Introduction. The interest in multibeam dielectric lens antenna arrays has been growing in recent years due to the development of millimeter-wave telecommunication and radar systems. Progress in the development of mobile communication systems based on adaptive beamforming technology is increasingly associated with multibeam systems based on lens antenna structures, providing an alternative to hard-to-implement and energy-consuming phased antenna arrays. In recent years, spherical and cylindrical Luneburg lens antennas implemented using additive manufacturing technology have attracted research attention. Despite their complexity of execution, these design exhibit excellent electromagnetic characteristics. This paper provides a review of Luneburg lens antennas manufactured using 3D printing, which can find application in 5G and 6G communication systems.

Aim. To review achievements in the design of lens antenna structures manufactured using additive manufacturing.

Materials and methods. Materials for analysis, comparison, and systematization were derived from various sources, including research articles, publications in proceedings of Russian and international conferences, and websites of manufacturers of lens antennas over the past 20 years. The material selection mechanism was based on the originality of the presented designs of printed Luneburg lens antennas.

Results. A review of Luneburg lens antennas manufactured using 3D printing, which differ from each other in terms of mechanical strength, complexity of execution, and electrodynamic characteristics, was carried out. The results of a comparative analysis of the key characteristics of these antennas are presented, along with examples of their practical implementation.

Conclusion. The disadvantage of Luneburg lens antennas has always been the complexity of their manufacture; however, additive manufacturing technologies open up new opportunities for their fast, high-quality, and automated production. Various 3D printing technologies can be used to create dielectric lens antennas, which differ in the resolution of printers, printing speed, and cost. Additive manufacturing methods are constantly developing, having reached the technological possibility of printing Luneburg lens for the sub-THz range with a high level of resolution and accuracy. In addition, 3D printers capable of printing multiple lenses simultaneously have also appeared.

Introduction. Radar stations for surveillance of airborne objects operating in the shortwave range are characterized by a number of limited technical characteristics, one of which is the azimuth viewing sector. The use of a linear antenna array (AR) as a receiving array provides the viewing angle of only up to 60°. When modernizing the station, this limitation was removed by applying an AR with a ring structure (currently a multi-ring AR is used). In practice, after performing a number of spatial processing algorithms, the operator obtains the azimuth, range, and speed of the observed object. However, due to the peculiarities of shortwave signal propagation, the accuracy of measuring these parameters does not ensure stable tracking of airborne objects. The use of multi-ring ARs also allows the elevation angle to be measured with a subsequent calculation of the height of the object.

Aim. Analysis of the phase distribution of the incident wave at the aperture of a multi-ring AR, as well as spatial processing of the received signal using the phase direction finding method to improve the accuracy of elevation angle measurements.

Materials and methods. Computer simulation in the MATLAB environment was carried out to form phase distributions on the elements of a multi-ring array, to calculate the elevation angle using the phase method, and to form portraits of the surveyed object. This environment has been successfully used to solve a wide range of problems of varying complexity in both industry and research fields.

Results. The possibility of using the phase method of direction finding of a radiation source to improve the accuracy of elevation angle measurement is demonstrated based on the conducted computer simulation. The obtained results were verified on the example of surveying an airborne object by a shortwave radar station.

Conclusion. The results obtained proved the relevance of using the phase method when performing spatial signal processing by a shortwave radar station. The proposed method made it possible to eliminate the ambiguity in measuring the elevation angle and to increase the accuracy of its determination, which is a new result in relation to the systems under consideration.

Introduction. Tightly coupled dipoles currently belong to one of the most popular types of antenna arrays. Their main advantages include an electrically low-profile height, the ability to scan the beam across a wide sector of angles without the onset of scan blindness, and a low level of cross-polarization. In recent years, the number of publications on the topic of antenna arrays of this type has increased significantly. The authors have paid sufficient attention to baluns included in the antenna array elements. However, the possibility of implementing a differentially-fed scheme in antenna arrays of this type remains poorly studied. This makes the study of this subject especially relevant in the development of radio devices where such feed technique is preferable.

Aim. Differentially-fed tightly-coupled dipole array design and study.

Materials and methods. The following materials were used to create the prototype: copper sheet, ceramic glass substrate ST-50-1, dielectric RO3003. A numerical study of the characteristics was carried out in the ANSYS HFSS environment; an experimental study of the prototype was carried out in an anechoic chamber using an automated measuring complex and a vector network analyzer.

Results. The results of designing a planar antenna array of tightly-coupled dipoles for the X-band are presented. In the antenna array, each of the dipole arms is fed using a separate coaxial cable, while the two arms of one dipole are fed out-of-phase. The results of a numerical study of the characteristics of an 8 × 8 antenna array made from the developed elements are presented. Across the range from 6.5 to 12.25 GHz, the average active VSWR does not exceed 3, while the gain varies from 21.5 to 25.7 dBi. The possibility of beam scanning in a sector of angles up to ±45° is shown. The results of an experimental study of the radiation characteristics and matching of the prototype of a single element are presented.

Conclusion. The importance of taking into account the effects that arise at the edges of finite antenna arrays during simulations is shown. The feasibility of manufacturing and measuring antenna array prototypes with a large number of elements is experimentally confirmed. The proposed element design demonstrates the possibility of implementing the differentially-fed scheme in tightly coupled antenna arrays.

Introduction. Polyharmonic signals with a line spectrum are often encountered in practical problems. Among the examples are signals from sensors monitoring rotating elements of mechanical systems, heart rate signals, or signals of radio systems with pulse-to-pulse repetition-period staggering. Due to instable frequencies of the signal harmonics or due to fluctuations in the sampling period, the line spectrum is disrupted. These distortions can be considered as a consequence of changes in the local time scale of the processed signal. This interpretation makes it possible to use scale-invariant transforms to reconstruct the signal spectrum. Methods for reconstructing the spectrum of a signal, the sampling moments of which are unknown a priori, are based on a preliminary reconstruction of the signal and subsequent estimation of its spectrum. Existing algorithms for reconstructing a signal sampled on an uneven time grid with unknown nodes are characterized by a high computational complexity due to their iterative nature and reliance on optimization search methods.

Aim. To synthesize a non-iteration algorithm for reconstructing the spectrum of a polyharmonic discrete signal under the assumption of slow changes in the sampling period.

Materials and methods. To solve the problem, the digital Lamperti transform is implemented. The quality assessment of the proposed algorithm is realized via computer simulation using a test signal known from the literature on the digital spectral analysis.

Results. The conducted computer simulation of the proposed algorithm has proven its feasibility. The line structure of the test signal spectrum, which was distorted by slow changes in the sampling period with an amplitude of 20 % of the mean value of the sampling period, was completely restored. Errors of the frequency and power estimates of individual signal harmonics exhibit values comparable with those derived from the spectrum estimate when the signal is evenly spaced. The error in estimating the sampling period comprised 5 % of its mean value.

Conclusion. A new iteration free algorithm for reconstructing the line spectrum of a discrete polyharmonic signal is proposed. The algorithm uses the scale invariant Lamperti transform. The synthesized algorithm can be used in a simple iterative procedure to estimate changes in the sampling period.

Introduction. The article presents a brief history of the European and international standardization process in the field of digital video broadcasting (DVB) systems as applied to satellite broadcasting. The advantages of new systems are considered. The results of a comparative analysis of their main characteristics in relation to previous versions are given.

Aim. To study the current progress in the standardization of digital satellite broadcasting systems, to analyze their new features and capabilities, to compare their main characteristics, and to distinguish the advantages of new systems in relation to previous versions.

Materials and methods. Documents of the European Telecommunications Standards Institute (ETSI) and the International Telecommunication Union related to the standardization of satellite broadcasting systems from DVB-S to DVB-S2X were studied. A comparative analysis of the main functions and characteristics of the systems under consideration was carried out.

Results. New functions and features introduced in the DVB-S2 system were analyzed, including the Time-Slicing (Annex M) option, which allows receivers to select and decode a specific stream carrying one or more services of interest without wasting resources on processing other streams. New DVB-S2X system options were considered, including the SuperFraming Structure (Annex E) option, which ensures increased immunity to co-channel interference from neighboring beam signals, as well as support for future developments related to beam hopping. The advantages of channel aggregation in DVB-S2X were illustrated, which allows the capacity of two or three transponders to be shared in order to increase the statistical multiplexing ratio in the case of UHDTV programs.

Conclusion. The standardization of digital satellite broadcasting systems provides an opportunity for equipment developers and manufacturers to use the latest technologies and methods, while relying on internationally recognized standards. This allows, on the one hand, the equipment of digital satellite broadcasting systems and the consumer quality of the services provided to be constantly improved, and, on the other hand, the cost of chips and equipment produced to be optimized.

Introduction. Parametric spectral estimation methods provide an improved level of frequency resolution compared to matched signal processing conventionally used in radar technology. This renders these methods promising for application in cases where the sample size of a spatial or temporal signal is strictly limited. At the same time, parametric methods are not optimal when receiving single signals against the background of normal uncolored additive noise. Therefore, parametric methods can be used as independent approaches provided that, first, working detection statistics are selected and justified and, second, that detection characteristics and noise immunity are constructed and analyzed.

Aim. This paper investigates modified detection statistics of the parametric Burg method, characterized by the simplicity of decision functions and the capacity to provide a constant false alarm probability under varying additive noise levels.

Materials and methods. Statistical computer simulation of the detection algorithms under consideration was conducted. This method is widely used in the analysis of parametric methods of signal processing. The detection characteristics obtained in the work were compared using the well-known Burg harmonic mean method, which involves the lowest computational costs.

Results. The paper presents original decision functions derived from the transformation of power spectral density estimates of the Burg method. The detection characteristics and immunity to signal-like interference of the modified Burg method are obtained and investigated, providing the basis for a comparative analysis of the proposed partial detection statistics. These are shown to retain the property of invariance of false alarm probability to the level of normal white noise.

Conclusion. The obtained detection and noise immunity characteristics for ultrashort and short signal samples allow us to recommend the parametric Burg harmonic mean method, implemented on the basis of a forward and backward linear prediction algorithm, as an independent signal processing method under strict restrictions imposed on the size of the analyzed sample of spatial-temporal signals.

Introduction. The use of non-equidistant pulse sequences as probing radar signals makes it possible to eliminate blind spots in speed and range. However, the implementation of multi-channel Doppler filtering (MDF) based on the classical fast Fourier transform (FFT) algorithm of non-equidistant signal samples in the signal detection problem is associated with energy losses. The use of modified FFT algorithms increases the efficiency of MDF against the background of white Gaussian noise, while reducing the efficiency of signal accumulation in the part of signal processing channels blocked by the narrow-band clutter. To eliminate this drawback, the authors previously proposed using combined classical and modified FFT algorithms. However, the use of the combined method does not lead to an optimal solution in terms of MDF efficiency.

Aim. Optimization of weight processing of non-equidistant signals to improve the efficiency of MDF.

Materials and methods. An MDF synthesis was carried out using optimization procedures, and the effectiveness of the algorithms was assessed using computer calculations.

Results. The results show that the Kaiser Bessel window with a window parameter of 4.42 provides the highest signal-(clutter+noise) ratio improvement coefficient averaged over frequency channels equal to 30.06 dB and the highest probability of correct signal detection averaged over MDF channels equal to 0.5 at processing of non-equidistant pulse sequences. Optimization of the weight processing of MDF under the specified conditions increased the average efficiency characteristics used of up to 53.18 dB and 0.92, respectively.

Conclusion. Separate optimization of weighting processing for each frequency channel can significantly improve the average efficiency characteristics of a multichannel Doppler filter and eliminate all the shortcomings of the classical and modified FFT algorithms when processing non-equidistant pulse sequences. However, these advantages are achieved at the cost of not using the FFT, i.e., implemented within the framework of the discrete Fourier transform (DFT) algorithm.

Introduction. Nanomaterials based on binary and multicomponent oxides are of interest for the development of catalysts, photocatalysts, gas sensors, solar cells, as well as in other fields. The most effective methods to produce oxide systems of various compositions are those of chemical co-deposition, as well as two-stage approaches.

Aim. To develop sensor nanomaterials based on ZnO, Zn–Fe–O, and Zn–Sn–O ternary oxide nanosystems, as well as to develop methods for assessing their properties.

Materials and methods. ZnO and ZnFe2O4 nanopowders were synthesized by chemical coprecipitation, and ZnFe₂O₄ and Zn₂SnO₄ nanostructures were produced by modifying ZnO nanowires. The surface chemical composition and microstructure were studied using scanning electron microscopy, backscattered electron diffraction, and Xray photoelectron spectroscopy. The sensor responses of the samples to vapors of organic solvents were analyzed.

Results. The response value of zinc oxide and zinc ferrite samples synthesized by chemical coprecipitation was found to be 2–4 orders of magnitude higher than that of modified zinc oxide nanowires. The formation of ternary oxide nanostructures led to an increase in the sensor response of zinc oxide nanowires. This effect can be explained by the formation of adsorption sites of various types during formation of such systems. The samples produced by chemical coprecipitation showed an extremely high sensor response. This may be due to the formation of fractal structures at the percolation threshold.

Conclusion. ZnO and ZnFe₂O₄ oxide nanostructures produced by chemical coprecipitation exhibit a high sensor response to acetone and ethanol vapors. Methods for the formation of multicomponent oxide systems with improved sensor properties compared to the original zinc oxide nanowires were developed. The resultant sensor nanomaterials are promising for use as sensitive layers of gas sensors for detecting organic solvent vapors.

Introduction. Silicon nitride is a highly promising material for fabrication of photonic integrated circuits (PICs). Plasma-enhanced chemical vapor deposition is a prospective method for large-scale industrial production of silicon nitride-based PICs. The disadvantage of this method, which limits its practical application, consists in high insertion losses in the telecommunication frequency band due to absorption on the Si–H and N–H bonds remaining from the film growth process. Thermal annealing is the most common method for breaking these bonds and reducing losses. Therefore, investigation of the impact of annealing on the optical properties of photonic integrated waveguides is an important research task.

Aim. To investigate the effect of annealing treatment on the optical properties of PICs based on the silicon nitride films with different thicknesses obtained by plasma-enhanced chemical vapor deposition.

Materials and methods. The work investigates the effect of annealing treatment on the optical properties of PICs based on the silicon nitride films with thicknesses of 200, 400 and 700 nm. To that end, the transmission characteristics of a set of test elements were measured using a high-definition component analyzer in the frequency range of 185…196 THz.

Results. Frequency dependencies of loss and coupling coefficients, as well as the group index before and after annealing were extracted from the measured transmission characteristics of the test elements. It was found that waveguides on a 200-nm-thick film exhibited higher losses in comparison with the waveguides on thicker films. The waveguides with cross sections of 900 × 400 and 900 × 700 nm2 demonstrate the losses below 5 dB in the frequency range of 185…190 THz. A rapid increase in losses due to absorption on the N–H bonds was observed at the frequencies above 190 THz. The work shows that thermal annealing reduces insertion losses across the frequency range from 185 to 196 THz. The adequacy of extracted optical parameters is confirmed by comparing theoretical and experimental transmission characteristics of the ring resonator.

Conclusion. The obtained results demonstrate that silicon nitride waveguides fabricated by the method of plasma-enhanced chemical vapor deposition require the stage of thermal annealing. Vacuum annealing at 600 °C for 30 min reduces insertion losses in the waveguides with cross sections of 900 × 400 and 900 × 700 nm² down to 4 dB/cm in the frequency band from 185 to 196 THz.