| Millimeter wave dielectric materials,which frequently work in variable temperature environments have already had various applications in satellite communications,military aerospace,precision guidance and other industries,etc.Materials like antenna radomes on a spacecraft with high-speed need to undergo a temperature variation of thousands of degrees Celsius,resulting in drastic changes of dielectric properties which directly affect the performance and reliability of electronic equipments and the whole system.Hence,the impact of dielectric properties changes on equipments needs to be known and controlled in the design stage.Complex permittivity is an extremely important parameter to characterize electromagnetic properties of dielectric materials,then it is indispensable and significant to design a set of complex permittivity measurement system under ultra-high temperatures.Traditional open cavity method and free space method which are the mainstream methods at present for high-temperature measurements of complex permittivity both have limitations and drawbacks to some extent.The traditional open cavity method has the advantages of high measurement accuracy,but can only measure low-loss materials and the test temperature is limited within 1200? because of metal oxidation.Free space method can achieve ultra-high temperature measurements over 1200? owing to the non-contact of the material,but is limited to measure materials with high losses with limited accuracy.Currently,there is no system that can fulfill both low-loss and high-loss material measurements under ultra-high temperatures up to 1600 degree Celsius in air.The multifunctional ellipsoid/cavity system carried out in this paper can achieve this purpose with the feature of frequency universality.The system is composed of ellipsoidal mirror system(based on free space method)and ellipsoidal cavity system(based on open cavity method)with the advantages of both methods,making up for their respective deficiencies,hence filling the gap that at present there is no system that is able to fulfill the measurements of both low loss and high loss materials with high precision under ultra-high temperatures.Firstly,in this paper,the complete test theory was set up for this multifunctional system.Based on the distribution expression of the electric and magnetic field in the ellipsoidal cavity,the transcendental equations for relative dielectric constant calculation and formula for loss tangent were deduced.Combined with free space method the ellipsoidal mirror system based on,the blameless testing theory was built.Secondly,gaussian beam parameters were selected by algorithm optimization referring to the previous theory.Then ellipsoidal mirror and excitation module were designed to fulfill the system design.Thirdly,the full-wave analysis was implemented to verify system design and the derived formula.Finally,two sets of physical system were designed successively and detailed tolerance analysis were carried out for experiments.Considering the cost and many other factors,the final design is chosen for physical machining.The whole process from formula deduction to system design,full wave analysis and verification,hardware system design and error analysis is accomplished.Due to limitations of objective factors,time is not enough for physical experiments.However,the feasibility is highlighted and demonstrated for complex measurements both for high-loss and low-loss materials under ultra-high temperatures over 1200? of the test scheme of this multifunctional ellipsoid/cavity system. |
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