A graded material structure is optimized for broadband radome application by using hybrid method in this paper. In the optimization, dynamic range ratio (DRR) of real permittivity and loss of material are taken into considerations. By using an analytical function, the optimization problem with the DRR constraint is converted to an unconstrained problem. The proposed hybrid method is a combination of trust region method (TRM) and genetic algorithm (GA). Firstly (TRM) is applied to optimize the dielectric constant distribution. Then the result of TRM is used as initial value of GA. GA is employed to improve the global property of the results provided by TRM. Because TRM has the advantage of fast searching speed and GA has the advantage of global property, the hybrid method has the feature of fast convergence speed. And the jitter property of GA permittivity distribution is moderated. The effectiveness of the hybrid is validated through the designs of two broadband radome walls. The minimum power transmission efficiency is 81.9% ranging from 1 GHz to 18 GHz for normal incidence.
In the following, numerical and experimental results for a line-shaped argon plasma source over a wide range of gas pressure (2 Torr-50 Torr) and microwave power (200-800 W) are presented. The line-shaped plasmas have been generated in a rectangular Pyrex tube, 15 mm in height and 5 mm inner width, placed-in a linear slot made in the upper wide wall of a custom-made narrow rectangular waveguide. The microwave power is coupled to the discharge gas via the slot. The effects of the waveguide width, power level (electron density, and discharge tube insertion depth on the excited axial (along x) electric field profile and hence the uniformity of the produced plasmas are investigated numerically using commercial software CST Microwave Studio®, and charge coupled device (CCD) camera. Results showed that, a uniform line-shaped plasma is generated as waveguide width decreased to 58 mm, plasma density value <<nres = 3.7 × 1011 cm-3, and discharge tube insertion depth = 0 mm. An optical emission spectroscopy study was also realized to deduce the relative density of argon species and electron excitation temperature Texc. In general, argon spectral lines intensity was increased enhanced markedly when microwave power increased, while the different lines showed different behavior as argon pressure increased. The electron excitation temperature Texc decreases with increasing argon pressure, but almost constant overall the whole plasma length.
This paper reports a neural-network-based methodology to estimate the amount of moisture content in soil at L, S and C frequency bands. A multilayered artificial neural network, using the Levenberg-Marquardt algorithm, is used as the ANN model. The input training data comprise the measured values of dielectric constant of soil in the dry and moist states. Dielectric constant is measured using microwave free-space transmission technique. Measurement has been performed using Vector Network Analyzer (VNA), microstrip patch antenna and soil sample holder. One great advantage with this method is that there is no need to test the pH value of the soil sample, and hence all the associated pre-processing steps, such as drying, pulverizing, can be avoided.
Rice husk and coal is an innovation in enhancing the microwave absorption properties of pyramidal microwave absorbers to be used in radio frequency anechoic chambers. An anechoic chamber consists of radar absorbing material (RAM) along its wall; floor and ceiling to eliminate unwanted reflections to create electromagnetically quite environment. To design the pyramidal microwave absorber, coal has been added to rice husk. This innovative material combination has been investigated to determine the best reflectivity performance of pyramidal microwave absorbers. In the commercial market, polyurethane and polystyrene are the most popular foam based material that has been used in pyramidal microwave absorber fabrication. Simulation tool that has been used is CST Microwave Studio. Simulation based comparison of rice husk and combination of rice husk plus coal is done in the frequency range of 0-20 GHz. Pyramidal microwave absorber has been tested using dielectric probe method and radar cross section method. Reflection loss performance is tested in the frequency range of 8.2 to 12.4 GHz.
Agricultural waste is produced at agricultural premises as a result of an agricultural activity. Agriculture residue is made up of organic compounds from the living plants like rice straw, oil palm empty fruit bunch, sugarcane bagasse, coconut shell, banana leaves and others. This research has highlighted their eco-friendly nature and high microwave absorption properties, developing a new and improved form of pyramidal absorbers including high carbon content coal in it. Software simulation is done using CST Microwave studio. Samples are developed using a new technique by adding cobalt as an accelerator, and its performance is analyzed in terms of its reflection loss performance using free space measurement method in the frequency range of 8.2 to 12.4 GHz.
The pattern synthesis for large antenna arrays is very important because of its wide applications. Several antenna array synthesis techniques for planar antenna array have been developed in the past years. In this paper, a hybrid method for solving antenna array pattern synthesis problem is introduced. The proposed method has three steps. Firstly, the iterative fast Fourier transforms (IFT) is used to generate a number of initial array excitations based on aim pattern. Then, the global optimization method differential evolution strategy (DES) is used to optimize these excitations based on initial excitations. After that, if the optimized pattern does not satisfy the goal, the simulated annealing (SA) method is applied to optimize the excitations until the goal is achieved or the maximum iteration is reached. Several simulation results show that the desired pattern can be effectively synthesized by using the proposed method.
A modified 1D finite-difference time-domain method is analyzed by using the surface boundary condition (SBC-FDTD) for oblique incidence. The SBC-FDTD iterative formulas are deduced in oblique incidence by TEz and TMz wave. The reflection and transmission coefficients of electromagnetic wave in the 1D photonic crystal (PC) including a graphene sheet are calculated by the SBC-FDTD method. The method is also validated by comparison with the existed analytic methods. Finally, this modified method is applied to simulate 1D graphene photonic crystal (GPC). By changing the position of grapheme sheet in GPC, the electromagnetic gap characterizations of 1D GPC by an oblique incidence plane wave in THz spectral range are studied. The computational results show that the graphene sheet can enhance the absorption in THz because of the localization of light and the surface defect formed by graphene.
Numerical solution is presented for light scattering from two kinds of free-standing periodic arrays, that is, disks made of noble-metal and circular apertures perforated in a thin noble-metal sheet. The shapes of them are complementary to each other, and the circular areas are allocated along two orthogonal coordinates with the same periodicity. Using the generalized boundary conditions of the surface impedance type, we formulate the boundary value problem into a set of integral equations for unknown electric and magnetic current densities defined over the circular area. Employment of the method of moments allows us to solve the integral equations and give the expansion coefficients of the current densities, from which we can find reflected, transmitted, and absorbed powers. Dependence of the powers on the array parameters and wavelength is discussed in detail from the viewpoint of grating resonance. Special attention is paid to the extraordinary transmission which occurs in the arrays of apertures of sub-wavelength size by analytical derivation of the quasi-static solutions.
This paper proposes a fast Minimum-Variance-Distortionless-Response (MVDR) beamforming algorithm for an antenna array for cancellation of multiple interference signals. The proposed algorithm uses Sample-Average Estimate (SAE) of the data covariance matrix and reduces its computational effort by applying the Matrix-Inversion-Lemma (MIL) to its covariance Matrix Inversion (MI) operation. The proposed algorithm is compared to two SAE-based algorithms: the Sample Matrix Inversion (SMI) algorithm that requires an MI operation and the Auxiliary Vector (AV) algorithm that does not need an MI operation. A non-SAE based algorithm using the Least Mean Square (LMS) method is also included for comparison. Simulation results show that the proposed algorithm converges slower than the SMI scheme but outperforms the AV and LMS schemes during the transient phase. Once convergence is achieved, the proposed algorithm converges to a better Mean Square Error than the rest of the algorithms evaluated.
In this paper, a cylindrical permanent magnet linear motor (PMLM), which has a high performance, was designed and developed, because the motor has a zero normal force and a higher thrust density. The structure of the motor plays a vital role at the stage of design. During the design stage, several models of the PMLM that had different structural parameters were simulated using FEM software, and the model that produced the high-performance was identified. The structural parameters involved include the radius and height of the permanent magnet, rpm, and hpm, the height of coil, hc, and the shaft radius, rs, within a fixed total radius, rtotal. Each model of the PMLM was simulated using FEM software and the model that produced the highest thrust was identified. To prove its high-performance characteristics, the performance of the PMLM was then compared to the commercialized PMLM using four performance indexes which are thrust F, thrust constant kf, motor constant km and motor constant square density G. About 200 commercialized PMLMs with three different types have been chosen which are the slot type PMLM, slotless type PMLM and shaft motor. Based on the comparisons, the designed PMLM had a better performance than the commercialized PMLM. In order to validate the simulation result, the PMLM was manufactured. The simulation and measurement static thrust characteristics were then compared, and it was found that the simulated thrust had a good agreement with the measured one.
Underground mines are challenging environments for off-body wireless communication, since the signal propagation is majorly affected by small scale and large scale fading. The use of multiple antennas at the transmitter and the receiver sides is a known technique to combat fading and enhance capacity. In this paper, the channel parameters of a 2×2 Multiple-Input Multiple-Output (MIMO) off-body system are investigated in an underground gold mine and compared to the Single-Input Single-Output (SISO) system parameters. Measurement campaigns were conducted using monopole antennas at a center frequency of 2.45 GHz for both Line Of Sight (LOS) and None Line of Sight (NLOS) scenarios. The measured frequency responses are converted into impulse responses through an Inverse Fourier Transform (IFT). The results show that for a constant transmitted power, the path loss exponents at NLOS are smaller than their counterpart values at LOS. The channel capacity values decrease as the propagation distance increases and when the link is obstructed at NLOS. The RMS delay spread is generally increasing with distance for both LOS and NLOS situations. When a fixed Signal-to-Noise Ratio (SNR) is assumed, MIMO topologies improved the SISO capacity by roughly 8 bps/Hz. The channel characterization results demonstrate that the MIMO configurations provided a remarkable improvement in terms of capacity, coherence bandwidth, and time delay spread compared to the SISO topologies.
Double-sided open periodic structures are analyzed using inhomogeneous plane wave scattering. The leaky and surface wave modes of several unit cells of different structures are computed using the poles of generalized reflection and transmission coefficients of inhomogeneous plane waves in the spectral domain. It is shown that the reflection and transmission coefficients of the zeroth order Floquet mode contain the poles of the Green's function of the complex stratified periodic structure. The properties of evanescent mode amplification as well as super resolution near field imaging in a wire medium are addressed. A balanced leaky wave antenna unit cell with double-sided radiation feature is introduced and it is shown that, in contrast to grounded structures, total absorption in lossless non-chiral double-sided open unit cells is not feasible as long as the behavior of the unit cell is well described by its fundamental mode.
New designs of all-optical logic gates based on spatial-soliton interactions in optical communication spectral regions were proposed. The proposed structures are composed of local nonlinear Mach-Zehnder interferometer (MZI) waveguide structures with multi-input ports and two nonlinear output ports. They can be used to design various all-optical logic gates. The nonlinear MZI waveguide structure with local nonlinear waveguides functions like a phase shifter. It employs angular deflection of spatial solitons controlled by the phase modulation created in the local nonlinear MZI. The light-induced index changes in the local nonlinear MZI waveguide structures break the symmetry of structure and make the output signal beam propagate through different nonlinear output waveguides. By properly choosing the input control power, the spatial solitons will be switched to different output ports. The numerical results show that the proposed local nonlinear MZI waveguide structures could really function as all-optical logic gates in the optical communication spectral region.
In this paper, we present an equivalent current-based numerical routine for calculating the diffraction of arbitrarily curved wedge modeled with non-uniform rational B-spline (NURBS) curves and surfaces. The NURBS curves and surfaces obtained from CAD systems need to be parameterized for numerical calculation; however, available parameterizing approaches in rendering computer graphics, which use straight line segments and flat facets for tessellation, are not suitable for the computation of the wedge diffraction. To make the full use of NURBS modeling technique in high-frequency asymptotic approaches, the proposed numerical routine utilizes a curvature adaptive tessellation scheme to parameterize the edge curve of the wedge with varying curvature as well as the method of parameter alignment to maintain the C0 continuity between the edge curve and the wedge surfaces, which is essential in evaluating the diffraction coefficients. Based on the proposed parameterizing method, the equivalent edge current can be implemented for diffraction computation of arbitrarily curved wedge modeled with NURBS curves and surfaces, complementing with the NURBS based physical optics (PO) as a fully NURBS-based high-frequency approach, which provides high geometrical accuracy and computational efficiency for calculating diffraction of electrically large curved wedges. Numerical examples are presented to validate the proposed method.
The concept of Scattering-Induced Mutual Coupling Effect (SIMCE) is proposed, and the mechanism of producing this phenomenon in Multiple-Input Multiple-Output (MIMO) communication systems at MilliMeter Wave (MMW) band is demonstrated. The model of estimating the scattering-induced mutual impedance in rain environment is derived, and the characteristics of Rain-Induced Scattering Mutual Impedance (RISMI) are discussed taking parabolic antennas as an example. The model of estimating the rain-induced mutual impedance is helpful for investigating the SIMCE in other discrete random media. And, the results given in this paper are significant for developing MMW MIMO communication systems.
This paper presents an effective joint range-DOA-frequency (JRDF) estimation method based on fourth-order cumulants for multiple mixed near-field sources and far-field sources impinging on a symmetric uniform linear array, named as JRDF algorithm. Making use of the proposed method, range-DOA-frequency can be effectively estimated by the same eigen-pair of a defined ``information matrix'' constructed by two fourth-order cumulant matrices. Compared with the related works, the proposed method can provide superior performance, such as higher estimation accuracy, without the procedure of parameter search or parameter matching. Simulation results are presented to demonstrate the efficacy of the proposed approach.
Based on the Collins integral formula and Lohmann optical system, we expand the hard edge aperture into complex Gauss function and derive an approximate analytic expression of intensity distribution theoretically for Hypergeometric-Gauss beams through the fractional Fourier transform (FRT) optical systems with hard edge aperture. The influences of FRT order, aperture size and other optical parameters on the light intensity distribution of Hypergeometric-Gauss beams passing through the FRT optical systems are discussed in detail. The results show that the FRT is an excellent beam-shaping method.
Perfectly Matched Layer (PML) is modeled by Split-Field FDTD (SF-FDTD) in order to simulate Radar Cross Section (RCS) of a plasma slab. PML is used as an absorbing boundary, and discrete plane wave (DPW) is employed to generate plane wave. DPW method has a power isolation of -300 dB between scattered-field and total-field regions. The dispersive media is modelled by shift-operator FDTD. In this article, the SO-FDTD and DPW are combined, and it is proved that this combination shows a good stability. Finally, two different plasma profiles (exponential and polynomial) are used to prove reflection coefficient of a conductive layer can be reduced by choosing true profile of covering layer. By using Near-to-Far-Field Transformation, all fields are transferred to far-field region to calculate RCS.
A more realistic model is developed to predict the specific attenuation when electromagnetic signals propagate through dusty media (dust storms). The model is based on Mie approximation for the scattering of electromagnetic signal by a spherical particle. Variation of the dust particles dimensions is considered in this model. Reliable published values for dust dielectric constant are used for computations over the frequency range from 2 GHz to 100 GHz, (i.e. S-band, X band, K-band, Ka-band, Ku-band and W-band). The model outcome is compared with the results from other models suggested in literature. The effect of air humidity on specific attenuation is also investigated.
Different solution techniques, computational aspects and the ways to improve the performance of 3D frequency dependent Crank Nicolson finite difference time domain (FD-CN-FDTD) method are extensively studied here. FD-CN-FDTD is an implicit unconditionally stable method allowing time discretization beyond the Courant-Friedrichs-Lewy (CFL) limit. For the solution of the method both direct and iterative solver approaches have been studied in detail in terms of computational time, memory requirements and the number of iteration requirements for convergence with different CFL numbers (CFLN). It is found that at higher CFLN more iterations are required to converge resulting in increased number of matrix-vector multiplications. Since matrix-vector multiplications account for the most significant part of the computations their efficient implementation has been studied in order to improve the overall efficiency. Also the scheme has been parallelized in shared memory architecture using OpenMP and the resulted improvement of performance at different CFLN is presented. It is found that better speed-up due to parallelization always comes at higher CFLN implying that the use of FD-CN-FDTD method is more appropriate while parallelized.
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