Study Of High Temperature And Broadband Measurements Of Dielectric Properties Of Microwave Transparent Materials

Microwave transparent materials are widely applied in radome, antenna windows, and other components. While aerospace crafts are flying at a very high speed in atmosphere, high temperature and high speed air flow will occur. In order to work properly in these situations, the microwave properties of radome and antenna windows must be guaranteed first. The microwave properties of these components are directly related to the dielectric properties of microwave transparent materials. Moreover, the transparent materials have different microwave dielectric properties at different temperatures and different frequencies. Therefore, it is very important that we study the dielectric properties of microwave transparent materials at high temperatures over broad frequency band in the researches, productions, and applications of these materials.According to the dielectric property test method of microwave transparent material and specification requirements, cylindrical cavity was chosen in the study of high temperature broadband dielectric property test techniques. The physical test model at high temperatures over broad band and the analysis theory of measurement error were researched. High temperature broadband resonant cavity and transmission system working at variable temperatures were designed and fabricated. High temperature heating protecting system was developed, and finally the test system was built and debugged. We solved the difficult technical problem of high temperature broadband measurement of microwave transparent material and filled the gap of this field in domestic.Subsection calculation method along the longitudinal direction of cavity was brought forward in the study of measurement theory. High temperature physical test model of the resonant cavity with unequal diameters was built and improved so as to reduce the effects on measurement results caused by the changes of diameter and length along the longitudinal direction of cavity.During the process of calibration and measurement, real-time temperature acquisition at different position and linear interpolation method were proposed and used to correct the key dimension and microwave parameters of the cavity. The heat instability of the cavity was reduced and the stability of the measurement results at high temperatures was improved.Supersonic induction heating method was first used in the microwave high temperature broadband dielectric property measurements. Operability and maintainability of the system were improved.Based on the theory of subsection calculation theory and the method of real-time correction, the high temperature broadband test system for microwave transparent material was built. Multimode resonant cavity, coupling device, and piston for holding the test sample were designed according to the composition of cavity and high temperature environment. The high temperature broadband resonant system was fabricated with high temperature metal. Due to the system needs, nonstandard double ridge waveguide used in variable temperature environment and adapter from double ridge waveguide to coaxial were designed and fabricated. They composed the variable temperature broadband microwave transmission system. Induction heating method was applied in the system for heating the resonant cavity and sample, and vacuum protecting system was developed. The induction ring and thermal insulation equipment were debugged, and temperature distribution of the resonant system satisfying the test requirement was obtained. Finally the test system was built. The microwave parameters at high temperatures and stability of the resonant system were tested. Samples made of quartz, sapphire are designed and fabricated. The complex permittivity of the samples was measured at 7-18GHz over the temperature range from room temperature to 1500℃.Finally, the measurement errors were analyzed based on the measurement theory. Error sources were analyzed and uncertainties were obtained combining with temperature properties. The errors caused by the deviation between the ideal physical test model and real test were discussed. The measurement errors were calculated with the program complied by using Matlab software. Error analysis provided evaluation approach for the accuracy of the measurement results.The measurement results and error analysis results show that the test system bases on subsection calculation method and induction heating method and built for the first time in domestic can be applied to the measurement of high temperature broadband dielectric properties at 7～18GHz over the temperature range from room temperature to 1500℃. The test system features good test repeatability and reaches the requirements of test function and measurement accuracy.