信号在时变等离子体中的传播特性

Propagation of electromagnetic signals in the time-varying plasma

高速飞行器等离子鞘套由于飞行姿态调整、湍流、非均匀烧蚀等因素的影响,使其等离子体参数存在时变特性,针对这种传输介质的快速时变特性引起的电波幅度、相位上的寄生调制效应,本文利用大面积辉光放电等离子体产生装置,搭建了等离子体中信号传输实验系统,进行了S频段的单频信号与调制信号传输实验,观测验证了调制效应的存在,且其调制频率与等离子体变化频率具有一致性,进一步分析了等离子参数与寄生调制效应的关系,理论和实验结果表明:即使当载波频率大于等离子频率时,时变等离子引起的寄生调制效应也会使传输信号的星座图发生旋转,造成其判决容差裕度变小,通信可靠性下降,并且载波频率越接近等离子频率时,其寄生调制效应越强烈。

Because the boundary layer of the plasma sheath formed around the hypersonic vehicle flying in atmosphere is turbulent, the parameters of plasma sheath, such as the electron density, become time-varying. Both the amplitude and phase of electromagnetic （EM） signal are modulated by the time-varying plasma. By using a large volume uniform plasma generator, an experimental system for the propagation of EM signals in the time-varying plasma is built. The propagation experiment of the monochromatic signals and binary phase shift keying signals in S band of plasma is conducted. The modulations of the amplitude and phase of EM signal are proved, and the rotation of constellation of the multiple phase shift keying （MPSK） signal is observed. The experimental and simulation results demonstrate that the frequency of parasitic modulation is the same as that of time-varying plasma and the parasitical modulation intensity varies in proportion to the ratio of the electron density profile to the carrier frequency. Even if the carrier frequency is higher than the plasma frequency, the parasitical modulation will make the constellation of the MPSK signals circumvolve, and the bit error rate higher.