Low Reflection Coefficient Research Articles

Monte Carlo simulation of low energy neutron reflection characteristic of Al/B4C composite material

To systematically analyze the neutron reflection characteristics of boron carbide reinforced with aluminium (Al/B4C) composite material, the neutron shielding calculation model of Al/B4C was established. Monte Carlo method had been used to simulate the low energy neutron transmission and reflection coefficients of Al/B4C composite with different mass content of B4C and different thickness of composite material. The results show that the neutron reflection coefficient of Al/B4C increases rapidly with the increase of composite shielding thickness and then tends to saturate when the shielding thickness increases to a certain value. The neutron saturation reflection coefficient and the corresponding saturation reflection thickness both decreases with the increase of B4C mass content in the composite material. The smaller the interaction cross section of the neutron with the material is, the greater the neutron saturation reflection coefficient of the material is, and the greater the saturation thickness required reaching the saturation reflection. In the energy range of 200 eV-20 keV, in addition to the resonance energy point, the neutron reflection coefficient increases with the increase of the neutron energy. The increase of the neutron reflection coefficient per unit of energy increases firstly and then decreases with the increase of energy. When the B4C mass content in the composite material is at the range of 10%-40%, the 100 eV neutron reflection coefficients are below 10%, and the 10 keV neutron reflection coefficients are above 25%.

Freeze-Dried Carbon Nanotube Aerogels for High-Frequency Absorber Applications

A novel technique for millimeter wave absorber material embedded in a metal waveguide is proposed. The absorber material is a highly porous carbon nanotube (CNT) aerogel prepared by a freeze-drying technique. CNT aerogel structures are shown to be good absorbers with a low reflection coefficient, less than -12 dB at 95 GHz. The reflection coefficient of the novel absorber is 3-4 times lower than that of commercial absorbers with identical geometry. Samples prepared by freeze-drying at -25 °C demonstrate resonance behavior, while those prepared at liquid nitrogen temperature (-196 °C) exhibit a significant decrease in reflection coefficient, with no resonant behavior. CNT absorbers of identical volume based on wet-phase drying preparation show significantly worse performance than the CNT aerogel absorbers prepared by freeze-drying. Treatment of the freeze-dried CNT aerogel with n- and p-dopants (monoethanolamine and iodine vapors, respectively) shows remarkable improvement in the performance of the waveguide embedded absorbers, reducing the reflection coefficient by 2 dB across the band.

Design of MIMO Antenna for Ultra-Wideband Applications

ABSTRACTIn this paper, a shorted planar elliptical metal antenna (SPEMA) is designed for ultra-wideband (UWB) applications. The shorting strip yields almost 20% size reduction and the impedance bandwidth for S11 < −10 dB of the SPEMA covers the entire UWB (3.1–10.6 GHz). The surface current distribution of the proposed antenna is observed. Two symmetrical antennas are placed very closely for multiple-input multiple-output (MIMO) applications. The proposed MIMO antenna exhibits low reflection coefficient (< −10 dB), low mutual coupling (< −13 dB), and low correlation coefficient (<0.02) in UWB range. The absolute peak gain and radiation patterns in E and H plane of the MIMO antenna are presented. Measured results are in good agreement with the simulated results.

A High Linearity High Gain Double Balanced Down Conversion Mixer Using Current Bleeding for Mode 1 of UWB System

This work presents high linearity and high gain CMOS down conversion Gilbert cell mixer for ultra wide band application, designed in 0.18𝜇𝑚 technology. Inductive source degeneration is used at the RF trans-conductance stage of Gilbert cell mixer to improve the linearity of the mixer. Differential LC matching is used at RF and LO stage and current bleeding is used to increases the conversion gain and linearity of the mixer. Output buffer is used at the load to achieve 50 ohm impedance match. The proposed mixer shows low reflection coefficient less than –10 dB for entire band 1, band 2, and band 3 ranges from 3.168 GHz to 4.752 GHz frequency. The proposed mixer works at DC supply of 1.5 volt for low power consumption. Simulation results shows that the mixer achieves the conversion gain of 11.03 dB, 1 dB compression point of -14.65 dB, IIP3 is 0.257 dBm, input return loss is –24.292 dB and measured matched RF (input) port impedance of 48.79 ohm.

Electromagnetic absorption behaviour of ferrite loaded three phase carbon fabric composites

This article investigates the electromagnetic absorption behaviours of carbon helical yarn fabric reinforced composites and manganese–zinc (Mn–Zn) ferrite particles loaded 3 phase fabric composites. A carbon helical yarn having stainless steel core was prepared and made into single jersey knitted fabric. The composite was prepared by sandwiching a fabric with polypropylene films and thermal pressed. The absorption values of helical yarn fabric composite was observed to be less in the C band region (4–8 GHz). For improving the absorption coefficients of composite, Mn–Zn ferrite particles were dispersed in the polypropylene (PP) composite. The ferrite loaded PP composites exhibited better permittivity and permeability values, hence the absorption loss of the composite was improved. The helical yarn fabric reinforced with Mn–Zn ferrite/PP composite showed larger absorption coefficients than virgin PP/fabric composite. The change in thermal stability and particle size distribution in the Mn–Zn ferrite/PP composite was also analyzed. At higher ferrite concentration, bimodal particle distribution was observed which increased the conductivity and shielding effectiveness (SE) of the composite. In addition, complex permittivity value was also increased for higher incident frequency (4–8 GHz). As the ferrite content increases, the dielectric loss and magnetic permeability of PP/ferrite increases due to increased magnetic loss. Hence, ferrite loaded PP composite showed the total SE of −14.2 dB with the absorption coefficients of 0.717. The S1C7 fabric composite having ferrite dispersion showed the better absorption loss and lower reflection coefficient of 14.2 dB and 0.345 respectively compared to virgin PP/helical yarn fabric composite. The increasing ferrite content (45 wt%) improved the absorption loss and total SE. Though, ferrite based fabric composite exhibits moderate absorptive shielding, it can be used as shielding panels in the electronic industries.

2D Gain Enhancement Based on Single-Layer SIW Corrugated Technique with Slot Array Feeder

Novel slot-array-based SIW corrugated technique is demonstrated to achieve 2D gain enhancement, namely, sharpening the beamwidths in both E-plane and H-plane. Compared to other metallic corrugated methods to realize 2D gain enhancement, the proposed design, with SIW grooves to reduce the beamwidth in E-plane and slot array to increase the directivity in H-plane, has a lower profile, weight, and design complexity, which can be easily fabricated with the common printed circuit board (PCB) technique. A prototype is designed and fabricated, with measurement presenting a low reflection coefficient less than −10 dB from 26.4 GHz to 28.2 GHz and an enhanced gain up to 18 dB. Overall, our proposed technique will be beneficial for the design of high-gain antenna in 5G wireless terminals.

Design of Radar Absorbing Structures Utilizing Carbon-Based Polymer Composites

Radar absorbing structure (RAS) is a composite laminate with a low reflection coefficient for the electromagnetic illumination in microwave frequency range, and thereby can be used in the stealth technology and electromagnetic compatibility (EMC). In this study, microwave absorbing properties of a two-layer composite laminate (carbon black impregnated rubber sheet attached to the carbon fiber–epoxy composite panel) has been investigated. Complex permittivity and permeability of the composite materials were measured in C- and X-band frequencies (4–12 GHz) by reflection/transmission technique using a coaxial waveguide and network analyzer. Complex permittivity can be controlled with the amount of carbon black in the rubber composite. High values of dielectric constant and dielectric loss are observed in the carbon fiber composite. Optimization of microwave absorption is conducted for the two-layer RAS on the basis of transmission line theory. It is found that microwave absorption is strongly sensitive to carbon black content in the rubber composite and its layer thickness. For the rubber sheet containing 10% carbon black (with dielectric constant ≍5), the maximum microwave absorption (30 dB) is predicted at 10 GHz.

Absorbing boundary layers for spin wave micromagnetics

Micromagnetic simulations are used to investigate the effects of different absorbing boundary layers (ABLs) on spin waves (SWs) reflected from the edges of a magnetic nano-structure. We define the conditions that a suitable ABL must fulfill and compare the performance of abrupt, linear, polynomial and tan hyperbolic damping profiles in the ABL. We first consider normal incidence in a permalloy stripe and propose a transmission line model to quantify reflections and calculate the loss introduced into the stripe due to the ABL. We find that a parabolic damping profile absorbs the SW energy efficiently and has a low reflection coefficient, thus performing much better than the commonly used abrupt damping profile. We then investigated SWs that are obliquely incident at 26.6°,45° and 63.4° on the edge of a yttrium-iron-garnet film. The parabolic damping profile again performs efficiently by showing a high SW energy transfer to the ABL and a low reflected SW amplitude.

Electromagnetic microwave absorption properties of carbon nanocoils/tissue

An economic electromagnetic wave absorber with carbon nanocoils (CNCs)/tissue and paraffin has been successfully synthesized and exhibits excellent microwave absorption property. The CNCs/tissue composite was obtained by growing spring-like CNCs on the tissue substrate with Fe2(SO4)3/SnCl2 catalyst in a chemical vapor deposition method. It is found that the absorbers containing 10wt% and 20wt% CNCs/tissue show strong microwave absorption in a frequency range of 2–18GHz. For the composite with 10wt% CNCs/tissue, the maximum reflection loss (RL) of −46.36dB at 14.72GHz and the bandwidths of 7.44 and 5.68GHz respectively corresponding to RL below −10 and −20dB can be obtained. In addition, the composite with 20wt% CNCs/tissue can also gain broad bandwidth corresponding to reflection loss below −10dB, which is as high as 7.52GHz. It is revealed that the composite with 10wt% CNCs/tissue owns suitable real part of permittivity, which helps to obtain better impedance match and lower reflection coefficient to enhance the absorption of electromagnetic wave.

Improved Design and Microfabrication of ${H}$ -Plane and ${E}$ -Plane Loaded Rectangular Slow-Wave Structure for THz TWT Amplifier

This paper reports further investigation on a recently proposed ${H}$ -plane and ${E}$ -plane loaded slow-wave structure (SWS) for terahertz traveling-wave tube (TWT) amplifier. An improved input–output coupler is designed to enable easy fabrication by microfabrication technology, UV-lithography, electroplating, and molding (LIGA), and deep reactive-ion etching. The coupler shows very low reflection coefficient, ${S}_{11} dB over a frequency range 360–450 GHz. To improve the beam-wave interaction and enhance the saturated output power of the TWT, the SWS with tapered design is implemented. By tuning the period, the wave is resynchronized with the beam at the end of the SWS, resulting in more than 60% increase in the saturated output power across the 80-GHz bandwidth. In addition, the sensitivity of the output power of the TWT to the fabrication tolerance of the individual geometrical parameters is also studied in detail. It is found that the output power reduces by 80% for approximately 2% variation in the synchronized beam voltage. The KMPR-based UV-LIGA technique is adopted to fabricate the ${H}$ -plane and ${E}$ -plane loaded SWS. The measured lateral dimensional accuracy within $\sim 2~\mu \text{m}$ and rms roughness of metal sidewall surface less than 80 nm is obtained.

Improvement of Transmission Distance by Using Ferrite in Superconductive Wireless Power Transfer

The wireless power transfer (WPT) characteristics between the transmitter and receiver coils according to the existence of ferrite were analyzed in this study to improve the transmission distance of superconductor resonance coils. To set the optimal reflection coefficient, the thickness increased by 1 mm, from 0 to 10 mm. It was found that the transfer efficiency improved with the lowest reflection coefficient at the thickness of 2 mm. The 2-mm-thick ferrite was applied to the transmitter coil, and the reflection coefficient was measured by the distance. As a result, the transfer efficiency with ferrite was highest at the distance of 2500 mm. Also, it was confirmed that the WPT characteristics were improved up to 500 mm by the application of ferrite. It is believed when the superconductive WPT system is organized using such advantageous ferrite, its transmission distance and efficiency improvement can be secured.

Low-Cost Wind-Insensitive Ultra-Wideband Balloon Antenna for Seismic Telemetry Systems

This letter presents a low-cost wind-insensitive ultrawideband conformal balloon antenna for wireless seismic telemetry applications. The proposed design consists of conformal conductive strips attached on two balloon surfaces, a lightweight three-dimensional printed bi-conical feeding structure, and a third large balloon for shielding protection and buoyancy enhancement. Experimental characterization of a fabricated prototype has demonstrated the balloon antenna exhibiting low reflection coefficients, stable quasi-omnidirectional patterns, and flat realized gain over a very wide bandwidth from 103 to 384 MHz. A scaled propagation experiment based on the balloon antenna was performed under a weather condition with wind-force scale of four to five. The received power by a virtual seismic acquisition unit is almost independent of its azimuthal location, and a statistical probability of over 95% was achieved for the power variation less than 5 dB.

Elastic mismatch between ePTFE and PLLA vascular grafts in relation to femoral and carotid arteries in humans: in vivo, in vitro and in silico assessment

Many factors have been held responsible for the elevated failure rates of vascular substitutes. Several studies have shown that the lower the mismatches regarding mechanical or functional properties between the native vessel and its vascular substitute, the lower the hemodynamic alterations. From a theoretical point of view, the best substitutes have similar characteristics to the tissue or organ to be replaced. In the present work, samples of electrospun nanofibrous Poly(L-lactic acid) (PLLA) and expanded polytetrafluoroethylene (ePTFE) vascular grafts were subjected to pulsated pressure conditions using an experimental setup. Graft dynamic response was obtained from experimental measurements applying an ultrasonic gold standard method (to measure graft diameter) and a high frequency response pressure transducer (to obtain intraluminal pressure). Similarly, femoral and carotid arteries of normotensive male control subjects and ambulatory male patients with mild to moderate hypertension were explored in vivo in order to relate their mechanical performance to that observed in the synthetic grafts. The obtained results verified the existence of important biomechanical differences between vascular grafts and human arteries. Under normal physiological conditions, the pressure-strain modulus (106dyn/cm2) of ePTFE and PLLA (1095 ± 21.5 and 12.4 ± 1.47, respectively) resulted significantly higher (p < 0.05) than the carotid and femoral modulus values (4.47 ± 1.36 and 5.9 ± 1.42, respectively). During the hypertensive condition (higher pressure levels) ePTFE and PLLA increased their elasticity values to 1136 ± 21.6 and 15.97 ± 2.6, respectively, with respect to an increase of 9.27 ± 3.59 and 15.70 ± 5.96 of carotid and femoral arteries. Additionally, the elastic mismatch generated by the interposition of both grafts between native arteries (carotid or femoral) was quantified by means of computational simulations. PLLA tubular structures manifested a mechanical behavior that mimics the response to pulsatile hypertensive pressure regimes (the lowest reflection coefficient was observed) which constitutes a highly desired property for vascular tissue-engineered scaffold production.

Low‐profile antenna for millimetre‐wave chip‐to‐chip communication

Here, the authors report a low-profile antenna for millimetre-wave chip-to-chip communication. The height of the proposed antenna is limited to 0.18 mm, 1/20 wavelength at 80 GHz. To realise such a low profile, a zeroth-order resonator antenna with a periodic array of four unit cells was employed, and its geometry was optimised using a full-wave simulation tool to cover an 8 GHz bandwidth from 76 to 84 GHz. With this, the antenna was capable of radiating in the end-fire direction parallel to the board length despite the short distance between the ground and the radiator. Simulation and measurement results showed that the optimised design had low reflection coefficients and consistent radiation patterns throughout the target bandwidth.

Wave interaction with a partially reflecting vertical wall protected by a submerged porous bar

This study gives an analytical solution for wave interaction with a partially reflecting vertical wall protected by a submerged porous bar based on linear potential theory. The whole study domain is divided into multiple sub-regions in relation to the structures. The velocity potential in each sub-region is written as a series solution by the separation of variables. A partially reflecting boundary condition is used to describe the partial reflection of a vertical wall. Unknown expansion coefficients in the series solutions are determined by matching velocity potentials among different sub-regions. The analytical solution is verified by an independently developed multi-domain boundary element method (BEM) solution and experimental data. The wave run-up and wave force on the partially reflecting vertical wall are estimated and examined, which can be effectively reduced by the submerged porous bar. The horizontal space between the vertical wall and the submerged porous bar is a key factor, which affects the sheltering function of the porous bar. The wave resonance between the porous bar and the vertical wall may disappear when the vertical wall has a low reflection coefficient. The present analytical solution may be used to determine the optimum parameters of structures at a preliminary engineering design stage.

Higher-order brick-tetrahedron hybrid method for Maxwell's equations in time domain

We present a higher-order brick-tetrahedron hybrid method for Maxwell's equations in time domain. Brick-shaped elements are used for large homogeneous parts of the computational domain, where we exploit mass-lumping and explicit time-stepping. In regions with complex geometry, we use an unstructured mesh of tetrahedrons that share an interface with the brick-shaped elements and, at the interface, tangential continuity of the electric field is imposed in the weak sense by means of Nitsche's method. Implicit time-stepping is used for the tetrahedrons together with the interface. For cavity resonators, the hybrid method reproduces the lowest non-zero eigenvalues with correct multiplicity and, for geometries without field singularities from sharp corners or edges, the numerical eigenvalues converge towards the analytical result with an error that is approximately proportional to h2p, where h is the cell size and p is the polynomial order of the elements. For a rectangular waveguide, a layer of tetrahedrons embedded in a grid of brick-shaped elements yields a low reflection coefficient that scales approximately as h2p. Finally, we demonstrate hybrid time-stepping for a lossless closed cavity resonator, where the time-domain response is computed for 300,000 time steps without any signs of instabilities.

Crystallization Mechanism of CVD Si3N4–SiCN Composite Ceramics Annealed in N2 Atmosphere and Their Excellent EMW Absorption Properties

To tailor a new electromagnetic wave (EMW) absorbing material with lower reflection coefficient (RC) and larger operating frequency band, the CVD Si3N4–SiCN composite ceramics were prepared from SiCl4–NH3–C3H6–H2–Ar system and then annealed at the temperatures of 1400–1700°C in N2 atmosphere. Effect of the annealing temperatures on the microstructure, phase composition, permittivity, and microwave‐absorbing properties of the ceramic were investigated. Results showed that the CVD Si3N4–SiCN ceramics gradually crystallized into nanosized SiC grains, Si3N4 grains and graphite (T ≤ 1600°C), and then the grains grew up at T = 1700°C. The permittivity, dielectric loss, and electrical conductivity of as‐annealed CVD Si3N4–SiCN ceramics (T ≤ 1600°C) increased firstly due to the formation of conductivity and polarity network and the increase in nanograin boundary, and then decreased at 1700°C because of the growth of nanograins and the disappearance of nanograin boundary. The minimal RC and effective absorption bandwidth of the as‐annealed CVD Si3N4–SiCN ceramic at 1600°C was −41.67 dB at the thickness of 2.55 mm and 3.95 GHz at the thickness of 3.05 mm, respectively, demonstrating that the totally crystallized CVD Si3N4–SiCN ceramic (T = 1600°C) had the superior microwave‐absorbing ability.

A 6:1 Bandwidth, Low-Profile, Dual-Polarized Ring Array of Spiral Antennas With Connecting Arms

In an earlier work, the design of a 2:1 bandwidth dual-polarized phased array of Archimedean spiral antennas with cavity was presented. The reflection coefficient, circular polarization, and grating lobes goals were obtained independently in the design process. The low end of the final bandwidth was constrained to the frequency cutoff of the spiral antenna, at the low end of its bandwidth. In this communication, an extension of the bandwidth of the array is achieved by connecting the spiral antennas of the array. Hence, with the same size, the bandwidth is enlarged almost three times (a total of 6:1). The measurements confirm the simulations. The array can scan up to $\theta=30^\circ$ keeping a low reflection coefficient, good right- and left-hand circular polarization, low-sidelobe levels, and using a low-profile cavity.

Key comparison SIM.EM.RF-K5b.CL: scattering coefficients by broad-band methods, 2 GHz–18 GHz — type N connector

The first key comparison in microwave frequencies within the SIM (Sistema Interamericano de Metrología) region has been carried out. The measurands were the S-parameters of 50 ohm coaxial devices with Type-N connectors and were measured at 2 GHz, 9 GHz and 18 GHz. SIM.EM.RF-K5b.CL was the identification assigned and it was based on a parent CCEM key comparison named CCEM.RF-K5b.CL. For this reason, the measurements standards and their nominal values were selected accordingly, i.e. two one-port devices (a matched and a mismatched load) to cover low and high reflection coefficients and two attenuators (3dB and 20 dB) to cover low and high transmission coefficients. This key comparison has met the need for ensuring traceability in high-frequency measurements across America by linking SIM's results to CCEM. Six NMIs have participated in this comparison which was piloted by the Instituto Nacional de Tecnología Industrial (Argentina). A linking method of multivariate values was proposed and implemented in order to allow the linking of 2-dimensional results.KEY WORDS FOR SEARCHMain textTo reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/.The final report has been peer-reviewed and approved for publication by the CCEM, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

Composite light-absorbent coatings based on hollow oxide microspheres and lead sulfide

This paper describes the formation of various optically absorbent, thermally insulating composite materials based on hollow oxide microspheres embedded in a variety of polymer matrices, with lead sulfide as an absorbing pigment. It is shown that the coatings developed have a low reflection coefficient (2%–10%) in the visible and near infrared.