Full Wave Method Research Articles

Modified Soret Lenses for Dual-Band Integrated Detectors at Millimetre and Submillimetre Wavelengths

This article presents the comparison of different modified Soret lenses suitable for a millimetre and submillimetre-wave dual-band integrated pixel detector. The approach is based on the modification of a printed planar Soret lens, designed to operate in the sub-mm range, to obtain an antenna in the millimetre region. Three modifications of a transmission-mode Soret Lens at 840 GHz based on spiral, logarithmic, and meander antennas geometries have been analyzed with a combination of Kirchhoff's diffraction and full-wave simulation methods. The performance of the designs has been experimentally demonstrated in the submillimetre-wave band, showing good agreement with the simulation results.

Effect of substitutional Cu atoms on the electronic and optical properties of KCl: A DFT approach

The electronic and optical properties of KCl and Cu doped KCl (KCl:Cu) were calculated within full potential augmented plane wave method based on Density Functional Theory (DFT), as implemented in WIEN2k software. Calculations were carried out using two energy functional approximations, the exchange-correlation functional of Perdew, Burke and Ernzerhof (PBE96) to optimize the KCl lattice parameter, and the modified Becke-Johnson (mBJ) exchange potential in combination with the local density approximation (LDA) for correlation, to obtain the band structure. The energy bands reveal the insulating properties of KCl with a direct band gap of 8.6 eV, and the semiconductor nature of KCl:Cu with a direct band gap of 3.4 eV. From the analysis of the total and partial density of states, it is apparent that the KCl and KCl:Cu states at the Fermi level arise from Cl p states and Cu d states, respectively. Also, the optical properties reported here (dielectric function and absorption spectra) show that Cu atoms are responsible of electronic excitations occurring from Cu d valence bands to conduction bands of Cu, Cl and K with p character.

Modifications on parametric models for distributed scattering centres on surfaces with arbitrary shapes

Distributed scattering centre (DSC) is a particular type of scattering centres which has the strongest scattering amplitude among all scattering centres. Furthermore, their distributed signatures in radar images have directly shown geometries of targets. Therefore, DSCs play an important role in target recognition. DSCs are currently described by the attributed scattering centre (ASC) model, which is developed out of the asymptotic expansion solutions to canonical geometries. However, rather than canonical geometries, real radar targets have various geometric structures, which show different signatures in radar images. In order to characterise these various signatures, the parametric model is modified. Scattered fields from planar and single-curved surfaces with arbitrary shapes are studied in this study and motivated by which, the parametric models of DSCs for more general structures are presented. To validate these models, the scattered waves and the post-imaging results simulated by these models are compared with those obtained by a rigorous full-wave numerical method, and those obtained by the conventional ASC model. The comparison results demonstrate these models have a higher accuracy over the conventional model in simulations of scattered waves, as well as inverse synthetic aperture radar image signatures of different geometric structures.

A Wide-Angle Broadband Converter: From Odd-Mode Spoof Surface Plasmon Polaritons to Spatial Waves

A wide-angle broadband converter from odd-mode spoof surface plasmon polaritons (SPPs) to spatial waves is proposed using spatially gradient Vivaldi-shaped flaring structure. We first show the dispersion, Eigen mode, and effective circuit topology of the designed SPP structure and show that it can support a propagating odd mode. A conversion structure between odd-mode spoof SPPs and spatial mode has been designed to realize smooth impedance and wavenumber transition. Through the full-wave simulation method, we obtained the optimized geometric parameters, from which we fabricate the converter using the print circuit board technology. The measured results from both far field and near field show that the proposed structure has successfully excited odd-mode spoof SPPs coming from an extremely wide-angle spatial radiation. The allowed azimuthal and pitched angle ranges are larger than 72°.

Solution box: SOLBOX-18: Near-field scattering from electromagnetically large boxes

High-frequency techniques [1] are excellent tools for analyzing electrically large structures, especially if their geometries do not possess complex interactions, e.g., those caused by resonances. However, as the technology (both in terms of software and hardware) develops, it becomes possible to solve increasingly large problems via full-wave solvers without resorting to approximations and assumptions. Specifically, full-wave methods enable the analysis of electrically large objects with more direct applications of Maxwell's equations, while fundamental mechanisms, such as diffractions, reflections, and ray-like behaviors, are expected to be revealed as simulation results rather than being introduced as initial assumptions. Such full-wave solvers not only supply reliable simulations for arbitrarily complex structures, but also support high-frequency techniques by providing reference results for further analyses.

Parametric Scattering Center Model for Canonical and Composite Dielectric Objects

The scattering center (SC) model provides a concise, explicit description of the object electromagnetic characteristics, significantly improves the efficiency of radar signal simulations, and plays an important role in model-based automatic target recognition. The existing models are mainly developed for conducting objects, and few are developed for dielectric ones. The SC models of dielectric objects are essentially different from those of their conducting counterparts due to the different scattering mechanisms. In this article, the backscattering mechanisms of canonical dielectric structures are studied, and their corresponding SC models are proposed. More importantly, the parametric SC models of the composite objects of canonical structures are first presented. These SC models are validated by comparing the radar cross section (RCS), the time–frequency representation (TFR), and the high-resolution range profile (HRRP) from the SC model with the results of the full-wave numerical method.

Permittivity Spectrum of Low-Loss Liquid and Powder Geomaterials Using Multipoint Reentrant Cavities

Permittivity is a useful tool to characterize the composition and quality of many geomaterials. In general, the non-resonant permittivity measurement methods exhibit a higher degree of uncertainty than their resonant counterparts. In resonant measurements, the reduction in uncertainty comes typically with a loss in broadband. This article describes the theory, design, and application of multipoint coaxial reentrant resonant cavities applied to low-loss geomaterials at different temperatures. Specifically, a full-wave method based on circuit analysis is developed and applied for a circular corrugated waveguide. Moreover, the mode-matching method is applied to calculate the generalized admittance matrix (GAM). Two multipoint cavities and software were built and validated. The first cavity has five resonant frequencies, between 170 MHz and 2.3 GHz, and the second has four resonant frequencies, between 1.3 and 8.6 GHz. Thus, this method allows for “broadband-resonant” measurements. The permittivity values of liquid hydrocarbons, powdered kerogen, and pyrite are shown.

Optical characteristics of dilute gallium phosphide bismide: Promising material for near-infra photonic device applications

We report the optical and electronic characteristics of GaP1−xBix bismide alloys with dilute composition 0≤x≤0.187 in the cubic zinc blende crystallographic phase calculated using a scalar relativistic Full Potential Linear Augmented Plane Wave method within Density Functional Theory. The equilibrium lattice constants are calculated using the improved generalized gradient approximation of Wu and Cohen in excellent agreement with experimental values. A quasi-linear dependence of the lattice parameters on Bi composition is obtained. Electronic structures such as the band gap, total density of states and partial density of states have been discussed using the recently developed approximate modified Becke–Johnson of Tran-Blaha with spin–orbit interaction (SOI) included. Electronic band structure analysis shows a significant decrease of band gap energy when P is replaced by Bi (by ∼548 meV per x=0.062). The band gap can be tuned in the emission wavelengths range from the visible to near-infra spectrum (∼0.414–1.633 μm for x∼0.187). The decrease in band gap energy is attributed to the both resonance interactions between Bi_p sates with the top of the valence band and Ga_s states at the host conduction band when the Bi content increased. The band gap undergoes an indirect (Γ–X) to direct (Γ–Γ) transition at x=0.05. The complex dielectric function, refractive index, reflectivity, extinction and absorption coefficient were calculated for incident radiation energy 0–8 eV. The critical-points in various optical spectra are identified in excellent agreement with measured values. The maximum absorption occurs within the energy range from ∼5 to 8 eV, corresponding to the ultraviolet spectrum region. Dilute GaP1−xBix alloys are a promising material system for the realization of the future photonic devices operating in the near-infra spectrum.

Performance prediction of electromagnetic band gap structure for microstrip antenna using FDTD-PBC unit cell analysis and Taguchi's multi-objective optimization method

Finite Difference Time Domain-Periodic Boundary Condition (FDTD-PBC) unit cell analysis of Electromagnetic Band Gap (EBG) structure has evolved as a versatile method due to its unique feature of solving Maxwell's equations in time and space domains. Providing a wideband characterization in a single simulation has been a prominent requirement in fields like wave interactions with high-speed electronics and Radar Cross Section (RCS) design. These requirements are otherwise difficult to attempt using full-wave simulation methods like Method of Moments (MoM) and Finite Element Method (FEM). In this work, a novel combination of Taguchi's multi-objective analysis method, along with FDTD-PBC unit cell analysis, is proposed for the optimization and prediction of EBG structure performance. Taguchi's method converges at a significantly faster rate compared to conventional optimization methods such as Particle Swarm Optimization (PSO) and Artificial Neural Networks (ANN). This study identifies important design variables and control factors such as the dimension of a square-shaped EBG unit cell like the width of the patch (w), height of the substrate (h), permittivity of the substrate (εr), gap width between the cells (g) and radius of the via (r). The impact of each of these control factors on the performance of the EBG structure is analyzed. The performance of the EBG structure is predicted using Taguchi's method for those experiments which are not included in orthogonal array (OA). The measured and predicted results are found to be closely matching with a 10% prediction error.

Structural, electronic, optical properties and first-principles calculations of Sr1-xCaxWO4 ceramics

A series of strontium calcium tungstates Sr1-xCaxWO4 powders with (x = 0; 0.25; 0.5; 0.75 and 1.0) were prepared by solid-state reaction method and analyzed by X-ray diffraction (XRD). All these compositions possess a tetragonal scheelite structure with I41/a space group. Raman active vibrational modes in the range from 20 to 1000 cm-1 of the series Sr1-xCaxWO4 with tetragonal structure exhibit 13 modes in arrangement with the Group theory analysis of structural Raman-active modes. The optical properties were investigated using the diffuse reflectance UV–visible absorbance spectrum. Based on Density Functional Theory (DFT) and using full Potential-linearized Augmented Plane Wave (FP-LAPW) method with the Local Density Approximation and the Generalized Gradient Approximation (GGA), implemented in the Wien2k package, we have investigated electronic and optical properties of all the compositions. The results indicate a decrease in the values of the optical direct bandgap (from 4.29 to 3.87 eV) with the increase of Ca into SrWO4 lattice, which is in good agreement with our experimental results.

Full-wave method for the analysis of the radiation characteristics of a VLF source in the atmosphere

The basis of the full-wave method is the propagation matrix method and a plane wave expansion, which is generally used for the analysis of radiation problems in layered media. The radiation field in the spatial domain is represented as an integral in the wavenumber domain. By integrating the vertical wavenumber, the radiation can be represented as a superposition of plane waves with different directions. If the atmosphere and ionosphere are regarded as layered media, the vertical wavenumber integral contains first-order singularities when the radiation source is located in the ionosphere, which are relatively easy to handle. When the radiation source is located in the air, the vertical wavenumber integral contains second-order singularities, which are more complicated. Therefore, there are few full-wave studies on radiation characteristics in the air and ionosphere when a very-low-frequency (VLF) source is located in the air. In this paper, air is considered as an isotropic medium with a micro-conductivity, the vertical wave number integral that has second-order singularities is solved, and the radiation fields in the air and the ionosphere are calculated. The numerical results show the correctness and effectiveness of the proposed algorithm.

An ab initio study on the transition path of carbon dioxide at high pressure: Evidence for a new intermediate [formula omitted] phase

The structural and dynamical behaviour of seventeen solid polymorph of the non-molecular CO2 compound was evaluated using the full potential linear augmented plane wave method (FP-LAPW) in the framework of density functional theory within the generalized gradient approximation (GGA-PBEsol). We showed the presence of stable non-molecular solid CO2 compound, which has the Pa3‾ phase as the most energetically stable phase. The Pa3‾ phase of the CO2 compound under pressure undergoes phase transition to the rutile P42/mnm phase at P ≈ 5 GPa, while the Pnnm phase was found to be dynamically unstable. Also, the α-cristobalite P41212 phase was found to be dynamically unstable with imaginary mode in the phonon dispersion curves. We showed that the I4‾2d phase exist at P ≈ 11 GPa and undergoes phase transition to the tetragonal P4‾m2 phase a high pressure of P ≈ 168 GPa Thus, the transition path for the non-molecular solid CO2 is given as shown below:Pa3‾→3.5 GPaPnnm→7.7 GPaP42/mnm→11 GPaI4‾2d→168 GPaP4‾m2

Characteristic mode-based compact circularly polarized metasurface antenna for in-band RCS reduction

AbstractThis paper presents a novel design of a low profile circularly polarized (CP) metasurface (MTS) antenna with in-band radar cross-section (RCS) reduction property. The MTS is loaded as a superstrate on slot antenna and it can be viewed as a polarization-dependent MTS (PDMTS). The rectangular patch-based PDMTS is analyzed using characteristic mode analysis to find two orthogonal degenerate modes, which produces CP waves. Linearly polarized slot antenna is used to excite the PDMTS. The performance of PDMTS loaded slot antenna is analyzed numerically using full-wave analysis method. The PDMTS CP antenna is fabricated and its performance is tested experimentally. The proposed antenna has a compact structure and it has an overall size of $0.52{\lambda _0}\times 0.52{\lambda _0} \times 0.078{\lambda _0}$ (where ${\lambda _0}$ is the free space wavelength). The measured results show that the PDMTS antenna achieves $-10\,{\rm dB}$ impedance bandwidth of 29.41$\%$, 3-dB axial ratio bandwidth of 9.05$\%$, broadside gain of 6.34 dB, and monostatic RCS reduction of $-30.2\,{\rm dBsm}$ at the resonant frequency of 5.86 GHz. The simulated results are in well agreement with the measured results and it is well suited for C-band Radar and Satellite communication.

Effect of hydrostatic pressure and the emergence of half metallic ferromagnetism in rhodium oxide - A DFT+U perspective

Electronic band structure calculation was carried out to investigate the half metallic ferromagnetism in RhO using GGA + U method. A Full Potential-Linearized Augmented Plane Wave (FP-LAPW) Method is employed to unravel the electronic properties of RhO. Using Generalized Gradient Approximation (GGA + U) method, the electronic properties such as density of electronic energy states(DOS) and band structure plots have been plotted. At high and low pressure the lattice constants vary and half metallic ferromagnetism with energy gap variations in the spins is observed. In GGA method, DOS and band structure plots of RhO predict metallic characteristics in both the spins, whereas in GGA + U method, half metallic ferromagnetism is predicted particularly at the Hubbard potential of U = 5 eV.

DFT study of optoelectronic spectra of barium cadmium chalcogenides (Ba2CdX3, X = S, Se and Te)

AbstractDFT analyses of electronic and optical spectra of barium cadmium chalcogenides (Ba2CdX3, X = S, Se, Te) have been carried out. The study of electronic spectra has been made in terms of band structure and density of states using full potential linear augmented plane wave plus local orbital method. Band structure calculations have been carried out under the approximations PBE-GGA, PBE-Sol, LDA and TB-mBJ. Band structures of these materials show that Ba2CdS3, Ba2CdSe3and Ba2CdTe3crystals possess a band gap less than 1 eV, underestimated relative to the experimental/theoretical literature values. Optical spectra of these chalcogenides have been analyzed in terms of real and imaginary parts of dielectric function, reflectivity, refractive index, extinction coefficient, absorption coefficient, optical conductivity and electron energy loss. Optical results show large anisotropy along different directions. These results provide a physical basis of barium cadmium chalcogenides for potential application in optoelectronic devices.

Multi‐polarised SAR imaging simulation of ship on heavy sea

Multi-polarisation synthetic aperture radar (SAR) imaging simulation of ship under heavy sea condition is presented here. Full-wave method is used to compute the fully polarimetric backscattered waves of the complex scene of ship and sea surface. The influence by heavy sea on SAR imaging results is investigated; furthermore, the suppression effects to sea waves by different polarisations are evaluated in detail.

Optimization of tower-footing grounding impedance for guyed-V transmission towers

Backflashover is one of the mechanisms by which lightning strikes can cause outages/damages in power systems. In this context, the tower-footing grounding system of transmission towers has an essential role for mitigating overvoltages that may result in backflashovers. The tower-footing grounding systems consists of long conductors, in different shapes and arrangements, buried in either homogeneous or stratified ground in order to obtain the lowest tower-footing grounding impedance. In this paper, the tower-footing grounding impedance of a typical guyed-V transmission tower is evaluated in the frequency domain using full-wave electromagnetic analysis and method of moments (MoM). Various scenarios where the tower-footing grounding system is buried in a homogeneous, a 2-, and a 3-layer stratified soil are studied. Also, the effect of the opening angle between the electrodes in the counterpoise arrangement in reducing the footing impedance is investigated. It is shown that the tower-footing impedance is notably reduced, especially when stratified soil is considered in the analysis.

Electronic structure and optical properties of TaNO: An ab initio study

We performed ab initio calculations to study the structural and optoelectronic properties of simple and slab phase TaNO using density functional theory (DFT), in which the full potential augmented plane wave (FP-LAPW) method was implemented using the computational code Wien 2k. The modified Becke-Johnson potential (mBJ-GGA) was used for these calculations. The calculated band structure and electronic properties revealed an indirect bandgap for simple TaNO (3.2 eV) and a direct bandgap for slab TaNO (1.5 eV). The interband electronic transitions were investigated from the band structure, and transition peaks were observed from the imaginary part of the dielectric function. These transitions are due to Ta-p, N-p and O-p orbitals for simple TaNO and Ta-p, N-s as well as O-p orbitals for slab TaNO. The plasmon energy was related to the main peak of the energy loss function, which was approximately 10 eV. The static value of the dielectric constant and the refraction were close to the experimental values. In general, slab TaNO shows different properties and is more suitable for optoelectronic applications due to direct bandgap.

Investigations on Tissue-Simulating Medium for MRI RF Safety Assessment for Patients With Active Implantable Medical Devices

In this paper, a full-wave simulation-based method is proposed to determine the optimal tissue-simulating medium for magnetic resonance imaging radio-frequency safety assessment for patients with active implantable medical devices (AIMDs). Compared to the standard medium, the proposed tissue-simulating medium is optimized for each AIMD lead path in human bodies. The reciprocity-based transfer function model is used as a criterion for optimizing the medium parameters. Numerical investigations and validations are performed for a neurostimulator to demonstrate the method.

Graphene‐based low side‐lobe microwave horn antenna

In this study, a novel method is proposed to control and reduce the side-lobe level (SLL) of the pyramidal horn antennas. In this method, graphene sheets are deposited on the antenna walls to taper the aperture field leading to pattern engineering with the goal of the SLL reduction. In essence, graphene sheet acts as a high-impedance surface and hinders electromagnetic power from reaching to the horn antenna aperture edges, in such a way that the diffraction phenomenon could be drastically diminished. The proposed design is numerically analysed and optimised using full-wave three-dimensional simulation methods. Numerical full-wave results illustrate more than 17.6 dB reduction in the SLL of the proposed antenna.