Radioelectronics and Communications Systems最新文献

Abstract

A detailed study of the problem of scattering of a plane-polarized electromagnetic wave by systems of conductive strips with impedance forming a plane grating is carried out. Two correct general mathematical models of scattering by conductive gratings were developed in a system of singular integral equations. The first modification of the method of integral equations is related to the simpler case of scattering of an E-polarized wave by a conducting strip grating. A simple and correct mathematical model of scattering in the form of a system of singular integral equations was obtained. The diagonal kernel functions of this system have a logarithmic feature, which is considered weak. The second modification is developed for the more complicated case of H-polarized wave scattering by a conducting strip grating. A more complex mathematical model of scattering in the form of a system of integral equations was obtained. The diagonal kernel functions of this system have a strong or Cauchy-type singularity. In this polarization case, two sets of additional conditions arise to ensure the correctness of the mathematical model. In order to check the correctness of the solution of electromagnetic wave scattering problems, it is important to obtain it in an explicit analytical form, even under certain assumptions that narrow the frequency range of the application of mathematical models. The asymptotic models of wave scattering by a single narrow strip with impedance and a weakly filled grating were developed. To show the asymptotic model effectiveness, the algorithms for calculating the directional characteristics are developed and simulations were performed.

Abstract

The paper is devoted to the optimization of the radiation characteristics of a linear focusing array, which consists of a set of semitransparent inclined elements (mirrors), an excitation source, and an end opaque mirror. The array geometry satisfies the quasi-optical conditions, while the inclination of mirrors is set in such a way that the array allows us to focus the scattered field in the Frenzel zone and the far zone. The formation of a radiation pattern (RP) close to the specified one, or the provision of maximum radiation in the given region of angular coordinates is achieved by choosing the reflection and transmission coefficients of semitransparent elements of the array. To ensure these requirements, a variational statement of the optimization problem is used. The maximization of corresponding functionals is achieved numerically by the methods of successive approximations. Numerical experiments demonstrate high accuracy in fulfilling the requirements of the radiation characteristics which is also confirmed by experimental data.

Abstract

The pinning of domain walls in cobalt micropowders, nanopowders, and nanowires is studied by the double-pulse spin-echo NMR method under an additional magnetic video pulse. Cobalt micropowders are obtained by melting in an induction furnace. Nanopowders are produced using electron beam technology and chemical deposition. This chemical deposition reaction is carried out in an external magnetic field to obtain nanowires.

The pinning in these systems is measured as a function of the long-term magnetic video pulse and its amplitude. It is established that the magnetic video pulse area is constant for all its threshold values corresponding to the beginning of the double-pulse echo signal suppression. The linear nature of the pinning dependence on the external magnetic field magnitude in cobalt micropowders in a wider range of the external magnetic field change compared to lithium ferrite is presented. Alternative information about the pinning strength of domain walls in magnets can also be obtained by studying the magnetic video pulse influence on the magnetic echo signal formed by the joint action of radio frequency and magnetic pulses. These NMR methods can be used for microscopic control of the domain wall properties in the studied magnets with the aim of their potential use in functional materials, memory devices, and sensors.

Abstract

The mathematical description of low-frequency vibrations of the magnetically soft precision microribbon Fe_73.5Cu₁Nb₃Si_13.5B₉ with the fixed one end is proposed in the paper. Resonant frequencies of induced mechanical vibrations of microribbons are found in the given frequency range of 15–80 Hz, for an alternating magnetic field. The attenuation coefficient, measured for the mechanical self-vibration of the annealed sample, is derived. This coefficient measured by laser turns out to be much smaller than the corresponding half-width of the amplitude-frequency characteristic (frequency response) of the described above forced vibrations The theoretical model of forced and free microribbon vibrations with finite thickness and width is proposed. One of the created partial solutions properties is a relatively wide resonance curve, which is confirmed experimentally. The given examples of partial solutions confirm the assumption about the frequency response shape below the maximum frequency at the specified excitation frequency and system parameters. The dependence of the mechanical parameter characterizing the force moment during bending deformation on the applied magnetic field intensity determines the mechanical properties of soft magnetic crystalline materials.

Abstract

In this paper, there are researched resonance effects occurring when an electromagnetic wave falls on a cylinder. The nature of the effects depends on the ratio between the diameter of the cylinder and the wavelength of the radiation and on the cylinder material. In thick cylinders (D ≫ λ) without absorption, surface wave resonances occur in the form of peaks regularly following each other on the curve of dependence of scattering on the diameter. The follow-up period is determined by the refractive index. In very thin metal wires (D ≪ λ) the effect of abnormally strong absorption of electromagnetic radiation is observed. The absorption efficiency factor of a metal wire with a diameter of several micrometers in the centimeter range of wavelengths reaches several hundred. Calculations of the distribution of the electromagnetic field inside and outside the wire showed a significant increase in the intensity of the electromagnetic field inside the cylinder at certain ratios between the diameter of the wire and the wavelength of radiation. The trajectories of the Poynting vector flow are distorted, and energy enters the wire from all sides. This effect is non-resonant. Fano resonances occur in thin cylindrical wires with a high refractive index and low absorption. At the same time, for some diameters, the scattering increases, and a very narrow resonant peak appears, while for others, the scattering is very small. The wire becomes invisible at a certain wavelength.

Abstract

N-order bandpass filter (BPF) with parallel resonators and cross and mixed couplings can have (N + 1) transmission zeros (TZ) located on the complex plane s = σ + jω. TZs at real frequencies (jω axis) are also called attenuation poles (AP). The authors propose an alternative possibility of AP forming in filters using resonators with special properties, which significantly increases the AP number from (N + 1) to (3N + 1). Increasing the AP number with fewer resonators allows us to increase the selectivity and rejection level and reduce BPF insertion losses. The special properties of resonators are that their input admittance Y has one or two poles (ω_p1, ω_p2) located next to the resonance frequency ω₀. This leads to the appearance of AP in a BPF. We propose and analyze three resonators with special properties. They are formed by a cascade connection of a quarter-wave resonator and lumped L and C. The input of the resonators is located on the side of the lumped elements. It was found that the Q-factors of lumped elements do not affect the filter losses in the passband. The reduction of Q_L and Q_C leads only to the decrease of the AP “depth.” For the first time, 7 APs were implemented in the experimental second-order microstrip BPF.

Abstract

Unlike the known modulation method, the formant-modulation method for the evaluation of speech intelligibility allows the evaluation of not only the speech transmission index but also the articulation index. This paper presents the results of a study of the accuracy of speech transmission index evaluation using the full and rapid variants of the formant-modulation method. The study is carried out by the computer simulation method for two cases, namely, the case of the predominant effect of noise interference and the combined effect of noise and reverberation in a room of medium volume. Dependences of the systematic and random components of the estimation error of speech transmission index on the duration of the test signal and signal-to-noise ratio are obtained. It is shown that although the fast version of the formant-modulation method is 4–5 times inferior to the full version in terms of measurement accuracy of the speech transmission index, the desired estimation accuracy can be achieved if the duration of the test signal is sufficiently long. In particular, the bias and standard deviation of the estimate of the speech transmission index obtained by the fast formant modulation method do not exceed 0.03 and 0.014, respectively, for the test signal duration of 16 s.

Abstract

In this paper, there are represented the features of acousto-optic interaction in the context of pulse signal detection. It specifies the necessity of the development of a simpler method for the calculation of a demodulator output signal parameters in case of its feeding with a pulse signal. Approximation models of modulating pulse and a pulse of the photodetector’s output were made based on geometric representation of the photo-elastic interaction. It is shown the main parameter defining the shape of the output pulse is the inertia of the acousto-optic demodulator. It is postulated that this parameter is shaped due to the influence of two factors: an acoustic-optic interaction and a photodetector inertia. For estimation of the degree of influence of these factors, a numerical analysis based on developed models is carried out. The results of theoretic research and numerical analysis are verified with experimental research. It is stated that for the application of a high-performance photodetector, the inertia of the acousto-optic demodulator is defined mainly by space-time parameters of acousto-optic interaction.

Abstract

The performance of the Fisher-Snedecor (F) fading model with selection combining (SC) diversity scheme over the interference-limited system is presented in this paper. The probability density function (PDF) and cumulative distribution function (CDF) are derived without SC diversity under single-user interference. After that, the expressions of CDF and PDF for SC diversity are computed. The PDF is presented for different desired and interferer multipath fading parameters. The results are also illustrated in terms of outage probability (OP) for independent but not necessarily identically distributed (INID) and independent identically distributed (IID) variates. The higher number of diversity branches shows better OP performance.

Abstract

The paper investigates the influence of the surface morphology of an array of silicon nanowires (SNW) on their humidity-sensitive characteristics. In this work, diode-type moisture sensors based on silicon nanowires were synthesized. SNWs were synthesized by the method of metal-stimulated chemical etching, and a p–n junction was formed in the SNW array by diffusion. The surface morphology was studied by the method of atomic force microscopy. The electrical and moisture-sensitive characteristics of the humidity sensors were measured. The influence of the root mean square value (RMS) of surface roughness and the volume porosity of material on the performance of the devices was determined. In particular, it has been shown that an increase in the surface roughness of the silicon nanowire array leads to a significant increase in the response (by up to 72.5 times), as well as a decrease in the response time and the recovery time of humidity sensors amounting to 20 and 36 s, respectively. On the other hand, the reduction of the roughness RMS leads to an improvement in reversibility (up to 11.3%), short-term stability (up to 1.61%), and repeatability of the sensor signal (up to 0.45%). The importance of taking into account the relationship between the surface morphology of the array of nanowires and their humidity-sensitive characteristics for the development of high-performance nanowire-based sensors has been demonstrated.