Volume 69, Nº 10 (2024)

The filtered orthogonal frequency division multiplexing method used for the transmission of the signals with spectra in adjacent frequency subbands has been analyzed. Three types of errors appearing when applying the method for quadrature amplitude modulation symbols transmission have been described. For the error reduction the modified algorithm based on cyclic convolution properties has been developed. In the numerical experiment, the classical and modified algorithms have been applied in combination with modulation schemes of large order.

An absorber of electromagnetic waves of a gradient type has been obtained using carbon fiber felt “Karbopon-V22M” with the following characteristics: reflection coefficient from -10 to -30 dB in the frequency range from 2 to 18 GHz, bending stress and tensile stress ≥ 300 MPa (deformation percentage -0.7%); density ≤ 2.5 g/cm³, thickness ≤ 6 mm. The prospects of using the created radio-absorbing material for concealing large-sized moving or stationary objects located underneath from radar detection systems, as well as for protecting electronic equipment, information carriers and biological objects from increased levels of electromagnetic radiation, localization of electromagnetic radiation are shown.

A dual-polarization ultra-wideband feed in the form of a metal four-ridge conical horn with a conical two-layer dielectric insert is proposed and investigated. Electrodynamic modeling and optimization of the transition parameters are carried out using finite element and finite difference methods in the time domain. It is shown that the proposed feed provides matching below the level of -10 dB in the frequency band of 6...40 GHz and the efficiency of a single-mirror offset system of more than 0.5 in the frequency band of 6...30 GHz.

A method for synthesizing efficient estimates of parameters of a random process whose distribution of samples has complete sufficient statistics is proposed. The method is based on the representation of the estimated parameters of the process in the form of a solution to the system of equations for mathematical expectations of functions derived from complete sufficient statistics, selected in such a way that the system of equations was solvable with respect to the estimated parameters. This solution is then replaced by the aforementioned functions in order to obtain the final estimate. The conditions under which the obtained estimates will be efficient are provided. Examples of parameter estimation for sample distributions from a uniform distribution and an additive mixture of Gaussian noise and a sequence of rectangular pulses with unknown amplitudes are presented, and their efficiency is demonstrated.

The problem of electromagnetic wave emission from a magnetic film under the influence of a short pulse of light from a femtosecond laser is considered. The model of accelerated motion of magnetic charges is proposed as the cause of radiation. The proportionality of the field of the emitted wave to the second derivative of the magnetization in time is revealed. Pulses of two types — rectangular and half-sinusoidal — are proposed for consideration. It is shown that the field of the radiated wave for a half-sinusoidal pulse exceeds the field from a rectangular pulse more than twice.

Electromagnetic parameters of the ladder-type slow-wave structures (SWS) formed by a metal plate with periodically arranged slots of a certain shape placed in a waveguide are studied. Modifications of the ladder-type SWS associated with the complication of the slot shape or the waveguide shape are proposed in such a way that the frequency of the slot resonance is lower than the cutoff frequency of the waveguide, and the SWS exhibits the properties of a double-negative metamaterial. It is shown that the fundamental spatial harmonic is backward, while the +1^st harmonic acquires normal dispersion and the beam-wave synchronism is possible in a sufficiently wide frequency band. SWS with dumbbell-shaped slots and SWS in a groove-loaded waveguide are designed for W-band traveling-wave tube (75…110 GHz) with a relative bandwidth of about 25% and operating voltages of 8…13 kV. In such structures, there is the possibility of interaction of a slow wave with two sheet electron beams propagating from above and below the plate.

The first-principle modeling of two different quasi-two-dimensional phases based on the volume phases Cr₂Te₃ and CrTe₃ is carried out. Structural relaxation of the obtained 2D compounds and their volumetric prototypes was performed within the framework of the density functional method and the projection plane wave method. Magnetic anisotropy in various crystallographic planes of quasi-two-dimensional structures and corresponding bulk materials has been studied. An increase in magnetic anisotropy was found during the transition from bulk phases to quasi-two-dimensional phases of Cr₂Te₃/CrTe₃. A charge density map is constructed and the density of electronic states is found for 2D Cr₂Te₃ and CrTe₃ materials.

Hafnium oxide films containing a non-centrosymmetric orthorhombic phase capable of spontaneous polarization were obtained by magnetron sputtering. It was shown that the presence of bombardment with negative oxygen ions promotes the formation of the orthorhombic phase.

Analysis and interpretation of exact solutions for oscillatory modes of 2-D dense electron beam with 3-D trajectories are carried out in the paper.

The article considers the features of determining the longitudinal T₁ and transverse T₂ relaxation times in the designs of NMR relaxometers with a flow-through and stationary liquid medium. The advantages of using the modulation technique for recording the NMR signal for flow-through NMR flowmeters-relaxometers and small-sized NMR relaxometers for express control of condensed media in comparison with other methods are noted. It is experimentally proven that the relationship obtained from the Bloch equations using approximations for determining T₁ based on the results of two measurements of the NMR signal amplitudes or resonance frequencies has a number of limitations in its application for monitoring the state of a flowing medium. Based on experimental data, a study was made of the relationship between the magnetic field modulation frequencies for determining T₁ and it was proven that for a number of relationships between them it is impossible to determine the T₁ value during measurements. In this case, the NMR signals from the flowing medium at these modulation frequencies are recorded, and the medium itself has relaxation times T₁ and T₂. The results we obtained allow us to eliminate the error in determining the state of the medium (whether it complies with the standard (absence of impurities in it) or not) using the measured values of relaxation times. This is extremely important a during conducting various experiments, especially using current flows, in which the use of other methods for solving this problem is difficult.