Study Of High Temperature Complex Permittivity Measurement Of Transparent Materials By Stripline Resonator Cavity

Wave-transparent materials are widely applied in electronic device. The complex permittivity of materatials plays an important role on performance of the device. When the wave-transparent materials are used in radomes, antenna windows, and high-power devices, due to the impact of the environment, the electromagnetic parameters of materials will change as a function of temperature. Particularly while aerospace are flying at a very high speed in atmosphere, high temperature and high speed air flow will occur. It is an important factor of instability or performance deterioration in those electronic systems.In order to work properly in these situations, we must know the different microwave dielectric properties of the transparent materials at different temperatures. Therefore, it is very necessary to build a test system to measure the dielectric properties of materials as a function of temperature and frequencies. Based on the development of dielectric property test methods at home and abroad, combined with the specific requirements of this research, stripline cavity was chosen in the study of high temperature broadband dielectric property test techniques.An improved resonance method at high temperature for determining complex dielectric properties of low-loss materials was developed. The calculation process was given by a physical model of the stripline resonator cavity at high temperature. The paper brought forward the method of segmentation calculation according to the temperature changes over the cavity, which matched the actual situation and offer theoretical foundation for high temperature measurements.High temperature coaxial line, cooling coaxial line and adapter were designed and fabricated. In this work, we have performed sensitivity analysis in an effort to find the optimum stripline aperture dimensions to obtain both the high quality factor and easy to distinguish working modes. The physical size of the cavity was obtained after optimization and simulation.Vacuum furnace was designed to avoid oxidation for high temperature measurements, supersonic induction heating method was used in the microwave high temperature complex permittivity measurements, making the test system easier to maintain and operate.During the calibration and testing process, temperatures were real-time measured by the thermocouples buried in the ground boards of the cavity, we use the new method of segmentation calculation to analyze the resonant frequency and quality factor considering the temperature differences along the stripline resonator cavity, conductivity and thermal expansion constant of the cavity metal as function of temperature are calculated. The heat instability of the caity was reduced, and the measurement accuracy and stability at high temperature were improved.The whole test system was built and the measurement software was compiled. The microwave parameters at high temperatures and stability of the resonant system were tested. We have verified the proposed method from measurements of some typical samples with the available reference data in the literature. The measurement errors were analyzed based on the measurement theory. Error sources were analyzed and uncertainties were obtained with the program.Finally a system has been developed for measuring the complex permittivity of low loss materials at frequencies from 500MHz to 8GHz and over a temperature range up to 1500℃using stripline resonator cavity method. We solved the difficult technical problem of high temperature broadband measurement of microwave transparent material and filled the gap of this field in domestic. Such a test system has a significant guiding effect to the theoretical research and manufacture of wave-transparent materials.