| When the reentry vehicle returns to the atmosphere at a speed of Mach 10 to 25,there will be a 5 to 10 minute communication interruption,the so-called "black obstacle" phenomenon.There are two main reasons for this phenomenon.On the one hand,the ablation and gas-dissolution of the antenna window occur due to high temperature,and the dielectric properties of the material change,which reduces the wave-transmission rate of the material.On the other hand,the air is subjected to the aircraft.The extrusion forms a shock wave,and the molecules in the air are ionized,forming a layer of plasma sheath on the surface of the aircraft,which seriously hinders the communication between the aircraft and the outside world.Plasma sheaths,high-temperature antenna windows,antennas,etc.are coupled to each other,resulting in increased attenuation of electromagnetic waves,increased phase delay,and reduced antenna gain,impedance mismatch,and distortion of the pattern,which seriously degrades the signal transmission quality.In order to explore the electromagnetic wave propagation characteristics and antenna radiation characteristics of the reentry aircraft antenna window,the following research work is carried out:1.The dielectric properties and transmission characteristics of the antenna window in high temperature environment are discussed.The thermoelectric behaviors of three kinds of transmissive materials,such as silicon dioxide,silicon nitride and C/C composites,were analyzed and compared.The law of dielectric properties with temperature was given.The antenna window was calculated when the temperature reached 2500 K.Wave transmission performance.2.The propagation characteristics of electromagnetic wave transmission plasma sheath and antenna window material are studied.Because the antenna window ablate and decompose at high temperature,some material molecules enter the air and ionize,which changes the parameters of the plasma sheath and affects the dielectric properties of the sheath.The complex process of electromagnetic wave propagation in plasma sheath and antenna window is analyzed.The influence of material ablation on plasma sheath parameters is discussed.The calculation model of electromagnetic wave propagation is proposed,and the amplitude and frequency characteristics of electromagnetic wave in typical reentry state are analyzed.The phase delay characteristics were calculated and analyzed.The calculation results show that the electromagnetic wave has multiple reflections and transmissions at the interface between the antenna window and the plasma sheath,showing a frequency resonance effect and a phase nonlinear effect.3.The antenna window local modeling simulation and antenna radiation characteristics calculation were carried out.The effects of plasma and antenna window on the radiation characteristics of the antenna are studied.The antenna window is partially modeled during the reentry process of the aircraft.The radiation characteristics of the antenna in the typical reentry state are simulated and analyzed.The effect of changes in the dielectric properties of the antenna window on antenna performance.In this paper,the theoretical study and simulation analysis of electromagnetic wave propagation characteristics and antenna radiation characteristics under the coupling of medium ion sheath and antenna window in the reentry process are carried out.Electromagnetic waves are reflected multiple times at the interface between the plasma sheath and the antenna window,causing serious Energy attenuation,at the same time,will increase the return loss of the antenna,reduce the gain,and deteriorate the pattern.The work of this paper makes a theoretical exploration of the propagation of electromagnetic waves in complex environments,and provides a certain data reference for the development of re-entry aircraft ground simulation experiments. |
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