Study On Polarization Characteristic Of Electromagnetic Wave And Radiation Property Of Antenna Under Plasma Sheath

With the human being to explore the outer space of the earth, as an important carrier of developing space, the reentry vehicle has been developed rapidly. While, when the reentry vehicle enters the dense atmosphere with the ultra fast 10~25 Maher, the enormous momentum of the vehicle is constantly converted into heat, the vehicle stagnation zone heat protection material and air molecules ionized, and a layer of high temperature plasma shock fluid is formatted around the spacecraft. The plasma shock fluid is called “plasma sheath”, and the shielding effect of plasma sheath on the electromagnetic signal from the aircraft’s measurement and control system is similar to that of a metal sheath. This serious affects on the vehicle of information transmission, even lead to a transmission interrupt of navigation, telemetry, remote control and the control and signal. Then the aircraft lost contact with the outside world, and "Blackout" phenomena is break out, which greatly threaten the flight safety of the aircraft.The essence of “Blackout” phenomenon is the block and distortion effects of reentry plasma sheath on the electromagnetic signal transmission, and has a great impact on the propagation characteristics of the electromagnetic wave and the characteristics of the aircraft antenna. Because there is a certain angle of flight trajectory and aircraft rolling motion, communication links between aircraft and navigation satellites and between aircraft and ground control station are usually in the squint direction of the vehicle antenna, which causes the signal of the aircraft antenna to transmit and receive usually incident on plasma sheath at a certain angle. Meanwhile, the plasma sheath has the characteristics of space inhomogeneous, frequency dispersion and the parameters in the whole reentry course are 3 to 4 orders of magnitude. The characteristics of the electromagnetic wave propagation in the plasma sheath and the characteristics of the antenna have a large gap with the conventional media. This paper on the basis of studying the basic physical characteristics of the actual reentry plasma sheath, the transmission amplitude-frequency characteristics of different polarization electromagnetic wave in the plasma sheath at any angle are studied. The depolarization effect of the plasma sheath on the incident electromagnetic wave at any angle and the polarization loss of the receiving antenna are studied. A comprehensive study on the impedance and the polarization characteristics of the antenna at different altitude in different altitude is carried out. In the end, two methods to reduce the reflection loss of the plasma are presented. The main innovation points of this paper include below:1. The equivalent transmission line calculation method for the incident plasma sheath at any angle is proposed. The influence of the incident angle on the transmission characteristics of different polarized electromagnetic waves is revealed. Based on Snell theorem, the propagation characteristics of the equiphase wave front and the equiamplitude wave front of non uniform refraction wave in the plasma sheath are studied. The transmission amplitude-frequency characteristic of different polarized electromagnetic waves in different angles are calculated by using the double Gauss distribution model, and the relationship of plasma electron density, collision frequency, electromagnetic wave frequency, incident angle and transmission characteristics of electromagnetic wave is presented.2. Based on the polarization decomposition and synthesis theory of electromagnetic wave, the polarization characteristics of electromagnetic waves in the plasma sheath are presented, and the polarization mismatch model is established. By adopting S, C, Ka and L band waves, the polarization characteristics of the electromagnetic wave with different angles in different frequency bands and the loss of polarization between the antenna and the receiving antenna are obtained. The influence of the plasma parameters and the incident angle on the polarization characteristics and the polarization loss of the electromagnetic wave are presented, and it is found that the polarization reversal phenomenon of electromagnetic wave from the right hand to the left hand. Numerical results indicate that the larger the incident angle and the lower the collision frequency is, the worse the deterioration of the polarization property, and the loss of polarization of electromagnetic wave is more serious.3. A layered plasma model which can simulate the non uniform distribution of the reentry plasma sheath is established. A method for calculating the characteristics of the antenna with frequency dispersion and high loss characteristics is proposed. The impedance characteristics, working frequency offset, spatial radiation power and polarization characteristics of GPS, S, C, and Ka band were calculated by the plasma sheath covering the typical reentry are calculated. Numerical results indicate apparent mismatch between the antenna and the feed system in the whole course of reentry is occurred, antenna impedance in Smith circle rotate in the direction of anti-clockwise, which experienced a great change in the capacitance and inductance reactance. At the same time, the problem of antenna resonance frequency offset is also experienced in the first direction towards low frequency then towards high frequency. Apparent deterioration of the polarization characteristics of the antenna in the whole course is occurred, the polarization characteristics of the antenna have even changed from the right to the left at some height. The antenna gain pattern is distorted in the forward direction, and the radiation performance of the antenna is drastically reduced. The height of the obstacle of the measurement and control is predicted, which is consistent with the measured data given in the NASA Report.4. Based on impedance matching theory, two plasma adaptive methods for reducing the reflection loss is proposed. Aiming at the problem of antenna impedance mismatch, a new method of measuring the impedance of the antenna is presented, which does not interfere with the receiver. On the basis of this, the adaptive compensation method for the impedance of the antenna is studied. Aiming at the problem of reflection loss of electromagnetic wave in plasma, a method use of electromagnetic materials to reduce the reflection loss of electromagnetic wave is presented. The simulation results show that these two methods are feasible. At the same time, these two kinds of plasma adaptive method can be used in combination, both increase the output of the feed system to the antenna, and reduce the plasma sheath reflect back signal. This method is conducive to the protection of aircraft antennas and transmitters. Above all, the research results of this paper can be more detailed analysis of the electromagnetic problem of reentry and may provide some reference data for the design of reentry electromagnetic system. the research results of this paper has important significance on the study of the mechanism of the plasma sheath to the measurement and control system of aircraft, the exploration of the problem of "black barrier" for the further.