Frequency Dependence Research Articles

Volume Structure Retrieval Using Drone-Based SAR Interferometry with Wide Fractional Bandwidth

Synthetic aperture radar (SAR) interferometry (InSAR) is a well-established remote sensing technique capable of providing accurate topographic information of an area. Multiple scattering occurring at different heights within a single resolution cell, however, cannot be resolved using a single baseline and results in a degradation of the height accuracy. Techniques such as polarimetric InSAR and SAR tomography tackle this problem using additional measurements to obtain the three-dimensional structure of the volume. However, polarimetry-based methods assume orthogonal and deterministic scattering mechanisms and tomography requires numerous observations. This paper presents a novel approach to retrieving the three-dimensional structure of semitransparent media using wide fractional bandwidth InSAR. The frequency dependency of the InSAR coherence is exploited to retrieve the three-dimensional structure, assuming that signals with wide fractional bandwidth are available, as is the case for unmanned aerial vehicles (UAVs) or drones. As the extinction is, in general, frequency-dependent, a third observation may improve the inversion results. Simulations of different scenarios show that the frequency profile can be inverted to obtain information about the three-dimensional scattering structure. The acquired data also enable retrieval of the phase center depth as a function of frequency. This method allows for frequent and accurate monitoring of semitransparent targets, e.g., forests and vegetation, using a swarm of two or three drones.

Mathematical modeling of microwave channels of a semi-active radar homing head

Objectives. Radar homing heads of guided missiles form a large group of radars which differ from other radars due to their specific purpose. The advantages of a semi-active radar homing head (SARH) include the ability to have a powerful irradiator at the command post and, as a result, a powerful reflected signal from the target. This results in an increase in the range of its detection and guidance. The absence of an emitter simplifies the missile’s onboard control equipment, reduces its weight and dimensions, thereby improving its maneuverability and increasing the guidance accuracy, resulting in the greatest distribution of this type of SARH. However, in order to determine the exact Doppler shift of the target signal as part of SARH, a reference signal with a frequency coinciding with the illumination transmitter signal must be supplied to the receiving path. This study aims to synthesize and analyze the SARH receiver circuit with improved accuracy characteristics.Methods. The following methods are used: statistical radio engineering; optimal signal reception theories; and computer modeling in CAD AWR Design Environment.Results. A mathematical model of the SARH receiver was obtained and analyzed. The proposed receiver model allows the spectral characteristics of signals to be calculated at any point of the microwave paths, as well as signal characteristics at the input of the head channel, at the output of the first conversion mixer, at the output of the first intermediate frequency selector, and at the output of the receiving path. The calculated values of the main characteristics of high-frequency channels are also given.Conclusions. The resulting model allows the frequency dependencies of main parameters of the receiving path, such as the gain factor, noise factor, decibel compression points, and third-order intermodulation intercept points to be estimated. The values obtained during the simulation are maximally close to existing systems, since the models of real-life and mass-used microcircuits thu created are used as the main elements when designing high- frequency paths. The model can be used to study methods of improving technical indicators, as well as to develop new principles and schemes for developing radioelectronic complexes, for example, when designing a receiving path using advanced radio photonics.

Variety of attenuation of RF suppression filters

Differential mode attenuation of RF suppression filters can be measured directly. Alternatively, this attenuation can be derived as modal S-Parameters from the S-Parameters measured for the single multiport paths which in fact the filter is. This paper outlines the differences between the two approaches. The first consists in direct measurement with separation transformers and the second with calculation from single ended measurement. In addition, the paper presents the method of determining the differential mode attenuation for the case of input and output impedance different from those used in direct measurements, i.e. by 50Ω/50Ω. At the same time, the presented method of determining the differential mode attenuation for different input and output impedances will require a change in the assumptions of the CISPR 17 document entitled ”Methods for measuring the attenuation characteristics of passive EMC filtering devices”, since the modal S-Parameter formulas presented in this document are only valid for the case when the RF suppression filter is in perfect balance. Otherwise, for different impedances it should be done taking into account the modification of the modal S-Parameters, as shown in this paper.The inclusion of this innovative change in the CISPR 17 document will allow the approach presented in this document to be used for various cases of the RF suppression filter, which is undoubtedly a significant progress in determining its attenuation.Frequency span by measurement of attenuation of RF Suppression filter is spanned from 10 kHz to 30 MHz.The comparison presented in this paper confirms that the S parameters are identical only if the filter is in equilibrium (ZL=ZN).From the results, three frequency areas can be distinguished. In the first interval between 10 kHz and 150 kHz the filter is in balance, which was demonstrated with identical measurement results. Between 150 kHz and 1.5 MHz, the attenuation is superimposed on the dynamics of the measurement system and therefore on the random frequency dependence of both measurements. Above 1.5 MHz there is a clear filter asymmetry, so the curves move away from each other.

Superparamagnetic effects in the linear magnetic response of polydisperse ensembles of nanoparticles suspended in a fluid.

Within a kinetic theory, the linear magnetic response of uniaxial single-domain particles suspended in a fluid is analyzed. The main qualitatively different types of frequency dependence of the longitudinal dynamic magnetic susceptibility of such particles are described. It is shown that superparamagnetic (related to orientation thermal fluctuations of the magnetic moment inside a particle) peculiarities of the response of a particle to a probing magnetic field are not fully determined by the ratio of anisotropy energy to thermal energy when a stationary bias field is applied. For a case where the indicated ratio is much greater than one, a simple approximate expression for the dynamic magnetic susceptibility of a particle is proposed. The developed approach is extended to polydisperse suspensions of noninteracting uniaxial nanoparticles. It is shown that polydispersity does not vanish away specific superparamagnetic features in the dynamic magnetic response of such systems. Quantitative estimates of the corresponding effects are performed in different frequency ranges of the applied field. It is demonstrated that under certain restrictions on the disperse composition of a suspension, the internal diffusion of the magnetic moment can lead to a splitting of the absorption spectrum of the system. The significant role of the bias field is revealed. In particular, it can cause an additional absorption maximum provided the particle-size distribution meets the outlined condition. Also, it enables one to assess how important it is to take into account superparamagnetism of particles: the effect of the biasing is stronger for particles with smaller anisotropy and thereby more pronounced superparamagnetic properties. A qualitative agreement of some of the inferences with the experimental data is briefly discussed.

Influence of design and annealing parameters on measurements of AC properties of NiFe-SiO2 multilayer structures

In this article, due to the exceptionally interesting electrical and magnetic properties of nanocomposites, which include ferromagnetic materials in a dielectric matrix, multilayer NiFe-SiO2 nanocomposites were obtained. The entire sequence of technological processes using magnetron sputtering equipment in the presence of argon was presented, the aim of which was to create structures with different numbers and thicknesses of layers, as well as in two measurement configurations. Measurements of AC electrical properties were carried out in a variable temperature range, i.e. from room temperature to 373 K, using the 4-point method. The obtained measurement results were compared in terms of design parameters, such as thickness and number of layers, as well as technological parameters, such as heating duration. Structural analysis was performed using scanning electron microscopy (SEM) to investigate the effect of oxidation on the multilayer structures and to confirm the granular structure of the nanocomposite. Additionally, the chemical composition of the obtained multilayers was examined using Energy-dispersive X-ray spectroscopy (EDX). Resistive parameters were tested in the range of magnetic field induction from 0 T to 0.3 T. Based on the conductivity relationships, it was proven that a hopping mechanism of current conduction occurs in the tested samples. Using the frequency dependencies of the phase shift angle, it is possible to adjust the equivalent RC electronic circuit, thereby determining the potential application of the tested materials.

Frequency-Dependent Microelectrophoresis Study of Colloids with Tunable Surface Charge.

Nonaqueous poly(methyl methacrylate) (PMMA) colloidal suspensions in a solvent that is simultaneously matched in both density and refractive index have been important for real-space studies of colloidal self-assembly, but their complex electrostatic character remains largely unexplored. Electrophoresis is a powerful tool for determining the surface potential and charge of the colloidal suspension; however, because of refractive index matching, standard electrophoresis measurements are not feasible. We carry out microscope-based microelectrophoresis measurements on PMMA colloids in cyclohexyl bromide and cis-trans decalin to measure particle charge as a function of salt concentration in both DC and frequency-variable AC fields. The colloid charge depends on salt concentration and reverses sign near 0.35 μM, providing evidence that solution ions are actively modifying the colloid surface. The frequency dependence of the electrophoretic mobility yields the characteristic time scale for electric double-layer polarization and provides intriguing evidence for Manning condensation and polyion formation.

A lattice pairing-field approach to ultracold Fermi gases

We develop a pairing-field formalism for ab initio studies of non-relativistic two-component fermions on a (d+1)-dimensional spacetime lattice. More specifically, we focus on theories where the interaction between the two components can be described by the exchange of a corresponding pairing field. The introduction of a pairing field may indeed be convenient for studies of, e.g., the finite-temperature phase structure and critical behavior of, e.g., ultracold atomic Fermi gases. Moreover, such a formalism allows to directly compute the momentum and frequency dependence of the pair propagator, from which the pair-correlation function can be extracted. For a first illustration of the application of our formalism, we compute the density equation of state and the superfluid order parameter for a gas of unpolarized fermions in (0+1) dimensions by employing the complex Langevin approach to surmount the sign problem.

Acoustic echo cancellation based on two‐stage BLSTM

AbstractAcoustic echo cancellation (AEC) methods aim to suppress the acoustic coupling for hands‐free speech communication. Traditional AEC works by identifying the acoustic impulse response using adaptive algorithms. With recent research advances, deep learning has become an attractive choice for AEC. This paper introduces a two‐stage bidirectional long short term memory (TS‐BLSTM) framework, incorporating multi‐head self‐attention mechanisms after each BLSTM block. This is aimed at better capturing contextual information and further enhancing ability of the model to handle complex acoustic scenarios. The BLSTM blocks are utilized to aggregate magnitude spectrum information, modelling both time and frequency dependencies. Additionally, dilation convolution is introduced to broaden the range of information in each convolution output. The magnitude decoder estimates a mask for the input, resulting in the generation of an estimated magnitude spectrum for near‐end speech. Experimental results indicate that the proposed method achieves promising outcomes.

Electrical characterization of Ag/MoO3−x/p-Si Schottky diodes based on MoO3−x synthesized via sol–gel method: an investigation on frequency and voltage dependence

MoO3−x is a commonly used buffer layer in organic based optoelectronic devices to align energy level between active semiconductor and metal layer. The purpose of this study is to show the effects of the MoO3−x interlayer by comparing with a reference device without MoO3−x interlayer. To evaluate the effect of MoO3−x interlayer synthesized by sol–gel method, basic important parameters such as ideality factor (n), barrier height (∅\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\varnothing$$\\end{document}B), interface states (Nss) and series resistance (Rs) of Ag/MoO3−x/p-Si and Ag/p-Si Schottky diodes were calculated by electrical characterization methods. The current–voltage (I–V) measurements show that the ideality factor for the p-Si/Ag reference diode was decreased from 2.4 to 1.9 while rectification factor increasing 44.6 times using MoO3−x interlayer, and frequency–dependent measurements (C–V–f, G–V–f) were carried out to elucidate this deviation at room temperature for MoO3−x-based diode. From the XPS analysis, it was seen that Mo+6 and Mo+5 oxidation states were intense on the surface of the MoO3−x film, and the Mo+4 oxidation state increased as it went into the bulk. The changes in Rs and Nss due to the energy levels formed by the Mo+6 and Mo+5 oxidation steps at the interface are depicted. In addition, the energy density distribution profile of Nss was obtained using the I–V characteristics for various forward bias voltages range from 2.1 × 1012 to 2.5 × 1012 eV−1 cm−2. Based on the experimental results, the sol–gel synthesized MoO3−x thin film exhibits favorable rectification characteristics and holds promise for application as a Schottky diode.

Multifrequency Very Long Baseline Interferometry Imaging of the Subparsec-scale Jet in the Sombrero Galaxy (M104)

We report multifrequency and multiepoch very long baseline interferometry studies of the subparsec jet in the Sombrero galaxy (M104, NGC 4594). Using Very Long Baseline Array data at 12, 22, 44, and 88 GHz, we study the kinematics of the jet and the properties of the compact core. The subparsec jet is clearly detected at 12 and 22 GHz, and the inner jet base is resolved down to ∼70 Schwarzschild radii (R s) at 44 GHz. The proper motions of the jet are measured with apparent subrelativistic speeds of 0.20 ± 0.08c and 0.05 ± 0.02c for the approaching and the receding jet, respectively. Based on the apparent speed and jet-to-counterjet brightness ratio, we estimate the jet viewing angle to be larger than ∼37°, and the intrinsic speed to be between ∼0.10c and 0.40c. Their joint probability distribution suggests the most probable values of the viewing angle and intrinsic speed to be 66°−6°+4° and 0.19 ± 0.04c, respectively. We also find that the measured brightness temperatures of the core at 12, 22, and 44 GHz are close to the equipartition brightness temperature, indicating that the energy density of the radiating particles is comparable to the energy density of the magnetic field in the subparsec jet region. Interestingly, the measured core size at 88 GHz (∼25 ± 5 R s) deviates from the expected frequency dependence seen at lower frequencies. This may indicate a different origin for the millimeter emission, which can be explained by an advection-dominated accretion flow (ADAF) model. This model further predicts that at 230 and 340 GHz, the ADAF may dominate the radio emission over the jet.

Preparation of Three Component PVDF‐Bi4V2O11‐SDS Composites with Enhanced Dielectric and Electrical Properties

AbstractIn this work, poly(vinylidene fluoride) (PVDF)‐bismuth vanadate (Bi4V2O11; BVO)‐sodium dodecyl sulfate (SDS) composite films are fabricated by the solution casting method. The frequency dependence of the dielectric and electric performances of the resultant PVDF‐BVO‐SDS composites on various weight percentages of SDS contents is investigated. The results demonstrate that the three‐component PVDF‐BVO‐SDS composites have a superior dielectric constant (96) and suppressed dielectric loss (<1) at 102 Hz for 20 wt% of SDS contents. The surface morphology of the PVDF‐BVO‐SDS composites shows that SDS is successfully reinforced on the PVDF‐BVO matrix with better homogeneity. The ac conductivity of the PVDF‐BVO‐SDS composites is also improved, which is helpful for the enhancement of the dielectric constant and relatively low loss in the composites. Moreover, the PVDF‐BVO‐SDS composite also exhibits a larger value of the dielectric constant (96) than the pristine PVDF matrix, which is 15 times higher than that of the pristine one. These significantly enhanced dielectric and electrical characteristics of PVDF‐BVO‐SDS composites will fulfill the practical criteria for their use in high dielectric‐constant capacitors and energy storage devices.

Application of continuous shear wave elastography method with multiple frequency selection to liver viscoelasticity measurement

In this study, we present a method for evaluating liver viscoelasticity using continuous shear wave elastography with an arbitrary frequency selection. In the Voigt model, viscosity depends on the frequency of shear waves, thus allowing viscosity evaluation by combining measurements at different frequencies. The shear wave frequency must satisfy the continuous shear-wave elastography frequency conditions. Therefore, the shear-wave frequency was selected as an integer fraction of the pulse repetition frequency of the ultrasound device. Low-viscosity phantoms were used in this study. The frequency dependence of shear-wave velocity was confirmed using a viscous phantom. The shear wave velocity dispersion measured using the proposed method was 5.41–8.98 m/s/kHz for four healthy liver volunteers, which are similar to that of a healthy liver reported by magnetic resonance elastography (5.76–7.52 m/s/kHz). It has been demonstrated that the frequency dependence of shear wave propagation can be measured.

Accurate numerical analysis of resonances in random waveguides: Effects of the waveguide length

In this study, the transmission frequency dependence of the random waveguide, which is the waveguide having a randomly corrugated boundary, on waveguide length is investigated using an accurate numerical method. While resonances in the low-frequency region are caused by the interference among the dominant modes, those in the high-frequency region are caused by the mode conversion from the dominant mode to the second mode. Furthermore, their characteristics have not been investigated in detail. Numerical analysis shows that the resonant frequencies in the high-frequency region are nearly independent of the length of the random waveguide within the numerical error. It indicates that random waveguides can be regarded as a stack of independent resonators in high-frequency regions.

Verification of effect of interference between multiple scatterers on the evaluation of backscattering coefficient

Backscatter coefficient analysis methods for biological tissues have been clinically applied but assume a homogeneous scattering medium. In addition, there are few examples of actual measurement studies in the HF band, and the consistency with theory has not been sufficiently confirmed. In this paper, the effect of correlations among scatterer positions on backscattering was investigated by performing experiments on inhomogeneous media having two types of scattering source with different structural and acoustic properties. In the echo data of phantoms containing two types of scatterer acquired by multiple sensors, the power and frequency dependence of the backscatter coefficient were different from theoretical calculations due to the interference effects of each scatterer. The effect of interference between the two types of scatterer was confirmed to be particularly strong for echoes acquired by the sensor at high intensity and HF, or for a higher number density of strong scatterers.

Exploring Dielectric and Magnetic Properties of Ni and Co Ferrites through Biopolymer Composite Films

This study explores the synthesis and characterization of chitosan/gelatine films incorporating nickel ferrite (NiFe2O4) and cobalt ferrite (CoFe2O4) nanoparticles. The magnetic nanoparticles exhibit superparamagnetic behaviour, making them attractive for various applications, including biomedical uses. The X-ray diffraction analysis confirmed the successful synthesis of NiFe2O4 and CoFe2O4 nanoparticles, and the scanning electron micrographs illustrated well-dispersed ferrite nanoparticles within the biopolymer network, despite the formation of some aggregates attributed to magnetic interactions. Magnetization loops revealed lower saturation magnetization values for the composites, attributed to the chitosan/gelatine coating and the dielectric studies, indicating increased dielectric losses in the presence of ferrites, particularly pronounced in the case of NiFe2O4, suggesting interactions at the interface region between the polymer and ferrite particles. The AC conductivity shows almost linear frequency dependence, associated with proton polarization and conduction processes, more significant at higher temperatures for samples with ferrite particles.

Modeling the Morphology of Fast Radio Bursts and Radio Pulsars with fitburst

We present a framework for modeling astrophysical pulses from radio pulsars and fast radio bursts (FRBs). This framework, called fitburst, generates synthetic representations of dynamic spectra that are functions of several physical and heuristic parameters; the heuristic parameters can nonetheless accommodate a vast range of distributions in spectral energy. fitburst is designed to optimize the modeling of features induced by effects that are intrinsic and extrinsic to the emission mechanism, including the magnitude and frequency dependence of pulse dispersion and scatter broadening. fitburst removes intrachannel smearing through two-dimensional upsampling, and can account for phase-wrapping of “folded” signals that are typically acquired during pulsar-timing observations. We demonstrate the effectiveness of fitburst in modeling data containing pulsars and FRBs observed with the Canadian Hydrogen Intensity Mapping Experiment telescope.

Piezoelectric and Pyroelectric Properties of Organic MDABCO-NH4Cl3 Perovskite for Flexible Energy Harvesting

This study describes the synthesis and characterization of the lead-free organic ferroelectric perovskite N-methyl-N’-diazabicyclo [2.2.2]octonium)-ammonium trichloride (MDABCO-NH4Cl3). The electrospinning technique was employed to obtain nanofibers embedded with this perovskite in a PVC polymer for hybrid fiber production. The dielectric, piezoelectric, and pyroelectric properties of these fibers were carefully examined. Based on measurements of the dielectric permittivity temperature and frequency dependence, together with the pyroelectric results, a transition from a high temperature paraelectric to a ferroelectric phase that persisted at room temperature was found to occur at 438 K. The measured pyroelectric coefficient yielded values as high as 290 μC K−1 m−2, which is in between the values reported for MDABCO-NH4I3 and the semiorganic ferroelectric triglycine sulfate (TGS). The hybrid nanofibers exhibited good morphological characteristics and demonstrated very good piezoelectric properties. Specifically, a piezoelectric coefficient of 42 pC/N was obtained when applying a periodical force of 3 N and a piezoelectric voltage coefficient of geff = 0.65 V mN−1. The performance of these fibers is on par with that of materials discussed in the existing literature for the fabrication of nano energy-harvesting generators. Importantly, the perovskite nanocrystals within the fibers are protected from degradation by the surrounding polymer, making them a promising environmentally friendly platform for flexible mechanical energy harvesting.

FEATURES OF MAGNETIC OSCILLATIONS IN A TWO-LAYER STRUCTURE WITH EXCHANGE COUPLING

Graphs of time and frequency dependences of the components of magnetic oscillations of the layers of a planar two-layer structure and their portraits of oscillations were constructed. The excitation of the structure layers for different interlayer coupling coefficients was carried out by a microwave magnetic field of circular polarization. The interlayer bond is characterized by an additional local energy minimum, the spatial position of which depends on the coupling coefficient.

Frequency-dependent ferroelectric and electrocaloric properties in barium titanate-based ceramics based on Maxwell relations

In this work, the frequency dependence of ferroelectric and electrocaloric properties in barium titanate-based ceramics was studied based on Maxwell relations. It is found that the maximum and remnant polarization will decrease while the coercive field increases a lot with rising frequency from 0.1 to 10[Formula: see text]Hz, indicating that polarization rotation and domain switching become difficult at high frequencies. The electrocaloric properties show the different frequency dependence at different phase structures. Isothermal entropy change ([Formula: see text]) and adiabatic temperature change ([Formula: see text]) are similar around/above Curie temperature ([Formula: see text], showing tiny frequency dependence. However, [Formula: see text] and [Formula: see text] display the obvious frequency dependence below [Formula: see text], especially in the orthorhombic–tetragonal phase-transition region with a stable ferroelectric phase, and this frequency dependence becomes more obvious under a low-electric field. It is also found that increasing the frequency can weaken the electric field dependence of electrocaloric strength. This work gives a general profile of frequency dependence for electrocaloric properties in ferroelectric ceramics.

New insights into early-stage time-dependent dielectric characteristics coupled with frequency dependency and hydration-driven reaction of hydraulic cement pastes: Experiments and statistical modelling

In the field of cement and concrete materials engineering, a comprehensive understanding of dielectric properties is crucial for optimising hydraulic cement paste mix designs. This study examines the relationships among the dielectric constant (ε’), loss tangent (tan δ), frequency, hydration time, water content and admixture dosage in different compositions of hydraulic cement paste. A consistent pattern can be observed across the frequency spectrum, where dielectric constants decrease as frequency increases. This aligns with the expected behaviour of cementitious materials. The most notable finding is the inversely proportional relationship between the loss tangent and frequency. As the frequency increases, the loss tangent consistently decreases, indicating improved energy retention and reduced dissipation. This characteristic is of significant value for various applications, including sustainable infrastructure and smart materials. Furthermore, the interactions among hydration time, water content and admixture dosage present an opportunity for dynamic optimisation within the cement paste matrix. These factors collectively impact dielectric properties, providing insights into the potential for tailored material performance and durability, especially in applications where dielectric properties are critical, such as sensing applications and structural health monitoring of concrete structures. This study emphasises the importance of customising cementitious compositions for specific applications in cement and concrete materials engineering. It achieves this through a detailed comparative analysis of different compositions, each representing varying water content and admixture dosage levels. This approach has the potential to significantly transform the field by enabling engineers to create hydraulic cement pastes with customised dielectric properties that meet the specific requirements of construction, infrastructure and other related applications.