太赫兹波在均匀磁化等离子体中的传播特性

研究了太赫兹波通过均匀磁化等离子体的传播特性,给出了太赫兹波衰减和相移随等离子体密度、碰撞频率、太赫兹波频率和磁感应强度的变化规律。等离子体电子密度越大,衰减和相移越大;随着碰撞频率的增大,电磁波的衰减先增加到峰值后逐渐减小,且随着磁感应强度的增加,衰减的峰值变大,峰值向碰撞频率减小的方向移动;当电子碰撞频率接近电磁波频率时等离子体对右旋极化波的衰减达到极大值。太赫兹波频率增大,衰减逐渐减小,而相移先增加后逐渐减小。     

太赫兹波斜入射磁化等离子体的传播特性

针对飞行器再入大气的“黑障”问题,基于等离子体电子密度Epstein分布,建立了太赫兹波穿过磁化等离子体鞘套的一维模型。计算了太赫兹波斜入射磁化等离子体鞘套的透射率和衰减;分析了等离子体碰撞频率、磁场强度、入射角等因素对太赫兹波传播特性的影响。研究结果表明,外加磁场强度增大,透射率的最小值和衰减峰值向较高太赫兹波频率方向移动;电磁波入射角变大,透射率变小,衰减变大;当入射角小于等于60°时,大气窗口0.22 THz处最大衰减为5.32 dB。研究结果为解决“黑障”问题提供了重要参考。     

等离子体鞘套中太赫兹波传输特性研究

采用散射矩阵方法(SMM)研究太赫兹波在非磁化、非均匀等离子体鞘套中的传输特性。在假定等离子体电子密度分布为双指数分布条件下,对太赫兹波斜入射等离子体时的功率反射系数、透射系数及吸收系数进行了仿真,还对其随太赫兹波的入射角度、太赫兹波的频率、等离子体的碰撞频率、等离子体分布形态的变化规律进行了总结。研究结果表明,在上述条件下,太赫兹波在等离子体中均有较好的传输特性。总体上来说,随着太赫兹波频率的提高,太赫兹波在等离子体中的透射性更好,可以考虑提升载频至太赫兹波段来解决通信黑障问题。     

沙尘环境下太赫兹波衰减特性

对沙尘环境下太赫兹波衰减特性进行研究,在国际电信联盟(ITU)大气衰减模型的基础上,增加了不同类型沙尘的影响。基于大气毫米波传播模型(MPM)预测了1 THz内大气衰减特性;根据Mie散射理论计算了太赫兹波在沙尘环境下因沙尘颗粒产生的衰减;结合以上两种因素得出沙尘环境下太赫兹的衰减特性。本文的研究结果对太赫兹在沙尘环境下传输应用具有重要的参考意义,也为其他颗粒环境太赫兹波传输特性的计算提供了参考。     

等离子体绕流场电磁波传输特性分析

本文分析了等离子体绕流场中电磁波传输特性随几个主要物理参数的变化规律,包括电子密度、谐振频率、碰撞频率等。通过时域有限差分(finite-difference time-domain, FDTD)方法进行了仿真,结果表明在典型仿真输入条件下,在米波至毫米波波段,随着谐振频率升高,等离子体绕流场反射率升高、透射率降低;在米波至厘米波波段,随着碰撞频率升高,等离子体绕流场反射率降低、透射率规律性不显著;在毫米波波段以上,等离子体绕流场的反射率和透射率与碰撞频率关系不大,而且反射很弱,以透射为主。目标飞行高度在30～50 km且速度在5 000 m/s以上时,目标驻点处等离子体绕流场对S、C、X波段有显著的衰减作用,影响雷达探测或产生黑障现象。开展了基于高焓激波风洞的等离子绕流场电磁特性测量实验,获取了C、X波段雷达散射截面(radar cross section,RCS)数据和绕流场电子密度数据,并利用其验证了仿真结果的正确性。     

镀膜刻槽型太赫兹聚合物波导管的传输特性研究

设计了外壁镀金膜环形槽型和内壁镀铜膜螺旋槽型两种太赫兹聚合物波导管。主要分析和比较这两种波导管的衰减与色散特性。在太赫兹频段,外壁镀金膜环形槽型比内壁镀铜膜螺旋槽型对太赫兹波的衰减大;内壁镀铜膜螺旋槽型比外壁镀金膜环形槽型的色散大,并且出现负色散现象。研究表明,内壁镀铜膜螺旋槽波导管适合传输太赫兹波,而太赫兹波在外壁镀金膜环形槽波导管内损耗较大,不适宜传输。分析二者的电场和磁场特性,均没有出现仿表面等离子体现象。     

不同行程下水蒸汽太赫兹传输特性

太赫兹波具有独特的性质和应用,却存在大气衰减等物理上的限制.由于太赫兹波在大气传输中主要衰减来自水蒸汽,文中采用太赫兹时域光谱技术,通过构建不同行程的太赫兹时域光谱系统(0.5m、1m、2m、3 m),在0.1~2.0 THz 频率范围内,分别对不同湿度的空气进行太赫兹时域光谱测量,获得了25 个水吸收峰和10 个太赫兹窗口.结果表明:随着传输行程或湿度的增加,吸收谱带被展宽、太赫兹窗口被压缩,为超宽谱太赫兹波的潜在应用提供依据.     

基于太赫兹时域光谱技术的水蒸气传输特性研究

为了评估太赫兹波在空间通信、雷达、遥感等方面的潜在应用,需要研究太赫兹波在大气中的传输特性。由于0.1～2 THz的太赫兹波在大气中传输的主要衰减来自水蒸气,采用太赫兹时域光谱技术(THz Time-Domain Spectroscopy,THz-TDS),在2 m长的太赫兹传输行程中对不同湿度的空气进行了透射光谱测量,获得了水蒸气在0.1～2THz频率范围内的吸收谱、传输率和弱衰减窗口。     

太赫兹雷达反等离子体隐身研究

针对等离子体隐身目标的探测问题,从等离子体隐身机理和太赫兹(THz)频段高频、宽频带的反隐身特性出发,分析了太赫兹波反等离子体折射隐身和吸收隐身的可行性和优越性,建立了一种内封闭式等离子体隐身的理论模型,并采用分层介质方法分析计算了太赫兹波在内封闭式等离子体模型中的传输特性。结果表明,太赫兹波可以穿透一定密度和厚度的等离子体且能量衰减很小,使反射波增强,增大了等离子体隐身目标的RCS和有效探测等离子体隐身目标的概率,为太赫兹雷达反等离子体隐身设计提供理论依据。     

太赫兹波大气分层传输特性

基于修正的Van-Vleck Weisskopf(VVW)线型,辐射传输色散理论和水汽连续体吸收模型,结合HITRAN数据库,建立了太赫兹(THz)波大气传输衰减和色散模型,并与毫米波传播模型(MPM)的结果进行了对比分析。加入新的天顶分层方法,形成了对宽频THz波在真实大气中纵向传输吸收衰减和色散的数值模拟能力。研究了不同传输距离下THz波大气分层传输衰减与色散特性、340 GHz和410 GHz窗口特性。研究结果表明：340 GHz和410 GHz大气窗口垂直传输10 km以上,衰减分别低于25 dB和45 dB,群速色散很低,相位变化在0.01个量级,信号传输失真度小。     

临近空间高超声速目标RCS模拟技术研究

针对雷达探测临近空间高超声速目标模拟试验中的雷达散射截面（radar cross section，RCS）逼真模拟问题，提出了一种适用于临近空间高超声速飞行器等离子体鞘套下目标RCS衰减模拟方法.首先利用不同高度、不同速度对应的等离子体频率和电子碰撞频率的相关数据，拟合得出不同速度、不同高度对应的等离子体频率和电子碰撞频率关系表；其次，实时查表得到给定雷达频率情况下不同目标高度与速度对应的等离子体频率和电子碰撞频率，建立目标等离子体包覆模型和电磁波传输模型，计算雷达电磁波的衰减系数和反射系数；最后，通过雷达电磁波的衰减系数和反射系数模拟出目标RCS衰减.通过与有关实测数据比对，证明了方法的合理性.仿真分析可知，利用高频率雷达探测临近空间高超声速飞行器将更容易得到连续的航迹，产生雷达"黑障"的时间更短.     

Attenuation of terahertz transmission through rain

Based on the Marshall-Palmer,Weibull raindrop size distribution and Mie electromagnetic scattering model,the relationships of attenuation coefficient of terahertz(THz) atmospheric window waves with precipitation rate and temperature are studied.Furthermore,combined with the loss of electromagnetic wave transmission in free space,the attenuation of THz communication and the transmission of current mobile communication signals through rain are compared and analyzed.The results show that the attenuation coefficient of THz transmission is increased with increasing precipitation rate,the difference of attenuation coefficient at different THz window waves is small,and the maximum difference is about 3 dB.The rain attenuation of THz wave is first decreased and then increased with increasing temperature,but the temperature has little effect on it.The attenuation of THz wave through rain is much larger than that of mobile communication signal.     

基于太赫兹片上系统微带线的设计与仿真

太赫兹片上系统是一种将太赫兹产生和探测装置以及波导传输装置集成在同一基片上的设计,应用于晶体材料的共振吸收以实现对太赫兹时域光谱的探测。太赫兹产生与探测装置都由光电导天线构成,波导传输装置由微带线构成。微带线是一种能够传输高频电磁波的波导结构,但相比于自由空间波导具有高损耗和散射特性。为了研究微带线的结构参数对太赫兹波传输损耗的影响,采用模拟仿真的方法,得出了传输损耗随着传输长度和频率的增加而增加,随着微带线金属层厚度与介质层厚度的增加而减少的规律,从而证明了传输损耗的减少能够通过合理设计微带线结构来实现。     

Strip dielectric wave guide antenna-for the measurement of dielectric constant of low-loss materials

The value of dielectric constant are the most important parameters in material science technology. In micro-wave and millimeter wave circuits using dielectric materials the values of this parameters should be known accurately. It is observed that the number of methods are reported in litrature, however these methods impose difficulties in experimentation and are not very accurate. In this paper a novel approach to the measurement of the dielectric constant of low loss materials at micro-wave and millimeter wave frequencies has been discussed. In this method by using antenna theory, a metallic strip dielectric guide is taken in to constideration and band reject phenomenon of dielectric antenna is used. Frequency response of an antenna in band reject mode is a function of the dimensional parameters, such as the metallic strip period, the profile of the metallic strip and the dielectric constant of the material used. Hence if one measure the frequency responce of the antenna in band reject mode, the dielectric constant of the material is determined provided all other parameters are known. This method gives a direct measure of dielectric constant and is quite accurate as computer techniques are used for evaluating the dielectric constant. This method verified experimentally also.     

A new low-loss mode in semiconductor magnetoplasmas

A new semiconductor plasma wave mode that can propagate with very little attenuation at frequencies smaller than the plasma collision frequency is described. The wave propagates at right angles to a dc magnetic field and may be used for modulation of lasers or for tunable filters in the microwave /millimeter wave region.     

Nonuniform Layer Model of a Millimeter-Wave Phase Shifter

The electromagnetic wave propagation of millimeter waves in dielectric waveguides with thin surface plasma layers is characterized. The phase and attenuation of a 94-GHz wave are computed for various surface plasma layer thicknesses as a function of earner density levels. The electron/hole pairs generated in the vicinity of the dielectric waveguide surface by photo excitation are assumed to have an exponential profile due to either carrier diffusion or the exponential absorption of the optical field. Field computations made for a uniform plasma layer are compared with those of the nonuniform plasma to illustrate the effects of the exponential tails of the carrier profiles on both the phase and attenuation of the millimeter wave. The thin plasma layers slightly affect the field profile of the transverse electric modes (fields polarized parallel to the plasma layer). The transverse magnetic fields are highly distorted at plasma densities greater than 10/sup 16/ cm/sup -3/.     

Analysis of millimeter wave phase shifters coupled to a fixedperiodic structure

The propagation characteristics of an active plasma-induced millimeter wave phase shifter coupled to a fixed periodic structure are discussed. The numerical calculation is based on an improved boundary value solution. Like a normal dielectric slab waveguide with a plasma layer, the grating waveguide displays a phase shift with the increase of the plasma density. This phase shift due to the significant change of the field distribution has an impact on the modal attenuation coefficient. It results in the scanning of the radiation beam in the vicinity of the second Bragg, Especially, at both weak and strong plasma densities when the mode losses are small, the resonances caused by the periodic structure do not appear to be weakened. The results can be used to design electronically controllable millimeter wave scanning antennas