The article presents the results of searching for relationships between the distribution type and content of multiwall carbon nanotubes (MWCNTs) in a polymer matrix and the values of permittivity of a composite material. Two types of composites are considered: a) with uniform distribution of MWCNTs in the volume of a polystyrene matrix (PS) and b) with nanotubes distributed in the volume of composites in the form of 3D interconnected cells surrounding PS spheres deformed by compression. It is shown that at mass concentrations of MWCNTs up to 6% and their uniform distribution in the volume of the composite, the Clausius–Mossotti model adequately describes the concentration dependence of the real parts of permittivity. The frequency dependences of the real parts of permittivity have a universal character of a power function with an exponent equal to −0.5 for all materials. The frequency dependences of the imaginary parts of permittivity are also power law. However, the exponent is not universal, as in the case of real parts of permittivity, but varies from −0.5 for composites with a uniform distribution of MWCNTs to −0.8 for composites with a packing of nanotubes on the surface of PS spheres.

For millimeter waves applications in the W-band 93.5-95.5 GHz, improved frequency synthesizers are highly demanded. Improved frequency synthesizer should be tunable synthesizer with high step resolution and possess high phase noise performance, low spurious harmonic levels and short locking time. These parameters are conflicting parameters in developing a high precision frequency oscillator with a pure spectrum. Therefore, it's necessary to make a compromisation between them. In this paper we present a frequency synthesizer based on a Gunn diode, two stable finely tuned frequency sources, a phase locked loop, and a 10th order subharmonic mixer in the feedback loop is built. The phase noise, spurious levels, and tuning step size of the phase locked loop generator are then analyzed, and the effect of each parameter on the others is introduced. To test the performance of the proposed frequency synthesizer using a spectrum analyzer, the output spectrum of the intermediate frequency was measured on a subharmonic mixer, which corresponds to the spectrum of a phase-locked loop generator. The results show that the proposed phase-locked loop generator can be tuned in steps of less than 1 Hz, and the first two spurious harmonics are far from the fundamental harmonic. These results can be used for designing an enhanced high precision frequency synthesizer with a low level of spurious harmonics and high phase noise performance in millimeter waves applications such as CW RADAR.

There are three main configurations for high-performance frequency synthesizers in millimeter waves i.e., traditional phase-locked synthesizer, harmonically-injected phase-locked loop synthesizer, and frequency-multiplication-based frequency synthesizer. In this work, we conduct a comparative analysis and simulation of these three configurations. The comparison is made in terms of phase-noise performance, tuning step resolution, frequency switching speed, maximum power, maximum frequency range, level of unwanted signals, and cost. Based on the results, the best configuration can be selected according to the needed requirements.

An experimental study has been carried out to model and to measure the resonance frequency shift introduced by inserting a small microwire segment in an open resonator. A hybrid frequency synthesizer based on a Gunn diode was used to perform measurements. The open resonator was excited using this hybrid frequency synthesizer together with a Schottky diode detector used to measure a resonance curve. The results have shown a small difference between the model and the measured frequency shifts of the small microwire segment using the hybrid frequency synthesizer and the open resonator in the 3 mm wavelength band.

Cold limb injury remains a serious and widespread condition both in countries with a cold climate and in regions located close to the equator, but having high-altitude territories. There is no medical equipment for the treatment of this condition. One of the ways to solve this problem is the use of microwave radiation at an early stage, which penetrates deep into the cooled volume and, accordingly, can accelerate the activation of internal vessels. The technical implementation of this approach involves the selection of the appropriate frequency and power of radiation, as well as the creation of a microwave chamber in which it is possible to ensure a sufficiently uniform heating of the entire volume of the frostbitten limb. The results of modeling the distributions of electromagnetic and thermal fields in the volume of the heated limb at microwave frequencies allowed for medical applications are presented.

In this work, the parameters of antireflection interference coatings based on alternating layers of ZnS/Al2O3 on the laser-induced damage threshold (LIDT) of ZGP crystals under the action of Ho:YAG laser radiation at a wavelength of 2.097 μm were determined. The coating deposition was carried out using the ion-beam sputtering method. The LIDT of the sample with a coating based on alternating layers ZnS and Al2O3 was equal to WoE = 3.45 J/cm2, and the LIDT of the uncoated sample was equal to WoE = 2.23 J/cm2. An increase in the optical breakdown threshold by ~55% was observed after the deposition of an AR coating based on ZnS and Al2O3 materials. An assumption was made about the absence of local fluctuations in the composition and mechanical stresses in the case of the coated sample, namely that this leads to good adhesion of the multilayer coating to the polished surface of the crystal, and as a result to an increase in the optical breakdown threshold as compared to the uncoated sample due to closure of the dangling chemical bonds and bulk defects emerging on the polished surface.

In this study, a new method is proposed for producing polymer composites via the adsorption of multiwalled carbon nanotubes (MWCNTs) on the surface of polystyrene spheres (PS) to provide a cellular distribution of MWCNTs in the composite. The method makes it possible to control the cell size down to the submicron level and the conductivity of the composite in a wide range. The effect of the MWCNT concentration on the surface of PS on the structure of the resulting composites and their electrophysical properties are studied in the frequency range of 115 GHz–1.4 THz. The percolation nature of the composite conductivity was established. It was shown that the obtained composite is a regularly chaotic medium, and its regularity scale corresponds to the sizes of the PS. MWCNTs on the PS surface form two subsystems: the first is a pseudo-regular subsystem on the edges of cuboid-like PS, while the second subsystem consists of chaotically scattered MWCNTs with unclosed ends on the faces of PS. The first subsystem belongs to the regular part of the structure and mainly determines the properties of the medium at frequencies above 500 GHz. The second subsystem of nanotubes is characterized by a small electrodynamic scale comparable with the size of one PS cell and determines the effective properties of the composites at frequencies below 500 GHz.

This article describes the performance of a backward wave oscillator, stabilized with phase locked loop. The backward wave oscillator is locked to the frequency of a direct digital synthesizer using an automatic phase locked loop. The direct digital synthesizer is playing the role of tuning part in the suggested frequency synthesizer. Mathematical evaluations of the stability, phase noise and locking speed of the suggested frequency synthesizer are obtained. The optimization technique of frequency synthesizers parameters is presented. This allows to get the maximum response speed and the lowest phase noise and level of spurious with fine tuning (less than 1 Hz). It was shown that the order and type of loop filter in the frequency synthesizer will affect simultaneously the response speed, phase noise and spurious level. The results of this paper shows that the frequency synthesizer can be used in small samples properties measurement using open resonators.

The creation of new composite materials with specified radio-frequency properties is often carried out using small-sized inclusions, while the polarizability parameters of these small bodies in electric and magnetic fields play a significant role. The paper considers the possibility of measuring the polarizability of small components of the composite in an open quasi-optical resonator. It is shown that the values of the electric and magnetic polarizability of small particles can be estimated from the shifts of the resonant frequencies that arise when a small scatterer is placed in the resonator.

The mechanism of formation of a terahertz jet by a dielectric cuboid and a sphere surrounded by ideally conducting screens is considered. The maximum screen influence is observed when the screen is located near the alight cuboid base. The screen influence eases when the screen is shifting along the dielectric object. Power flux density localization area is almost completely shifted inside the object when the screen is situated in the center. The minimum influence is observed when screen placed in the shadow plane of the cuboid base. This effect caused by the screen influence on electric field component tangential to the side edges and thus on the power flux directed to the central axis of the object. The screen influence on the terahertz jet in spherical object has been compared. Value of the power flux density after passing the object is higher in a cuboid, but the focusing characteristics are better (appearing on shorter distances) in a sphere. (C) 2020 Society of Photo-Optical Instrumentation Engineers (SPIE)

The paper presents the modeling of the reflectance in the terahertz range from the plane-parallel aerogel layers formed by multi-walled carbon nanotubes (MWCNTs), which were synthesized via the original one-step technique using a preformed Fe–Co/CaO catalyst at 650–700°C in C2H4/Ar feedstock. With an increase in frequency from 100 GHz to 1.5 THz, the reflectance decreases by more than a factor of two. Based on the experimental data and the well-known wire medium model formed by a three-dimensional lattice of intersecting conductors, a model of a regular wire medium is constructed using MWCNTs with effective electrophysical characteristics equivalent to those of an irregular aerogel medium. This model reveals that a significant drop in the reflectance in the terahertz frequency range occurs because the plasma frequency of the equivalent medium is located here. The model allows estimating the properties of the aerogel environment in the frequency range extending up to 5 THz and with variations in the packing density of MWCNTs in aerogel. Estimates of the reflection, transmission, and absorption are presented for the case of normal EMI incidence on thin (submillimeter) layers of such a medium.

The paper discusses the possibilities of using 3D-printing technology to create radar absorbing coatings and elements of electronic equipment. The frequency dependences of the relative changes in the dielectric constant of a filament for a 3D printer measured at an open resonator are presented for 1-3% nanotube concentrations in the acrylonitrile butadiene styrene in the range of 8-14 GHz. By the quasi-optical method, the relative changes in the dielectric constant in the range 115-258 GHz were measured. The electromagnetic response for shield in the EHF range was calculated.

Results of research of lateral scattering of electromagnetic radiation by aerogel of MWCNTs are presented. Frequency dependences of lateral scattering of THz radiation of spherical MWCNTs aerogels with diameter of 4.5 and 6 mm at frequency range 43-970 GHz are given.

Abstract In the paper quasi-optical resonant single-frequency flaw detector for investigation of heterogeneity of 3D printer polymer filament are described. A scheme of quasi-optical flaw detector based on 10 GHz open resonator with a filament broaching mechanism is given. Industrial filaments based on polystyrene, polyethylene terephthalate glycol and acrylonitrile styrene acrylate were tested in a flaw detector. The measured amplitudes at the maximum of the resonance curve in the presence of a filament in the inside of the resonator are presented.

Numerical modeling is used to study the distribution of the electric field excited by microwave radiation in a rectangular chamber in which a two-layer dielectric cylindrical phantom is placed whose electrophysical parameters correspond to muscle and fat tissues of human extremities. The electric field in the fat layer is shown to be noticeably higher than in the muscle layer and is inhomogeneous in both layers. Measurements have demonstrated that due to heat transfer, the resulting temperature inhomogeneity in the phantom volume is noticeably weaker than the electric field inhomogeneity. Thus, despite the significant skin effect and the presence of weak standing waves, a fairly uniform heating of the phantom is possible.

Results of research of scattering of electromagnetic radiation by aerogel of MWCNTs in a state of acoustic levitation are shown. A probe method for measuring the diameter of THz beams based on a spherical aerogel MWCNTs in the state of acoustic levitation is proposed. Comparison of frequency dependents of extinction coefficient of MWCNTs aerogel with ideal conductive ball at the range 63-124 GHz is presented.

This article presents the measurements of the reflectance from a plane‐parallel aerogel sample formed by multiwalled carbon nanotubes (MWCNTs) in the terahertz range. With an increase in frequency from 100 GHz to 1.5 THz, the reflectance decreases by more than a factor of two. Based on the experimental data and the well‐known wire medium model formed by a three‐dimensional lattice of intersecting conductors, a model of a regular wire medium is constructed using MWCNTs with effective electrophysical characteristics equivalent to those of an irregular aerogel medium. This model reveals that a significant drop in the reflectance in the terahertz frequency range occurs because the plasma frequency of the equivalent medium is located here. The model allows estimating the properties of the aerogel environment in the frequency range extending up to 5 THz and with variations in the packing density of MWCNTs in aerogel. Estimates of the reflection, transmission, and absorption are presented for the case of normal incidence on a thin (submillimeter) layer of such a medium. The aerogel absorption at a given conductivity of CNTs is determined by their packing density.

Results of approbation of model of thin dielectric cylinder in open quasioptical resonator to diagnostic filamentary dielectric objects are presented. Comparison of calculated frequency shift depending on filaments diameter with the results of experimental researches in open resonator in the frequency range 80-131 GHz was carried out. Its spectral characteristics with samples of polymer filaments are investigated.

The method of measurement of reflection coefficient behind an orifice in one of reflectors of the quasioptical open resonator is considered. According to the applied mathematical model, the method has the restriction which is consist in use of a short-wave approximation for the orifice size (it dimension is much more than a wavelength). For evaluation of limits of applicability of this method the experimental setup in the threecentimetric range of wavelengths consist of the quasioptical open resonator and the system of positioning of the measuring probe is created. By the experimental study of distribution of fields behind an orifice in the reflector of the open resonator it is found that the method, except restriction of mathematical model, has also restrictions, due to the wave’s interference, reflected from edges of this orifice. It causes a deviation from the uniform amplitude distribution of the field in an opening that can lead to increase in an error of a reflection coefficient measuring. It is also shown that by decrease of the sizes of an orifice formation of a bunch with small divergence, but with significantly more expressed distribution of the field localized about a bunch axis in an aperture is possible. It can be used for diagnostics of inhomogeneity of samples with higher locality.

A highly sensitive method of contactless microwave diagnostics is considered that allows to examine the properties of small objects of different shapes and permittivities and small local inhomogeneities of artificial (composite) and natural materials. The method is based on multiple interactions of radio waves with an examined object in an open quasi-optical resonator. It can be used in a wide frequency range, from units of gigahertz to several tens of terahertz, unachievable by other methods.The capabilities of the method are illustrated by detection of local nanostructured inhomogeneities of super thin threadlike wires under a glass insulation layer, measurement of polarizabilities of small spherical aerogels from carbon nanotubes, technological diagnostics of thin film thickness, and measurement of inhomogeneities of the reflection coefficient of sheet composites.