. These instabilities are attributed to the generation of thermal avalanches in the test sample. Differences in the measured permittivities by the two methods (TDR and AFT) are attributed to the presence of thermal avalanches and the use of a larger pipette (25 µl; Φ = 0.6 mm) than normally required (5 µl; Φ = 0.3 mm) for precision cavity perturbation measurements.