Free-space Conditions Research Articles

A Novel Target Detection and Identifying Approach Using Polarimetric Radar Cross-Section and Matrix Correlation Coefficient

This paper presents a novel target detection and identifying approach using polarimetric radar cross-section and matrix correlation coefficient. We have adopted a polarimetric radar cross-section matrix correlation strategy (PRMC) algorithm using a matrix correlation approach based on the polarimetric radar cross-section. It is projected as an inverse scattering problem under the electromagnetic scattering model using polarimetric Physical Optics approximation. The experimental measurements using canonical targets carried out under semicontrolled conditions verify the performance of the developed procedures. Finally, the identification strategies' effectiveness is demonstrated in free-space conditions and a scene with a brick and autoclaved aerated concrete wall.

A metamaterial unit-cell based patch radiator for brain-machine interface technology

This paper presents a novel approach to the design of a brain implantable antenna tailored for brain-machine interface (BMI) technology. The design is based on a U-shaped unit-cell metamaterial (MTM), introducing innovative features to enhance performance and address specific challenges associated with BMI applications. The motivation behind the use of the unit-cell structure is to elongate the electric path within the antenna patch, diverging from a reliance on the electrical properties of the MTM. Consequently, the unit cell is connected to an inset-fed transmission line and shorted to the ground. This configuration serves the dual purpose of reducing the size of the antenna and enabling resonance at the 2.442 GHz band within a seven-layer brain phantom. The antenna is designed using a FR-4 substrate (εr = 4.3 and tan δ = 0.025) of 1.5 mm thickness, and it is coated with a biocompatible polyamide material (εr = 4.3 and tan δ = 0.004) of 0.05 mm thickness. The proposed antenna achieves a compact dimension of 20 × 20 × 1.6 mm3 (0.338 × 0.338 × 0.027 λg3) and demonstrates a high bandwidth of 974 MHz with its gain of −14.6 dBi in the 2.442 GHz band. It also exhibits a matched impedance of 49.41-j1.32 Ω in the implantable condition, corresponding to a 50 Ω source impedance. In comparison to a selection of relevant research works, the proposed antenna has a low specific absorption rate (SAR) of 218 W/kg and 68 W/kg at 1g and 10g brain tissue standards, respectively. An antenna prototype has been fabricated and measured for return loss in both free space and in-vivo conditions using sheep's brain. The measurement results are found to be in close agreement with the simulation results for both conditions, showing the practical applicability of the proposed antenna for BMI applications.

Design of gradient coils based on the target field method in a high-permeability shield considering sidewall apertures

The combination of high-permeability ferrite and coils is often used to nullify ambient magnetic fields and is widely used in applications such as magnetic resonance imaging and high-sensitivity magnetic sensors. We propose the design method of transverse gradient coils based on the target field method in a high-permeability shield considering sidewall apertures (TFM-HPSCSA), which are installed in the ferrite shield to zero the residual magnetic field gradient. TFM-HPSCSA coils enable an innovative combination of magnetic field gradient linearity and structural luminescence. The coil current density distribution is set to avoid the central sidewall apertures. As a result, the contour distribution of the stream function under the optimized weight coefficients is calculated to obtain the wire alignments of the dBy/dx, dBy/dy and dBy/dz gradient coils. The deviations of the experimentally measured values from the finite element simulations of the three types coils are 0.63%, 1.17% and 0.48%, respectively, which verified the validity and the accuracy of the proposed method. In comparison to the traditional saddle gradient coil designed under the free space condition, the proposed dBy/dx coil produces a 30.5% improvement in the percentage range of regions with 5% magnetic field gradient nonlinearity. The increased linear range of the gradient magnetic field contributes to the improvement of the residual field nulling range in magnetic sensors and magnetic shielding systems in practical applications.

Evaluation of the Resolution in Inverse Scattering of Dielectric Cylinders for Medical Applications.

The inverse scattering problem has numerous significant applications, including in geophysical explorations, medical imaging, and radar imaging. To achieve better performance of the imaging system, theoretical knowledge of the resolution of the algorithm is required for most of these applications. However, analytical investigations about the resolution presently feel inadequate. In order to estimate the achievable resolution, we address the point spread function (PSF) evaluation of the scattered field for a single frequency and the multi-view case both for the near and the far fields and the scalar case when the angular domain of the incident field and observation ranges is a round angle. Instead of the common free space condition, an inhomogeneous background medium, consisting of a homogeneous dielectric cylinder with a circular cross-section in free space, is assumed. In addition, since the exact evaluation of the PSF can only be accomplished numerically, an analytical approximation of the resolution is also considered. For the sake of its comparison, the truncated singular value decomposition (TSVD) algorithm can be used to implement the exact PSF. We show how the behavior of the singular values and the resolution change by varying the permittivity of the background medium. The usefulness of the theoretical discussion is demonstrated in localizing point-like scatterers within a dielectric cylinder, so mimicking a scenario that may occur in breast cancer imaging. Numerical results are provided to validate the analytical investigations.

Computing hydrodynamic interactions in confined doubly periodic geometries in linear time.

We develop a linearly scaling variant of the force coupling method [K. Yeo and M. R. Maxey, J. Fluid Mech. 649, 205-231 (2010)] for computing hydrodynamic interactions among particles confined to a doubly periodic geometry with either a single bottom wall or two walls (slit channel) in the aperiodic direction. Our spectrally accurate Stokes solver uses the fast Fourier transform in the periodic xy plane and Chebyshev polynomials in the aperiodic z direction normal to the wall(s). We decompose the problem into two problems. The first is a doubly periodic subproblem in the presence of particles (source terms) with free-space boundary conditions in the z direction, which we solve by borrowing ideas from a recent method for rapid evaluation of electrostatic interactions in doubly periodic geometries [Maxian et al., J. Chem. Phys. 154, 204107 (2021)]. The second is a correction subproblem to impose the boundary conditions on the wall(s). Instead of the traditional Gaussian kernel, we use the exponential of a semicircle kernel to model the source terms (body force) due to the presence of particles and provide optimum values for the kernel parameters that ensure a given hydrodynamic radius with at least two digits of accuracy and rotational and translational invariance. The computation time of our solver, which is implemented in graphical processing units, scales linearly with the number of particles, and allows computations with about a million particles in less than a second for a sedimented layer of colloidal microrollers. We find that in a slit channel, a driven dense suspension of microrollers maintains the same two-layer structure as above a single wall, but moves at a substantially lower collective speed due to increased confinement.

A square ring and single split resonator based wearable antenna for Microwave energy harvesting for IoT nodes

A wearable antenna utilising metamaterial radiating elements, which operates at two frequency bands, are presented in this paper. A closed-ring resonator and a single split-ring resonator of square shape have been used to provide dual resonance. The work develops a compact microstrip patch antenna having radiating patch dimensions of 24.8 × 24.8 mm2, that generates electromagnetic energy exhibiting dual band from 2.3 to 2.5 Giga Hertz (WiMAX, ISM) and 3.4 to 3.6 Giga Hertz (WiMAX band). This work provides relevant parametric analyses, such as free space, on-body, and antenna bending effects. Microstrip antenna is designed to be built on flexible dielectric substrate named Rogers RT-5880 Material with dielectric constant 2.2 and thickness of 0.787 mm. The design is validated by a comparison of numerical and actual antenna measurements. Between simulated and measured findings, good equivalence has been discovered. The fabricated antenna provides an impedance bandwidth of 1036 Mega Hertz in lower and 352 Mega Hertz in upper-frequency range, a gain of 2.7 dB,2.35 dB, the directivity of 3 dB, 2.9 dB, the radiation efficiency of 93.32%,88.21 % at 2.4 and 3.6 Giga Hertz respectively in free space condition. The results highlight the applicability of an energy efficient dual-band proposed antenna in wearable applications and microwave energy harvesting for IoT nodes within the ISM and WiMAX bands.

Performance Comparison of Cotton and Silk Substrates on 1.575 GHz Frequency Textile Antenna

Textile antennas are currently being researched and developed so that they can be integrated into clothing. In this paper, two textile antennas with cotton and silk substrate are presented. The antennas operate at GPS L1 frequency, which is 1.575 GHz. This paper discusses the effect of antenna parameters on free space and on-body conditions, and the effect of antenna bending. Based on simulation and measurement results, the parameter values obtained indicate that the antenna can operate properly at a frequency of 1.575 GHz and is safe to use on the body.

A Bra Monitoring System Using a Miniaturized Wearable Ultra-Wideband MIMO Antenna for Breast Cancer Imaging

This paper represents a miniaturized, dual-polarized, multiple input–multiple output (MIMO) wearable antenna. A vertically polarized, leaf-shaped antenna and a horizontally polarized, tree-shaped antenna are designed, and the performance of each antenna is investigated. After designing the MIMO antenna, it is loaded with stubs, parasitic spiral, and shorting pins to reduce the coupling effects and remove the unwanted resonances. Afterward, the two-port MIMO cells are spaced by 2 mm and rotated by 90° to create three more cells. The antennas are designed using two layers of denim and felt substrates with dielectric constants of 1.2 and 1.8, and thicknesses of 0.5 mm and 0.9 mm, respectively, along with the ShieldIt™ conductive textile. The antenna covers a bandwidth of 4.8–30 GHz when the specific absorption rate (SAR) meets the 1 g and 10 g standards. Isolation greater than 18 dB was obtained and mutual coupling was reduced after integrating shorting pins and spiral parasitic loadings. A maximum radiation efficiency and directive gain of 96% and 5.72 dBi were obtained, respectively, with the relatively small size of 11 × 11 × 1.4 mm3 for the single element and final dimensions of 24 × 24 × 1.4 mm3 for the full assembly. The antenna’s performance was examined for both on-body (breast) and free space conditions using near-field microwave imaging. The achieved results such as high fidelity, low SAR, and accuracy in localization of the tumour indicate that the MIMO antenna is a decent candidate for breast cancer imaging.

Experimental comparison of absorber and conductive floor automotive near field antenna measurement systems

AbstractLarge truncated spherical near-field systems with conductive or absorbing floors are typically used in the measurement of the performances of vehicle-installed antennas. The main advantage of conductive floor systems is the ease of accommodation of the vehicle under test, but their performances are affected by the interaction with the reflecting ground floor. Instead, absorbing-based systems emulating free-space conditions minimize the effect of the interaction with the floor, but generally require longer setup times, especially at lower frequencies (70–400 MHz), where bulky absorbers are typically used to improve reflectivity levels. Considering scaled measurements of a vehicle model, the performances of these two typical implementations are analyzed in the 84–1500 MHz range and compared to free-space measurements. Absorbers with different dimensions and reflectivity have been installed in the scaled measurement setup, and measured data have been investigated with proper post-processing to verify the applicability to realistic systems. Figures of merit of interest for automotive applications, like gain and partial radiated powers, have been compared to free-space to evaluate the impact of different scenarios.

Optimization of the Measurement Technique for Emissions in Reverberation Chamber Using the Equivalence Principle

This paper presents an optimization of a method to reconstruct the radiated emissions of an equipment under test by the measurement of the electric field samples collected on the walls of a reverberation chamber. This means that only the orthogonal component of the electric field is necessary to obtain the radiative behavior of the device in free space conditions. The use of the equivalence principle allows one to reduce the number of equivalent sources used to reconstruct the radiation of the device. In fact, in the previous version of the method, the sources are placed into the entirety of working volume of the reverberation chamber. In the current version of the method, only the surface surrounding the equipment under test is discretized. The analytical implementation of the method is proposed for a particular stirring action: the multiple monopole source stirring technique. This technique is based on an array of monopoles placed onto the walls of the cavity, and therefore no further hardware is needed for the reconstruction of the radiated emissions. The method is experimentally validated in a real scenario.

Measurement and Prediction of Received Signal Level and Path Loss through Vegetation

This paper presents the measurement and prediction of received signal level and path loss through vegetation. Results were estimated under free space, single tree, and vegetation conditions which revealed that the presence of isolated trees along a radio path can affect signal propagation leading to reduction in signal strength (attenuation). The attenuation was found to be dependent on many factors and parameters of the trees e.g. Geometry of measurement, (either trunk or canopy path), state of trees foliation, frequency, canopy thickness among others. In the measured data, high loss values were recorded at canopy geometry which is due to high presence of interacting and attenuating elements at the canopy. Also, high variation in Received signal strength (RSS) was noted at canopy geometry. However, the trend shows variation from path to path. The main factor is the density of tree parameters along the chosen path. Depending on the density of tree parameters along the chosen path, the depth dependence may be extremely slow as seen in the measure data. With this evidence, for radio communication inside woodlands or forests, localizing the two nodes inside the vegetation will certainly give overall best performance in terms of signal impairment. The propagation mechanism in each case are the same at the antenna geometries used following the trend of signal decay. Different transmit antenna inclination angles were used and this has not shown any significant contribution to the excess loss. However, antenna height with respect to the trees canopy showed significant contribution to the excess loss. This information will be useful to network planning engineers in link budget estimation.

Ground-state phase diagram of ultrasoft bosons

In two-dimensional bosonic systems ultrasoft interactions develop an interesting phenomenology that ultimately leads to the appearance of supersolid phases in free space conditions. While suggested in early theoretical works and despite many further analytical efforts, the appearance of these exotic phases as well as the detailed shape of the ground-state phase diagrams have not been established yet. Here we develop a variational mean-field calculation for a generic quantum system with cluster-forming interactions. We show that by including the restriction of a fixed integer number of particles per cluster the ground-state phase diagram can be obtained in great detail. The great detail includes the determination of coexistence regimes of crystals of different occupancy as well as crystals with superfluid phases. To illustrate the application of the method we consider the softened van der Waals potential, for which the phase diagram is known via quantum Monte Carlo simulations. For densities other than that corresponding to the single-particle crystal, our results show very good quantitative agreement with the simulations regarding the location of the superfluid transitions. Additionally, the phase diagrams suggest that the solid-superfluid coexistence could be a reliable marker to locate supersolidity.

Sparsity‐based autoencoders for denoising cluttered radar signatures

Narrowband and broadband indoor radar images significantly deteriorate in the presence of target dependent and independent static and dynamic clutter arising from walls. A stacked and sparse denoising autoencoder (StackedSDAE) is proposed for mitigating wall clutter in indoor radar images. The algorithm relies on the availability of clean images and corresponding noisy images during training and requires no additional information regarding the wall characteristics. The algorithm is evaluated on simulated Doppler-time spectrograms and high range resolution profiles generated for diverse radar frequencies and wall characteristics in around-the-corner radar (ACR) scenarios. Additional experiments are performed on range-enhanced frontal images generated from measurements gathered from a wideband RF imaging sensor. The results from the experiments show that the StackedSDAE successfully reconstructs images that closely resemble those that would be obtained in free space conditions. Further, the incorporation of sparsity and depth in the hidden layer representations within the autoencoder makes the algorithm more robust to low signal to noise ratio (SNR) and label mismatch between clean and corrupt data during training than the conventional single layer DAE. For example, the denoised ACR signatures show a structural similarity above 0.75 to clean free space images at SNR of -10dB and label mismatch error of 50%.

Mechanism of colluvial landslide induction by rainfall and slope construction: A case study

The landslide hazards occurring in the complex geological genesis accumulation body are usually controlled by the coupling action of many internal and external factors. Therefore, this paper takes the dam-front Danbo accumulation body landslide of Yangfanggou hydropower station on the Yalong River as the geological prototype, and discusses the process and mechanism of slope stability degradation under the combined action of rainfall and slope construction. Based on the detailed understanding of the basic characteristics of the accumulation body, the development characteristics of the landslide and the construction situation of the slope engineering, the study conducted correlation analysis between rainfall and landslide displacement, the physical and mechanical tests of all types of rock-soil masses, and the numerical simulation testing of seepage field variation of the landslide section. It is found that the special slope structure and material composition of the old landslide accumulation layer on the upper part of the Danbo accumulation body are the internal factors for the occurrence of thrust load-induced landslide, and the construction of the slope engineering not only creates free space conditions for sliding, but also provides channels for the infiltration of rainfall into the slope after confluence, which is an external factor that caused the mechanical properties of the sliding zone soil to gradually weaken from the trailing edge to the leading edge. The geomechanical model of such landslide is that the active section of the trailing edge produces the “source of force”, the transition section of the middle section affects the occurrence of sliding, and the anti-sliding section of the leading edge controls the occurrence of landslide hazards. The results of this research provide not only a useful supplement to the theory of landslide formation mechanisms but also a scientific basis for guiding the prevention and control of similar hazards.

High efficient dual band stacked antennas integrated into rescue helmets for indoor communication

AbstractA dual-band stacked antenna integrated with a rescue helmet is proposed for WLAN applications. The cross slot aperture feeding technique enables the antenna to support dual-band operation at 2.4 and 5 GHz. On the ground plane, two slots are formed, perpendicular to each other and of different lengths. Four dielectric layers of different permittivity are stacked over this cross slot to attain the desired frequency bands. Under free space condition, the radiator yields 7% (2.31–2.48 GHz), 15.87% (5.12–6 GHz) 10 dB return loss bandwidth (BW). The use of low loss dielectric layers on the slots also provides a high antenna efficiency and moderate gain at both frequency bands. The small dimension of antenna encourages its use as a wearable helmet antenna. Antenna performances are observed under wearable condition. For assessing human exposure to RF electromagnetic fields, SAR evaluations are conducted at both 2.4 and 5 GHz WLAN bands.

Influence Analysis of mm-Wave DUT Mounting Fixture in 5G OTA Measurement

All of the test cases in the current version of TR 38.810 in 3GPP and CTIA millimeter-Wave (mm- Wave) test plan are limited to the free space configuration. However, the truly free-space condition does not exist for mm-Wave testing of 5G user equipment since no device can float in the air. Mounting fixtures and supporting structures are needed to fix the device under test (DUT) and move it in two axes. The influence of mounting fixture on 5G mm-Wave wireless device performance is analyzed in this paper. First, a common 4×1 patch array at 28 GHz is simulated as the performance baseline. Various mounting fixture thickness & various spacing between the DUT and the mounting fixture are simulated to get a quick understanding of the mounting fixture’s influence on DUT performance. In different configurations, the working frequency of the antenna would have different degrees of deviation, while the gain could decrease or increase accordingly. Then, to explain these phenomena, an equivalent circuit is extracted utilizing the transmission line theory. Finally, according to the findings, it is recommended that the thickness of the mounting structure should be equal to an integer time of 0.5 λg to minimize the impact of the fixture for practical mounting structure design.

Dynamics and decay of a spherical region of turbulence in free space

Abstract

Updated deterministic radiation transport for future deep space missions

NASA's deterministic transport code HZETRN, and its three-dimensional (3D) counterpart, 3DHZETRN, are being used to characterize the space radiation environment over a wide range of scenarios, including future planned missions to the moon or Mars. Combined with available spaceflight measurements, these tools provide the fundamental input for risk models used to quantify possible astronaut health decrements and satisfy agency limits in support of exploration initiatives. Further research is therefore needed to improve radiation transport and nuclear physics models while at the same time continuing to expand the available measurement database (ground-based and spaceflight) to validate such efforts. In this work, significant updates to the deterministic radiation transport models are presented. Charged muons and pions are fully coupled with the existing solutions developed for neutron and light ion (Z ≤ 2) transport. This update includes the 3D nature of pion production as well as the pion interactions, resulting in further production of energetic nucleons within shielding. Additional updates related to low energy proton recoils in hydrogenous materials and capture/decay processes associated with charged pions at rest are also described. Included in this work is the coupling of single and double-differential cross sections from Geant4 into HZETRN and 3DHZETRN. This enables a direct comparison of deterministic and Monte Carlo transport methodologies using the same nuclear databases for specific interactions. Comparisons between Geant4 and 3DHZETRN are shown and establish that the transport methodologies are in excellent agreement when the same cross sections are used. The deterministic codes are also compared to ISS data, and it is found that the updated 3D procedures are within measurement uncertainty (±5%) at cutoff rigidities below 1 GV, which approaches free space conditions.

Dual-Band Accelerating Beams Enabled Full Duplex Free-Space Optical Interconnection

High speed free-space optical interconnection (FSOI) is an effective strategy to overcome the bottleneck of short-reach electronic interconnection from chip scale to board level, due to its wide bandwidth, high energy efficiency, low latency and high interconnect density. Here, we propose an accelerating beam enabled reconfigurable FSOI scheme by harnessing its characteristics of diffraction-free, self-healing, and specifically designed trajectory. Then we experimentally demonstrate a proof-of-concept 100 Gbps full duplex FSOI enabled by dual-band 2D Airy beams over 40 cm free-space optical link with a receiver sensitivity of -17.3 dBm at bit error ratio (BER) of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-9</sup> . Both O-band and C-band 2D Airy beams can be successfully generated and detected with the help of two C-band spatial light modulators (SLMs). In addition, we experimentally investigate the impact of an obstacle occurring at the free-space optical link on the BER performance. Our experimental results indicate that, the performance of accelerating beam based FSOI is superior to the conventional Gaussian beam enabled FSOI under the same free-space optical link condition.

Caracterização arbórea da principal praça de Almenara, Minas Gerais

A arborização urbana é imprescindível para um melhor conforto da cidade. Contudo, existem aspectos que devem ser levados em consideração para que atinja todo seu potencial ambiental e social, como escolha de espécies e técnicas adequadas de plantio. O presente trabalho objetivou analisar a composição florística e a estrutura fitossociológica das árvores da praça Dr. Hélio Rocha Guimarães, em Almenara (MG), visando a colaborar com informações que possam auxiliar no correto planejamento e gestão da arborização da praça. Na pesquisa, foram levantados dados como número de árvores com padrão de altura mínima de dois metros, nome comum, nome científico, família, origem, porte, condição fitossanitária, além de aspectos dendrométricos e podas sofridas, condição do espaço livre da planta e do sistema radicular, e espaçamento entre árvores. Foram encontradas 47 árvores entre nove diferentes espécies, das quais 55,6% de origem exótica. Quanto ao número de árvores, a predominância é de nativas (72,3%); dentre elas, a Licania tomentosa (Benth.) Fritsch ou oiti (55,3%). As árvores são em sua maioria de grande porte (87%), com média de altura total de 12 m, ocupando área média de copa de 132,6 m2 e espaçamento entre si de 9,46 m. A praça apresentou um conforto do ponto de vista ambiental, considerando os aspectos estéticos e de conforto térmico. Contudo, deficitário em termos de diversidade biológica e, considerando que as árvores possuem bom espaçamento entre si, a mobilidade dos seus frequentadores fica comprometida apenas em relação aos danos causados por afloramento de raízes em alguns locais.