A paper discusses the need to perform accurate dielectric property measurements on larger sized samples, particularly liquids at microwave frequencies. These types of measurements cannot be obtained using conventional cavity perturbation methods, particularly for liquids or powdered or granulated solids that require a surrounding container. To solve this problem, a model has been developed for the resonant frequency and quality factor of a cylindrical
microwave cavity containing concentric cylindrical samples. This model can then be inverted to obtain the real and imaginary dielectric constants of the material of interest.
This approach is based on using exact solutions to Maxwell’s equations for the resonant properties of a cylindrical microwave cavity and also using the effective electrical conductivity of the cavity walls that is estimated from the measured empty cavity quality factor. This new approach calculates the complex resonant frequency and associated electromagnetic fields for a cylindrical microwave cavity with lossy walls that is loaded with concentric, axially aligned, lossy dielectric cylindrical samples. In this approach, the calculated complex resonant frequency, consisting of real and imaginary parts, is related to the experimentally measured quantities.
Because this approach uses Maxwell’s equations to determine the perturbed electromagnetic fields in the cavity with the material(s) inserted, one can calculate the expected wall losses using the fields for the loaded cavity rather than just depending on the value of the fields obtained from the empty cavity quality factor. These additional calculations provide a more accurate determination of the complex dielectric constant of the material being studied. The improved approach will be particularly important when working with larger samples or samples with larger dielectric constants that will further perturb the cavity electromagnetic fields. Also, this approach enables the ability to have a larger sample of interest, such as a liquid or powdered or granulated solid, inside a cylindrical container.
This work was done by Martin B. Barmatz of Caltech and Henry W. Jackson for NASA’s Jet Propulsion Laboratory. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp under the Information Sciences category.
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Refer to NPO-47163, volume and number of this NASA Tech Briefs issue, and the page number.
This Brief includes a Technical Support Package (TSP).
Technique for Performing Dielectric Property Measurements at Microwave Frequencies
(reference NPO-47163) is currently available for download from the TSP library.
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Overview
The document outlines a new technique developed at NASA's Jet Propulsion Laboratory (JPL) for measuring the dielectric properties of materials at microwave frequencies, particularly relevant for studying icy objects in space. The primary objective of this research is to validate a method that can accurately assess the dielectric properties of liquids and solids of various sizes within a cylindrical container inside a microwave cavity. This technique is particularly applicable to cryogenic hydrocarbons, such as liquid methane and ethane, which are significant for interpreting radar measurements from missions like Cassini on Titan.
The document highlights the challenges faced in interpreting radar signals from icy satellites of Jupiter and Saturn due to the lack of accurate dielectric property measurements at low temperatures. The study emphasizes the need for a comprehensive database to interpret radiometric measurements obtained from icy surfaces probed by microwave radar in the 1 to 15 GHz range. This database would enable scientists to construct models that align with radar echo observations, providing reliable insights into the surface and near-surface compositions of these celestial bodies.
Traditional microwave dielectric property measurements often rely on cavity perturbation methods, which require small solid samples and have several assumptions that can limit accuracy. The new technique overcomes these limitations by utilizing a theoretical model based on exact solutions to Maxwell’s equations for the resonant properties of a cylindrical microwave cavity. This allows for the use of larger cylindrical samples, enhancing the accuracy of the measurements.
The document also acknowledges the contributions of various individuals and groups at JPL who supported the research, particularly in the calibration and measurement processes. It notes the importance of discussions with experts in the field and the assistance provided by students and staff in preparing the necessary samples and components for the experiments.
In summary, this innovative technique for dielectric property measurement at microwave frequencies represents a significant advancement in the study of icy materials, with potential applications in future space missions and a broader understanding of the composition of celestial bodies. The research aims to fill critical gaps in data needed for interpreting radar signals and enhancing our knowledge of the solar system's icy environments.
