Intra-pulse modulation of the linear frequency modulated pulse caused by time-varying plasma

The effects of time-varying plasma on the propagation of a linear frequency modulated (LFM) pulse are investigated. An experiment on the propagation of LFM pulse in continuous time-varying plasma is described and a spectrum broadening phenomenon is observed. The phenomenon is due to intra-pulse amplitude and phase modulation of the LFM pulse by time-varying plasma; this explanation is verified by a comparison of the results of the experiment and theoretical analysis. The theoretical result is obtained by measuring the time-varying electron density that the intra-pulse modulation depends on and applying the measured time-varying electron density into calculation method presented in this paper. The effects of the bandwidth of the LFM pulse and oscillation frequency of electron density on the intra-pulse modulation phenomenon are then explored by another two experiments. Time-frequency analysis employing a smooth pseudo Wigner–Ville distribution (SPWD) is adopted to further analyze the phenomenon and results of time-frequency analysis confirm the earlier theoretical analysis and conclusions. The effect of intra-pulse modulation on the LFM pulse due to time-varying plasma should be emphasized in the application of the radar and communication systems on a vehicle under a plasma sheath.