The ship Kelvin-wake models on two-dimensional (2-D) linear and nonlinear sea surfaces are combined with the second-order small-slope approximation method (SSA-II) to comparatively study the corresponding electromagnetic (EM) scattering characteristics. The nonlinear sea-surface models include the Choppy Wave Model (CWM) and the second-order Creamer model (Creamer (2)). Considering the limitations of using the ideal plane EM wave incident upon a rough sea surface of the limited size, the expressions of the scattered field and scattering amplitude are derived by utilizing the modified tapered incident field. Due to the fact that the nonlinear effects of Creamer (2) surfaces is obviously stronger than those of CWM surfaces, the bistatic normalized radar cross section (NRCS) calculated from Creamer (2) surfaces is significantly greater than that of its linear and CWM surfaces for scattering angles departing from the specular direction, and the backscattering coefficients from Creamer (2) surfaces are also the greatest except within quasi-specular (near vertical incidence) region. In addition, for the linear and nonlinear sea surfaces, the influences of different wind speeds and directions on scattering characteristics are also calculated and analyzed in detail. However, taking the ship Kelvin wakes into account, the corresponding scattering features are obviously distinct from those of the single linear and nonlinear sea surfaces. This helps to provide a basis to extract the related ship information through the scattering characteristics of ship Kelvin wakes. Also it shows that the small-slope approximation method is a very effective analysis method to deal with the EM scattering from the rough sea surface.
In this paper, an ultra wideband antenna employing a defected ground structure is presented. The radiating element is a circular patch on which a fractal based geometry is inscribed in the form of slots and excited by a tapered feed-line for impedance matching. The antenna has an impedance bandwidth of 8.2 GHz (117% at centre frequency of 7 GHz) and a peak gain around 6 dB. To improve the impedance bandwidth and gain, a Swastika shape Electromagnetic band gap (EBG) structure is proposed. The unit cell of the proposed EBG has a compact size of 3 mm × 3 mm and is obtained by introducing discontinuities in the outer ring of the Cross-Hair type EBG. The stop band (-20 dB) achieved with this EBG is 3.6 GHz (7.5 GHz-11.1 GHz) which is 1.6 GHz more than that achieved by a standard mushroom-type EBG of the same size and same number of elements. After application of the proposed EBG, there is an improvement of 12% in the impedance bandwidth while the peak gain increases by about 2-3 dB. The radiation of the antenna shows a dumb-bell shaped pattern in the E-plane and Omni-directional pattern in the H-plane. All the measured results are in good agreement with simulated results.
In this paper, the difference scheme of the alternating-direction-implicit finite-difference time-domain (ADI-FDTD) method is replaced by the quasi isotropic (QI) spatial difference scheme to improve its numerical dispersion characteristics. The unconditional stability advantage of QI-ADI-FDTD is analytically proven and numerically verified. The numerical dispersion of the novel method can be dramatically reduced by choosing proper weighting factor. An example is simulated to demonstrate the accuracy and efficiency of the proposed method.
A subspace self-calibration ESPRIT algorithm for mutual coupling across an electromagnetic vector sensor is proposed in this paper. By introducing an auxiliary array element, the mutual coupling is calibrated. The whole array's mutual coupling matrix can be obtained simultaneously. A mathematic model for mutual coupling across the six collocated antennas of an electromagnetic vector sensor is established. And the solution of mutual coupling matrix was transformed into the solution of several matrix elements. The Cramer-Rao Lower Bound (CRLB) is also derived in the end of this paper to verify the efficacy of the proposed algorithm. The simulation results demonstrate that this approach is correct and effective.
This paper introduces a simple but effective scattering mechanism identification scheme for analyzing mixed scattering mechanisms obtained by model-based decomposition. Using the normalized scattering vector, each pixel is represented by a point in a standard 2-simplex in R3. Seven scattering category centers are represented by the three vertices, the three midpoints of sides and the centroid of the 2-simplex. The scattering category partitioning problem is then solved by minimizing the Euclidean distance between the image pixels and these category centers. The proposed scattering mechanism identification scheme is finally used for data analyzing and unsupervised classification. Experiments on AIRSAR and E-SAR L-band PolSAR images demonstrate the effectiveness of the proposed method.
This paper presents a simple mathematical model to determine the force, stiffness and moment parameters in Permanent Magnet (PM) bearings made of radial magnetized ring magnets using Coulombian model and vector approach for five degrees of freedom. MATLAB codes are written to evaluate the bearing characteristics for three translational (x, y and z) and two angular (ξ and γ) degrees of freedom of the rotor magnet. The results of the mathematical model are compared with the results of Finite Element Analysis (FEA) using ANSYS and experiments for a PM bearing with one ring pair, thereby the presented mathematical model is validated. Furthermore, the PM bearing with three ring pairs with alternate radial polarizations are analysed by extending the presented mathematical model and also using ANSYS. Finally, the 5×5 stiffness matrix consisting of principal and cross coupled values is presented for the elementary structure as well as for the stacked structure with three ring pairs.
In Part I of this work, travelling-wave modelling of uniform multi-conductor transmission line networks was analytically established with direct applicability to narrowband transmission, covering any network formed of lossy (in the general case), diagonalisable uniform multi-conductor transmission lines of either distinct or degenerate eigenvalues. As the whole work applies especially in the field of Power-Line Communications, in Part II, a validating experimental paradigm is first provided using a common cable type for indoor power electric networks. Then, direct applicability to narrowband transmission is addressed, along with potential expandability towards wideband signalling. A comparative evaluation with other existing methods of time-domain modelling is also included and relevant directions for future research are suggested.
In Part I of this work, analysis of uniform multi-conductor transmission line networks is performed on travelling-wave basis, via "quasi-TEM" approach. Narrowband interpretation of the modal theory in the time domain and quantification of the multiple reflections effect are both included. Theoretical demonstration and analytical formulation are provided, along with guidelines towards computational implementation. Any network formed of lossy, diagonalisable uniform multi-conductor transmission lines of either distinct or degenerate eigenvalues is covered. This work applies especially in the field of Power-Line Communications, as High-Frequency transmission over the power electric network is dominated by multipath propagation.
This study presents a novel wideband absorber for radar cross section (RCS) reduction. Unlike previous absorber designs that use multilayer lossy materials, this study proposes a design based on a planar antenna array that adopts a bowtie dipole structure as the unit cell. The complete design procedure was investigated by using examples for single- and dual-polarized incident wave designs. The measurement results show that the bandwidth of both designs exceeded 81% of 10 dB RCS reduction when the thickness is less than 12% of the free space wavelength at the lowest operating frequency. The high RCS reduction of the proposed absorbers was demonstrated using commercial ground-penetrating radar. Results show that the proposed absorber is invisible to radar.
A nonlinear electrical transmission line with an intersite circuit element acting as a nonlinear resistance is introduced and investigated. In the continuum limit, the dynamics of localized signals is described by a non-linear evolution equation belonging to the family of nonlinear diffusive Burgers' equations. This equation admits compact pulse solutions and shares some symmetry properties with the Rosenau-Hyman K(2,2) equation. An exact discrete compactly-supported signal voltage is found for the network and the dissipative effects on the pulse motion analytically studied. Numerical simulations confirm the validity of analytical results and the robustness of these compact pulse signals which may have important applications in signal processing systems.
We investigate the design of Concentric Circular Antenna Arrays (CCAAs) with λ/2 uniform inter-element spacing, non-uniform radial separation, and non-uniform excitation across different rings, from the perspective of Multi-objective Optimization (MO). Unlike the existing single-objective design approaches that try to minimize a weighted sum of the design objectives like Maximum Side Lobe Level (MSLL) and principal lobe Beam-Width (BW), we treat these two objectives individually and use Multiobjective Evolutionary Algorithm based on Decomposition (MOEA/D) with Differential Evolution (DE), called MOEA/D-DE, to achieve the best tradeoff between the two objectives. Unlike the single-objective approaches, the MO approach provides greater flexibility in the design by yielding a set of equivalent final (non-dominated) solutions, from which the user can choose one that attains a suitable trade-off margin as per requirements. We illustrate that the best compromise solution attained by MOEA/D-DE can comfortably outperform state-of-the-art variants of single-objective algorithms like Particle Swarm Optimization (PSO) and Differential Evolution. In addition, we compared the results obtained by MOEA/D-DE with those obtained by one of the most widely used MO algorithm called NSGA-2 and a multi-objective DE variant, on the basis of the R-indicator, hypervolume indicator, and quality of the best trade-off solutions obtained. Our simulation results clearly indicate the superiority of the design based on MOEA/D-DE.
In this paper, we have addressed three major problems of uniform linear array in case of a sensor failure at any position. We assume that sensor position is known. The problems include increase in sidelobe levels, displacement of nulls and diminishing of null depth. The desired null depth is achieved by making the weight of symmetrical counterpart element passive. Genetic algorithm (GA) along with pattern search (PS) is used for reduction of sidelobe levels, and adjustment of nulls. Fitness function minimizing the error between the desired and estimated beam pattern along with null constraints is used. Simulation results for diversified scenarios have been given to demonstrate the validity and performance of the proposed algorithm.
Space time adaptive processing (STAP) is a signal processing technique for detecting slowly moving targets using airborne radars. The traditional STAP algorithm uses a lot of training cells to estimate the space-time covariance matrix, which occupies large computer memory and is time-consuming. Recently, a number of compressed sensing based STAP algorithms are proposed to detect moving target in strong clutter situation. However, the coherence of the sensing matrix is not low due to the high resolution of the DOA (direction of arrival)-Doppler plane, which does not guarantee a good reconstruction of the sparse vector with large probability. Consequently, the direct estimation of the target amplitude may be unreliable using sparse representation when locating a moving target from the surrounding strong clutter. In this study, a novel method named similar sensing matrix pursuit is proposed to reconstruct the sparse radar scene directly based on the test cell, which reduces the computing complexity efficiently. The proposed method can efficiently cope with the deterministic sensing matrix with high coherence. The proposed method can estimate the weak elements (targets) as well as the prominent elements (clutter) in the DOA-Doppler plane accurately, and distinguish the targets from clutter successfully.
The emission of electromagnetic radiation from charged particles spiraling around magnetic fields is an important effect in astrophysical and laboratory plasmas. In theoretical modeling, issues still not fully resolved are, among others, the inclusion of the recoil force on the relativistic electron motion and the detailed computation of the radiation power spectrum. In this paper, the cyclotron radiation emitted during the nonlinear interaction of relativistic electrons with a plane electromagnetic wave in a uniform magnetic field is examined, by analyzing the radiated power in both time and frequency domain. The dynamics of the instantaneous radiation and the emitted power spectrum from one particle, as well as from monoenergetic electron ensembles (towards a picture of the radiation properties independent of the initial conditions) is thoroughly studied. The analysis is performed for several values of the wave amplitude, focusing near the threshold for the onset of nonlinear chaos, in order to determine the alteration of the radiation in the transition from regular to chaotic motion.
Electronically switchable microwave filters are attracting more attention for research and development because of their importance in increasing the capability of wireless communication and cognitive radios. In this paper, novel switchable microwave band-stop to all pass filters are designed by using stepped impedance resonator. Commercially available Pin diodes are used in order to allow the fastest switching between band-stop and all pass responses. The theoretical analysis is presented in this paper, and its feasibility has been experimentally verified with a micro-strip prototype. The design was also characterized by measuring the filter performance with increasing power levels of 20, 15, 10, 5, and 0 dBm. The results have shown that the switchable filter is immune to power saturation effects. Nonlinear measurements at higher power levels are also performed and the switchable filter produced low power inter-modulation product. The main advantage of this filter is its capability to switch between band-stop and all pass mode of operation. Other advantages include being small in size, and low in cost.
The goal of this paper is to present a semi analytical method which makes it possible the calculation of the dynamic iron losses in a three phase induction machine taking the slotting effect into account. The particularity of this method is that it allows the distinction of the stator and the rotor slot openings contribution in the dynamic and, consequently, in the total iron losses. This analytical study shows that a convenient choice of the stator and the rotor slot openings leads to an iron loss reduction, due to the cancellation of particular flux density slotting harmonics. Theoretical results are confirmed numerically.
The time delay, space shift and widening of wave packet transmitted and reflected by structures with Bragg mirrors have been investigated. The specific structures such as Bragg mirrors, resonators, and structures with chirp variation of thickness of the ``period'' have been considered. The calculation has been carried out under the conditions that carrier frequency, and incidence angle is in the vicinity of the Bragg resonance. Integral (mass center) and differential (group) estimates of the delay time and space shift have been compared. The conditions for the appearance of anomalous (negative) mass center delay or mass center shift (Goos-Hänchen shift) of the reflected wave packet have been determined. The shape transformations of the wave packet illuminating periodic and quasiperiodic apodized Bragg reflectors have been under consideration. Spatial apodization of permittivity contrast yields much smaller shape deformation of the transmitted wave packet upon incidence at angles and carrier frequency near the edges of reflection band, as well in Bragg reflection band, in comparison with phenomena in similar periodic structures. The values of group delay for layered structures with a small chirp variation of optical (electrical) thickness of the period along longitudinal coordinates have been experimentally obtained in microwave range.
A design and analysis of a novel proximity-fed printed slot antenna with 3.5/5.5 GHz dual band-notched characteristics are presented. To obtain an ultra-wideband (UWB) response, a circular patch with a rectangular conjunction arm is etched concentrically inside a ground plane aperture. The antenna is proximity-fed by a microstrip line with an open shunt stub on the other side of the substrate. The designed antenna satisfies a -10 dB return loss requirement in the frequency band from 2.7 to 17 GHz. In order to obtain dual band-notched properties at 3.5 and 5.5 GHz, an open ring slot is etched off the circular patch and a π-shaped slot is etched off the microstrip feeding line, respectively. A curve fitting formulation is obtained to describe the influences of the notched resonators on the corresponding notched frequencies. The proposed antenna is designed, simulated and fabricated. The measured data show a good agreement with the simulated results and the equivalent circuit results through the use of a modified Vector Fitting technique for a rational function approximation. The proposed antenna provides almost omnidirectional radiation patterns, relatively flat gain and high radiation efficiency over the entire UWB frequency excluding the two rejected bands.
This paper introduces a method for canceling the parasitic capacitance of integrated common mode (CM) filter by optimizing the layout of ground winding. Firstly, the CM filter with positive or negative coupling between the ground and inductor winding is researched, respectively. Then, the two coupling polarizes are combined to form the bi-directional coupling, simulation and measured results show bi-directional coupling ground can effectively improve the high frequency (HF) filtering performance. The equivalent circuits are given to demonstrate the cancellation mechanism, and modelling is derived for the design of ground winding. To further validate the application of proposed technique, CM noise and input/output signals for PFC (Power Factor Correction) converter with bi-directional coupling ground CM filter is simulated. The noise spectrums show conductive interference at high frequencies is effectively suppressed and meets the required electromagnetic interference (EMI) standard.
We show that the optical modes of a periodic nanostructure with frequency dependent dielectric constant (i.e., a dispersive optical nanostructure), in general can be written as an ordinary eigenvalue problem of a "dielectric function operator", for each distinct symmetry representation of the periodic nanostructure. For a frequency dependence in the form of polynomial rational function, the problem translates to a polynomial eigenvalue equation in the frequency of the mode. The resulting problem can be solved using the basis functions of a dielectric backbone structure, which has a frequency independent dielectric constant. Rapid convergence is achieved when the basis functions are selected to be the modes of a dielectric backbone structure that minimizes the frequency perturbation of the dielectric function of the optical nanostructure. In particular, using a two dimensional photonic crystal constructed with a polar crystal as an example, we demonstrate that, remarkable simple cubic equations are sufficient to obtain accurate descriptions of eigenfrequencies.