## 1 Introduction  
The effects arising during the propagation of electromagnetic pulses (EMPs) in plasma media with dispersion and resonance absorption are of importance for fundamental science and various applications. Particularly, the EMPs excited by man-made [1, 2] and natural sources, e.g., lightning discharges [3], can be used for diagnostics of the near-Earth plasma. The prospects of developing new transionospheric communication channels are associated with the technology of ultrawideband (UWB) EMPs. In any case, it is necessary to predict distortions in the shape of the signal during propagation in the near-Earth plasma [4, 5, 6]. The ionospheric plasma in the magnetic field of the Earth is an anisotropic and gyrotropic medium characterized by a whole set of resonance frequencies and cutoff windows of the electromagnetic radiation [7]. An EMP excited by a ground-based, atmospheric, or ionospheric source and propagating in such a medium can experience strong distortions in its shape and spectrum in its ionospheric path. Interpretation of the observations can be complicated significantly by the nonlinear effects stimulated by powerful EMPs, primarily the ionization effects [8, 9]. At present, numerical models are actively being developed, which allow predicting the shape of an EMP as the pulse propagates through the atmosphere and ionosphere [10, 11, 12]. At the same time, a rather promising approach, which is important for both verification of the developed models and physical demonstration of possible effects, is laboratory simulation performed in special laboratory plasma devices and based on the similarity parameters [13]. Currently, the technologies of generation and direct (real-time) oscilloscopic registration of UWB EMPs are available to researchers range in pulse duration from 0.1 ns to 1 ns with an amplitude up to several hundreds of kV [1; 14]. However, laboratory simulation of the propagation of EMPs excited by the antennas used in field experiments [15] runs into fundamental and even insurmountable difficulties caused by the refraction and parasitic reflections of the EMPs from the walls of the plasma chambers [16] even in a large column of highly uniform plasma. On the other hand, it is possible to ensure EMP propagation in plasma along an extended path without refraction losses by using symmetric or coaxial transmission lines installed in the plasma chamber and excited by a pulse generator. This model experiment scenario was first implemented in the late 1970s with compact--about 1 m long--strip lines [17]. However, even in this case estimates and experiments show that the length of the propagation path of nano- and subnanosecond UWB EMPs in plasma should be at least several meters to allow observation of the dispersion and nonlinear effects [16]. Therefore, handling the problems to solve requires large-scale plasma devices anyway. It was only recently possible to create a transmission line that makes it possible to experimentally simulate the propagation of UWB EMPs along a long path in a transparent plasma. In Russia, the largest device for simulation of physical phenomena in the atmosphere and ionosphere is the Krot device at the Institute of Applied Physics of the Russian Academy of Sciences [18]. Recently, a new scientific instrument (a \"gigantic\" plasma-filled coaxial line, GCL) has been added to this device. The GCL was described in full detail in [19] and allows one to study the interaction of EMPs with durations from 300 ps with gases and plasmas including magnetized ones. Today, this is the only instrument of its kind that makes it possible to experimentally study linear and nonlinear effects in plasma during the passage of a UWB EMP along a path with a total length of about 10 m. The preliminary experimental results obtained by using the GCL were published in [20].