Abstract: We consider a technique for determining the ultrasonic wave propagation delay upon mechanical stress measurements by using multichannel matched filtering. The expediency of using an “interval–delay” surface as a graphic representation of the results has been substantiated. The experimental results are reported. PubDate: 2015-07-01

Abstract: We estimate the accuracy of measurement of the instantaneous frequency ω(t) by the autonomous and mutual (i.e., using the reference signal) Wigner–Ville transform. It is shown that the estimation of the frequency ω(t) by the autonomous Wigner–Ville transform depends on the type of nonlinearity of the function ω(t) to a greater degree and on its time-variation rate to a lesser degree. The mechanism of increasing the frequency-time resolution of the mutual Wigner–Ville transform due to using the matched signal is described and the method for measuring the instantaneous frequency and the phase, which is based on revealing the character of modulation of the real and imaginary parts of the mutual power Pmut(t, ω) determined by the reference oscillation, is considered. Stability of the mutual Wigner–Ville transform with respect to normal white noise is estimated. PubDate: 2015-07-01

Abstract: We consider the possibility of using illumination of the HF broadcast stations for location of air objects. The relationships for estimation of the detection range are obtained and requirements for the degree of suppression of a direct signal from the broadcast station are determined. Spectral characteristics of the signals from HF broadcast stations are studied experimentally for different polarizations of the received radiation. The possibility of air object detection using the Doppler effect is shown. Theoretical estimates of the radar cross section of air objects for different polarizations of the incident radiation are given. It is found experimentally that the radar cross section is about the same for the vertical and horizontal polarizations. PubDate: 2015-07-01

Abstract: We consider the boundary condition for horizontal ionospheric currents at the terminator with allowance for the possibility of flowing of a part of the current into the upper ionosphere and magnetosphere. Theoretical models of extremely low frequency field propagation presented in the literature assume that the horizontal ionospheric current component orthogonal to the terminator is continuous at the terminator. A well-known boundary condition for Alfvén waves in a horizontally inhomogeneous ionosphere is used as the basis for theoretical constructs. PubDate: 2015-07-01

Abstract: We present the results of statistical analysis of an adaptive antenna array tuned using the least-mean-square error algorithm with quadratic constraint on the useful-signal amplification with allowance for the weight-coefficient fluctuations. Using the perturbation theory, the expressions for the correlation function and power of the output signal of the adaptive antenna array, as well as the formula for the weight-vector covariance matrix are obtained in the first approximation. The fluctuations are shown to lead to the signal distortions at the antenna-array output. The weight-coefficient fluctuations result in the appearance of additional terms in the statistical characteristics of the antenna array. It is also shown that the weight-vector fluctuations are isotropic, i.e., identical in all directions of the weight-coefficient space. PubDate: 2015-07-01

Abstract: We study propagation of periodic saw-tooth waves in nondispersive media with quadratic elastic and hysteresis nonlinearity. By comparing exact solutions for saw-tooth waves and their spectral characteristics, we identified regularities in the behavior of nonlinear acoustic effects in such media. It is shown that as distinct from the case of a medium with quadratic elastic nonlinearity, the effects of nonlinear dispersion can manifest themselves in hysteresis media. PubDate: 2015-07-01

Abstract: Some algorithms for estimating the number of radio signals with known and unknown amplitudes and phases are synthesized and analyzed. Modified maximum-likelihood algorithms are used to obtain consistent algorithms for estimating the number of radio signals in the case of unknown amplitudes and phases. Efficiency of the estimation algorithms is quantitatively characterized by the abridged probability of the signal-number estimation error. The studied-algorithm parameters are optimized according to the analysis results. PubDate: 2015-07-01

Abstract: We consider specific features of reflection of microwaves from the surface of a water basin for the two-layer model of oil spills, which are determined by a water-cut-oil film. Within the spill model, the dielectric properties of water were allowed for in accordance with the Debye theory, and the dielectric properties of the water-cut oil, in accordance with the theory developed for binary systems. The data about variations in the values of reflection coefficients depending on the frequency, viewing angle, thickness of the oil film, and moisture content in the film are obtained. The dependences of reflection coefficients on the film thickness are determined for various values of volume content of the water fraction in oil. Complex values of the dielectric permittivity of oil-water emulsions with preset volume moisture content are found. Describing the obtained dependences of the complex dielectric permittivity of the emulsion on the volume moisture content requires application of asymmetrical formulas for the mixture of polar and nonpolar fluids. PubDate: 2015-07-01

Abstract: We study the stability of a dense nonequilibrium plasma of the electron cyclotron resonance (ECR) discharge in an open magnetic trap immediately after the end of heating. The observed instability is accompanied by pulse-periodic generation of high-power electromagnetic radiation at a frequency that is close to the frequency of the upper hybrid resonance and the double gyrofrequency of electrons and by synchronous precipitations of fast electrons from the trap. It is shown that the observed instability is connected with excitation of plasma waves under the conditions of double plasma resonance in the decaying plasma of the ECR discharge. PubDate: 2015-05-26

Abstract: We consider a modification of the method for determining the mutual time delays of signals on the basis of the joint measurement of the time and frequency shifts. The modification allows one to increase the degree of manifestation of the correlation maximum for the frequency-modulated signals. The results of studying the proposed algorithm during processing of the phase- and frequency-modulated signals under the conditions of a priori uncertainty with respect to the carrier frequency of the signals are presented. PubDate: 2015-05-19

Abstract: We present the results of studying a microwave discharge produced by continuous-wave gyrotron radiation with a frequency of 30 GHz in a mixture of argon and hydrogen with an admixture of methane in the region of crossing of two wave beams. The discharge wave maintained over a substrate and had the form of a thin plasma layer. The gas temperature and electron density in the plasma were measured by the methods of optical spectroscopy of radiation. Features of determining Stark broadening of spectral lines of atomic hydrogen at a relatively low (of about 1013 cm−3) electron density are discussed. Dependences of the electron density and gas temperature on the pressure and gas composition, as well as the power of the incident microwave radiation are presented. The prospects of using the discharge under consideration for plasmochemical deposition of diamond films from the gaseous phase are considered. PubDate: 2015-05-19

Abstract: The basic sizes characterizing the morphology of a few practically important synthetic organic polymers in the glassy and viscoelastic states are confronted with the maxima of the absorption spectra in the terahertz frequency range. These sizes are determined using the X-ray diffraction technique. The absorption spectra of these polymers have been measured by the method of terahertz time-domain spectroscopy in the temperature range from helium to room temperatures. PubDate: 2015-05-19

Abstract: Some features of the electromagnetic wave propagation near the electron cyclotron resonance region in axisymmetric magnetic traps are studied both analytically and numerically. The ray paths are obtained with allowance for the plasma density inhomogeneities, magnetic field direction, and magnetic field strength. Criteria for the limiting cases of constructive refraction (the ray paths are attracted to the trap axis, where effective absorption takes place) and destructive refraction (the ray paths are expelled from the trap axis, and efficient heating of the central part of the plasma column is not possible) are found. It is shown that the inhomogeneities of the magnetic field direction and plasma density, which are typical of axisymmetric magnetic traps, as well as the spatial dispersion effects, can strongly affect the electromagnetic wave propagation near the electron cyclotron resonance surface. PubDate: 2015-05-19

Abstract: In this work, we study the spectrum S
iD of natural (thermal and shot) current noise of the p-n junction and the Schottky barrier in the case where their current-voltage characteristic has a nonideality coefficient η exceeding unity. The results obtained with the help of a special experimental setup are presented and explained using an equivalent scheme, which takes into account the presence of the series resistance of the diode base and contacts, as well as the possibility of existence of the leakage current. It is theoretically proven and experimentally confirmed that the van der Ziel relation S
iD = 2q (I
D + 2I
s), which is used for calculating the current-noise spectrum of an “ideal” junction with η = 1, cannot be used for η > 1 (here, q is the elementary charge, I
D is the current across the junction, and I
s is the junction saturation current). The previously obtained results are generalized using the Gupta theorem for the thermal-noise spectrum in the nonlinear resistive systems and are confirmed experimentally. It is found that the noise-current spectrum is given by the formula S
iD = (2q/η) (I
D + 2I
s), which is a modification of the van der Ziel relation for the case of an arbitrary value of the nonideality coefficient. The obtained formula for S
iD is experimentally tested by measuring the noise spectrum of the δ-doped Schottky diode, which is in thermodynamic equilibrium with the surrounding medium. PubDate: 2015-05-19

Abstract: We study the effect of the masking factor from the local thunderstorm cells on ULF magnetic field spectra with the inhomogeneous electron-density structures existing in the local ionosphere (ionospheric and lower ionospheric Alfvén resonators). Using an original data-processing technique for recording of horizontal magnetic components at the midlatitude reception point Novaya Zhizn’, we have examined the contribution of the sources located at different distances from the reception point to the formation of the background noise spectra. The ULF signal processing technique permitted us to reduce the pulse component of magnetic noise in amplitude above a certain threshold and thus rule out the effect of a local thunderstorm activity. Frequency dependences of the azimuthal angle of the principal axis of the magnetic noise polarization ellipse are also analyzed. It is shown that the presence of the lower ionospheric Alfvén resonator leads to a nonmonotonic dependence of the azimuthal angle on the frequency. It was found that the local thunderstorms within 60–80 km from the reception point completely mask the manifestation of the lower ionospheric Alfvén resonator in the ULF noise polarization parameters. To spot the local thunderstorm cells, we used the data from the meteorological radar facility MRL-4 in Nizhny Novgorod. PubDate: 2015-04-26

Abstract: We present the results of analysis of the dependence of the ionospheric airglow in the red (630 nm) and green (557.7 nm) lines of atomic oxygen on the pump-wave frequency f
0 near the fourth electron gyroharmonic 4f
ce. The experimental data were obtained in 2012 using the SURA heating facility. Stimulated electromagnetic emission spectra were used to determine the relation between f
0 and 4f
ce. It is found that at f
0 > 4f
ce and δf = f
0 − 4f
ce ≈ 15–20 kHz the red-line airglow intensity is about a factor of 1.5 higher for the magnetic zenith pumping (when the pump beam is inclined 12° south of the magnetic field direction) than for the vertical pumping. In the green line for the same offsets δf, the airglow was recorded with confidence only during magnetic zenith pumping. During vertical pumping, no regular dependence of the red-line airglow intensity on δf in the range −15 < δf < 280 kHz was obtained, while the green-line airglow was observed at 15 < δf < +5 kHz and 230 < δf < 280 kHz. In the red line during vertical pumping, a change from the artificial airglow generation to the ionospheric background suppression was detected when the F-layer cutoff frequency was decreased. During magnetic zenith pumping, a wide (about 30°) background suppression zone was observed around the airglow spot with an about 6° angular width. PubDate: 2015-04-26

Abstract: Using the neuronal-excitability model, we study an analog of the saddle-node bifurcation of the limit cycles in the case of slow variation in the control parameter, i.e., the so-called dynamic saddle-node bifurcation of cycles. It is shown that the stable oscillations in such a system occur on the two-dimensional invariant manifold which also exists after the passage through the “static” bifurcation value. It is found that oscillations disappear with a significant delay with respect to the oscillation-termination time which is predicted by the classical bifurcation theory. It is shown that the nonlocal oscillatory properties of the model and, in particular, the threshold properties of the two-dimensional invariant surface of the saddle trajectory play an important role in the delay formation. PubDate: 2015-04-26

Abstract: On the basis of the results of long-term measurements on the calibrated mid-latitude highfrequency paths, it is concluded that the processes responsible for the energy characteristics of the high-frequency wave field have inertial properties. Slow (one-hour) absolute variations in the average signal and effective-noise levels in the daytime and the night-time hours do not exceed 2.0 dB. Similar variations in the average signal levels in the twilight hours after removal of the trend, which is caused by the diurnal variation in the ionospheric characteristics, are also below 2.0 dB. Analysis of the time behavior of the relative error-appearance frequency during transmission of the binary sequences with the amplitude manipulation indicates that the above frequency varies in the range 20–75% for an hour if the reception results are averaged for 20 min. PubDate: 2015-04-26

Abstract: We study the electrodynamic characteristics of an antenna having the form of an infinitesimally thin, perfectly conducting narrow strip located on a plane interface of a resonant magnetoplasma and an isotropic medium. The antenna is perpendicular to an external magnetic field and is excited by a given voltage. Singular integral equations for the antenna current, on the basis of which the current distribution is found in the case of an infinitely long radiator, are obtained. The limits of applicability of an approximate method based on the transmission line theory for determining the current distribution and input impedance of the antenna are established. Within the framework of this method, the results obtained are generalized to the case of a finite-length strip antenna. PubDate: 2015-04-26

Abstract: We present the results of experimental and theoretical studies of generation of terahertz radiation during reflection of a femtosecond-long laser pulse from a metal surface. It is shown that a greater part of the experimental results (narrow directivity of the terahertz waves and dependence of its energy on the polarization and incidence angle of the laser radiation) can be interpreted within the model of Čerenkov radiation. Some new features of generation of terahertz radiation are considered, which appear during an optical breakdown of the medium near the metal surface. Structural changes in the terahertz signal, which take place in the process of formation of the near-surface plasma, are studied. PubDate: 2015-04-01