Broadband Measurement of Complex Permittivity for Liquids Using the Open-ended Cut-ofi Circular Waveguide Re∞ection Method

Various studies on speciflc absorption rates (SARs) have been carried out with liquid phantoms representing human body tissues in the fleld of electromagnetic compatibility (EMC) research. As a faithful representation of human body tissue is needed to establish an accurate SAR for measurement, knowledge regarding the precise measurement of sample materials with high permittivity and high loss is very important. In this study, the complex permittivity of a liquid phantom material was measured using the open-ended cut-ofi circular waveguide re∞ection method. The efiectiveness of the proposed approach for measuring a liquid phantom with high permittivity and high loss was also conflrmed by comparing the measurement results with those obtained using the TM010 circular cavity resonator method. In response to recent health concerns regarding the application of electromagnetic waves to the human body, the efiects of such waves and methods for measuring complex permittivity in relation to liquids used for humanoid phantoms have been studied (1). Meanwhile, the development of techniques for creating new chemical materials based on exposure to electromagnetic waves has accelerated. In research on such microwave chemistry applications, liquids are generally used as solvents for the synthesis of new chemical materials. Accordingly, knowledge of exact material constants for objects to be heated in the frequency band of electro- magnetic wave exposure is very important. In general, re∞ection constant determination method using a coaxial probe is a popular technique for measuring the complex permittivity of liquids (2). Although this method readily supports broadband estimation of material constants for liquids, a large amount of water is needed to avoid the in∞uence of re∞ective waves from the vessel bottom. The cavity resonator method can also be used for high-precision measurement (3{5). A transmission constant determination technique involving the insertion of a liquid sample into a Te∞on tube with a waveguide can be used for measurement at a continuous frequency (6). A cross-shaped coaxial probe can be used for simple measurement of complex permittivity, but material constant data on a known material is required as a standard with this approach (8). A transmission constant determination method involving the insertion of sample materials into a coaxial line (7) and a re∞ection constant determination method involving the insertion of sample materials into a cylindrical cavity (9) can be adopted for broadband measurement. In this study, the complex permittivity of tap water, methanol, ethanol (99.5) and isopropanol with high permittivity and high loss was measured via the re∞ection method using a coaxial feed open-ended cut-ofi circular waveguide (10) to reduce the efiects of liquid spillage at the time of sam- ple material insertion. The efiectiveness of the proposed method was then evaluated by comparing its results with those of the TM010 cylindrical cavity resonator method (4). Next, the complex permittivity of the liquids was measured at frequencies ranging from 50MHz to 3.0GHz using the procedure outlined above. In this case, the measurement limit was set at a very low frequency.