Abstract: We study collective dynamics of a small-sized chain of the unidirectionally coupled phase-locked loop. The conditions for the synchronous-regime existence are found, the asynchronous selfoscillation regimes and the transitions among them are studied, and the property of inheriting the structure of the parameter space of the chain when a new element is added to it is established. PubDate: 2016-07-25

Abstract: We solve analytically the problem about ion compensation for the space charge of a helical electron beam in a gyrotron operated in the long-pulse regime. Elementary processes, which take place during ionization of residual gas in the tube under typical pressures of 10−6–10−7 mm Hg, are considered. It is shown that distribution of the space charge is affected mainly by the electrons of the initial beam and slow-moving ions produced by ionization of the residual gas. Steady-state density of ions in the operating space of the gyrotron after the end of the transitional processes is found, as well as the electron density profile in the channel of electron beam transportation. The results obtained allow us to evaluate the pitch-factor variations caused by partial compensations for the potential “sagging” in the gyrotron cavity, thus being useful for analysis of starting currents, efficiency, and mode competition in high-power gyrotrons. PubDate: 2016-07-21

Abstract: We study the electrical properties of multilayer radar absorbing materials obtained by adding nonwoven sheets of dielectric fibers with an intermediate layer of electrically conductive carbon fibers. Multilayer materials that absorb electromagnetic radiation in a wide frequency range are obtained by varying the content of the carbon fibers. The carbon-fiber content dependent mechanism of absorption of electromagnetic radiation by sheets and multilayer materials is considered. PubDate: 2016-07-20

Abstract: We analyze the dynamics of the plasma-wave channels excited in magnetized plasmas in the whistler frequency range. A linear theory of excitation of a plasma waveguide by an external source is developed using the quasistatic approximation. Self-consistent spatio-temporal distributions of the electric field of quasipotential waves and plasma density, which are solutions of the nonlinear nonstationary problem of the ionizing self-channeling of waves in plasmas are found on the basis of the linear theory. PubDate: 2016-07-20

Abstract: We consider a method based on the generalized maximum-likelihood rule for solving the problem of reception of the signals with orthogonal frequency division multiplexing of their harmonic components (OFDM signals) in the time- and frequency-scattering channels. The coherent and incoherent demodulators effectively using the time scattering due to the fast fading of the signal are developed. Using computer simulation, we performed comparative analysis of the proposed algorithms and well-known signal-reception algorithms with equalizers. The proposed symbolby-symbol detector with decision feedback and restriction of the number of searched variants is shown to have the best bit-error-rate performance. It is shown that under conditions of the limited accuracy of estimating the communication-channel parameters, the incoherent OFDMsignal detectors with differential phase-shift keying can ensure a better bit-error-rate performance compared with the coherent OFDM-signal detectors with absolute phase-shift keying. PubDate: 2016-07-20

Abstract: Fundamentals of the method of transverse displacements for calculating the HF radio-wave propagation paths are presented. The method is based on the direct variational principle for the optical path functional, but is not reduced to solving the Euler—Lagrange equations. Instead, the initial guess given by an ordered set of points is transformed successively into a ray path, while its endpoints corresponding to the positions of the transmitter and the receiver are kept fixed throughout the entire iteration process. The results of calculation by the method of transverse displacements are compared with known analytical solutions. The importance of using only transverse displacements of the ray path in the optimization procedure is also demonstrated. PubDate: 2016-07-19

Abstract: We study the super-low-frequency field of a grounded horizontal electric dipole on a high-latitude submeridional path having a length of up to 1000 km. Unlike most of the earlier works, this paper considers the case of wave propagation along the dipole axis, where the main component of the magnetic field is radial. The frequency dispersion of the ground conductivity under the source, which determines the coefficient of excitation of the Earth—ionosphere waveguide, is found from the measurements of the field at a distance of three heights of the ionosphere. Typical parameters of propagation of super-low-frequency waves are refined allowing for specific features of the path used in the experiment. The significance of allowing for ground conductivity as the radiation propagates along low-conductivity regions is demonstrated. The regions where one of the magnetic-field components is predominant are calculated in a polar coordinate system linked to the source, as well as the regions where the both components should be allowed for. The theoretical possibility of determining the phase velocity of the wave based on the ratios of the magnetic-field components measured at longer lengths is noted. PubDate: 2016-07-19

Abstract: The dynamics of the microwave breakdown in a gas is simulated numerically within a simple 1D model which takes into account such processes as the impact ionization of gas molecules, the attachment of electrons to neutral molecules, and plasma diffusion. Calculations are carried out for different spatial distributions of seed electrons with account for reflection of the incident electromagnetic wave from the plasma. The results reveal considerable dependence of the ionization wave evolution on the relation between the field frequency and gas pressure, as well as on the existence of extended rarefied halo of seed electrons. At relatively low gas pressures (or high field frequencies), the breakdown process is accompanied by the stationary ionization wave moving towards the incident electromagnetic wave. In the case of a high gas pressure (or a relatively low field frequency), the peculiarities of the breakdown are associated with the formation of repetitive jumps of the ionization front. PubDate: 2016-06-29

Abstract: This work is devoted to studying the processes of activation of the neurons whose excitation thresholds are not constant and vary in time (the so-called dynamic thresholds). The neuron dynamics is described by the FitzHugh–Nagumo model with nonlinear behavior of the recovery variable. The neuron response to the external pulsed activating action in the presence of a slowly varying synaptic current is studied within the framework of this model. The structure of the dynamic threshold is studied and its properties depending on the external-action parameters are established. It is found that the formation of the “folds” in the separatrix threshold manifold in the model phase space is a typical feature of the complex dynamic threshold. High neuron sensitivity to the action of the comparatively weak slow control signals is established. This explains the capability of the neurons to perform flexible tuning of their selective properties for detecting various external signals in sufficiently short times (of the order of duration of several spikes). PubDate: 2016-06-29

Abstract: We consider a gas-dynamic model describing the formation of a plasma with multiply ionized ions under the conditions of resonant heating of the electron component. Based on the isothermal approximation, possible regimes of the plasma flow are classified, the influence of the geometric divergence of the flow on the formation of the ion charge distribution is studied, and optimal regimes for the achievement of the maximum ion charge are identified. The model can be used for optimization and interpretation of modern experiments on generation of the extreme ultraviolet radiation due to the excitation of lines of multiply ionized atoms in a gas flow heated by strong millimeter or submillimeter waves. PubDate: 2016-06-27

Abstract: We show that a significant energy redistribution occurs in the spectrum of reflected nonlinear waves, when an intense acoustic beam is reflected from an acoustically soft boundary, which manifests itself at short wave distances from a reflecting boundary. This effect leads to the appearance of extrema in the distributions of the amplitude and intensity of the field of the reflected acoustic beam near the reflecting boundary. The results of physical experiments are confirmed by numerical modeling of the process of transformation of nonlinear waves reflected from an acoustically soft boundary. Numerical modeling was performed by means of the Khokhlov—Zabolotskaya—Kuznetsov (KZK) equation. PubDate: 2016-06-27

Abstract: We analyze spatial correlations of the surface reverberation in a plane-layered acoustic channel. The horizontal correlation function of the wind reverberation for the developed waves with an isotropic spectrum is theoretically studied within the framework of the mode representation of an acoustic field. The correlation function of monostatic reverberation is shown to have a universal form, while in the case of a bistatic radiation regime, the characteristic correlation scale of the reverberation significantly depends on its delay time. PubDate: 2016-06-27

Abstract: We study locking of the oscillation frequency of the operating TE28,12 mode by an external monochromatic signal in a multimode gyrotron operated at a frequency of 170 GHz in the switch-on regime close to the real one. Locking zones, i.e., regions of single-mode generation at the external-signal frequency are found on the “current—detuning” plane of parameters. It is shown that as the number of competing modes increases, the maximum achievable current decreases, and the locking zones contract at sufficiently high currents. PubDate: 2016-06-27

Abstract: We consider an adaptive multi-channel interference canceller, which ensures the minimum value of the average output power of interference. It is proposed to form the weight vector of such a canceller as the power-vector expansion. It is shown that this approach allows one to obtain an exact analytical solution for the optimal weight vector by using the procedure of the power-vector orthogonalization. In the case of a limited number of the input-process samples, the solution becomes ill-defined and its regularization is required. An effective regularization method, which ensures a high degree of the interference suppression and does not involve the procedure of inversion of the correlation matrix of interference, is proposed, which significantly reduces the computational cost of the weight-vector estimation. PubDate: 2016-06-27

Abstract: We show that the discrete frequency spectrum of a plane hydrodynamic flow of ideal incompressible liquid with localized trajectories of the liquid particles can contain only one, two, or an infinite number of harmonics. PubDate: 2016-05-10

Abstract: This paper is devoted to an analytical study of the one-dimensional stationary system of equations for modeling of the electrode effect in the Earth’s atmospheric layer with aerosols. New integrals of the system are derived. Using these integrals, the expressions for solutions of the system and estimates of the electrode layer’s thickness as a function of the aerosol concentration are obtained for numerical parameters close to real. PubDate: 2016-05-10

Abstract: We present the results of simulating numerically the operation of a high-power active compressor of microwave radiation pulses in the 3-cm wavelength band with a waveguide switch activated by an electron beam. The influence of the nonlinear character of electron beam interaction with the high-frequency field in the switch on the output parameters of the compressor is discussed, and the simulation results are compared with the experimental data. It is shown that at a high power of the input radiation, the effect of electron expulsion from the region of the inhomogeneous field in the switch can lead to a decrease in the compressor gain and elongation of the leading front of the compressed pulse. PubDate: 2016-05-10

Abstract: We analyze a matching system for the scanning aperture antenna radiating through a layer with unpredictably changing parameters. Improved matching has been achieved by adaptive motion of a dielectric plate in the gap between the aperture and the radome. The system is described within the framework of an infinite layered structure. The validity of the model has been confirmed by numerical simulation using CST Microwave Studio software and by an experiment. It is shown that the reflection coefficient at the input of some types of a matching device, which is due to the deviation of the load impedance from the nominal value, is determined by a compact and versatile formula. The potential efficiency of the proposed matching system is shown by a specific example, and its dependence on the choice of the starting position of the dielectric plate is demonstrated. PubDate: 2016-05-10

Abstract: In part I of this work [1], we study the dispersion characteristics of low-frequency waves in a relativistic electron–positron plasma. In part II, we examine the electromagnetic wave instability in this plasma caused by an admixture of nonrelativistic protons with energy comparable with the energy of relativistic low-mass particles. The instability occurs in the frequency band between the fundamental harmonic of proton gyrofrequency and the fundamental harmonic of relativistic electron gyrofrequency. The results can be used for the interpretation of known observations of the pulsar emissions obtained with a high time and frequency resolution. The considered instability can probably be the initial stage of the microwave radio emission nanoshots typical of the pulsar in the Crab Nebula. PubDate: 2016-05-10

Abstract: We study the dynamical regimes in the system of two identical interacting phase oscillators with plastic couplings. The joint evolution of the states of the elements and the interelement couplings is a feature of the system studied. It is shown that the introduction of plastic couplings leads to a multistable behavior of the system and emergence of the asynchronous regimes which are not observed for the considered parameter values in the case of static couplings. The parameter plane is divided into regions with different dynamic regimes of the system. In particular, the regions in which the system demonstrates bistable synchronous behavior and the region in which the coexistence of many various asynchronous regimes is observed are singled out. PubDate: 2016-05-10