This paper presents a bacterial foraging algorithm (BFA) for null steering of linear antenna arrays by controlling only the element amplitudes. The BFA is a new evolutionary computing technique based on the foraging behavior of Escherichia (E.) coli bacteria in human intestine. To show the accuracy and flexibility of the proposed BFA, several examples of Chebyshev array pattern with the imposed single, multiple and broad nulls are given. It is found that the nulling method based on BFA is capable of steering the array nulls precisely to the undesired interference directions.
This paper presents a hybrid numerical approach combining an improved Time Domain Finite Element-Boundary Integral (FE-BI) method with Time Domain Physical Optics (TDPO) for calculations of electromagnetic scattering of 3-D combinativecomplex objects. For complex-combined objects containing a small size and large size parts, using TDPO is an appropriate approach for coupling between two regions. Therefore, our technique calculates the objects complexity with the help of FE-BI and the combinatory structures by using of the TDPO. The hybridization algorithm for restrictive object is implemented and the numerical results validate the superiority of the proposed algorithm via realistic electromagnetic applications.
New fractional boundary conditions (FBC) on plane boundaries are introduced. FBC act as intermediate case between perfect electric conductor and perfect magnetic conductor. In certain sense FBC are analogue of commonly used impedance boundary conditions with pure imaginary impedance. The relation between fractional order and impedance is shown. Plane wave diffraction problem by a strip described by FBC is formulated and solved using new method which extends known methods. Numerical results for physical characteristics are presented. Analyzing the scattering properties of the fractional strip new features are observed. FBC has one important special case where the fractional order equals to 1/2. For this special case the solution of diffraction problem can be found in analytical form for any value of wavenumber. Also for small values of wavenumber monostatic radar cross section has new specific resonances which are absent for other values of fractional order.
This paper proposes a displaced sensor array (DSA) configuration for estimating the angles of arrival of narrowband sources arriving at grazing incidence directions. Unlike the conventional uniform linear array (ULA) where all the array elements are aligned along one axis, the proposed DSA configuration comprises two displaced ULAs aligned on two parallel axes in the vertical plane. The steering vectors of the two parallel arrays differ from each other by only two multiplicative phase terms that represent the space factors due to the vertical separation and horizontal displacement of the two arrays. This makes the computational load of using MUSIC algorithm with the proposed DSA configuration identical to that of ULA yet the accuracy is much higher especially for cases involving narrowband sources arriving at grazing incidence angles. Simulation results obtained show that the proposed DSA configuration outperforms the conventional ULA in terms numerical accuracy and angular resolution.
In order to overcome drawbacks of standard particle swarm optimization (PSO) algorithm, such as prematurity and easily trapping in local optimum, a modified PSO algorithm which adopts a global best perturbation, is used to optimize the pattern of cylindrical conformal antenna array for sidelobe level (SLL) suppression and null control in certain directions.The convergence speed and accuracy of the algorithm are improved.Compared with genetic algorithm and simulated annealing, The PSO algorithm is much easier to understand and implement.Firstlypattern formula of conformal array is provided, then, the standard and modified PSO algorithm are introduced, at last, application examples and simulation results are presented.The results show that the Modified PSO algorithm is an effective and efficient method to solve multi-dimension and nonlinear problem.
In this paper, the bit-error-rates performance of impulse radio ultra-wideband (IR-UWB) wireless communication system based on time reversal technique is investigated in both single-input singleoutput and multiple-input single-output situations. Simulations indicate that the good result is obtained as we expect it and IR-UWB based on time reversal technique is very promising for high bit rates wireless communication applications.
Recently developed multiresolution frequency domain (MRFD) technique is applied to two-dimensional electromagnetic scattering problems. Scattered field formulation and perfectly matched layer is implemented into the MRFD formulation. Far field distributions of dielectric and perfectly electric conductor (PEC) bodies are calculated and bistatic echo widths of these structures are presented. Good agreement between MRFD and FDFD results is recognized. It is observed that the MRFD technique demonstrates superior computational efficiency characteristics compared to the traditional FDFD technique.
In this paper,a nondestructive technique for determining the complex permittivity and permeability of magnetic sheet materials using two flanged rectangular waveguides is presented. The technique extends existing single probe methods by its ability to simultaneously measure reflection and transmission coefficients imperative for extracting both permittivity and permeability over all frequencies. Using Love's Equivalence Principle,a system of coupled magnetic field integral equations (MFIEs) is formed. Evaluation of one of the two resulting spectral domain integrals via complex plane integration is discussed. The system,solv ed via the Method of Moments (MoM),yields theoretical values for the reflection and transmission coefficients. These values are compared to measured values and the error minimized using nonlinear least squares to find the complex permittivity and permeability of a material. Measurement results for two magnetic materials are presented and compared to traditional methods for the purpose of validating the new technique. The technique's sensitivity to uncertainties in material thickness and waveguide alignment is also examined.
Use of Multi-Objective Particle Swarm Optimization for designing of planar multilayered electromagnetic absorbers and finding optimal Pareto front is described. The achieved Pareto presents optimal possible trade-offs between thickness and reflection coefficient of absorbers. Particle swarm optimization method in comparison with most of optimization algorithms such as genetic algorithms is simple and fast. But the basic form of Multi-objective Particle Swarm Optimization may not obtain the best Pareto. We applied some modifications to make it more efficient in finding optimal Pareto front. Comparison with reported results in previous articles confirms the ability of this algorithm in finding better solutions.
In this work, we present a marching-on in degree finite difference method (MOD-FDM) to solve the time domain Helmholtz wave equation. This formulation includes electric and magnetic current densities that are expressed in terms of the incident field for scattering problems for an open region to implement a plane wave excitation. The unknown time domain functional variations for the electric field are approximated by an orthogonal basis function set that is derived using the Laguerre polynomials. These temporal basis functions are also used to expand current densities. With the representation of the derivatives of the time domain variable in an analytic form, all the time derivatives of the field and current density can be handled analytically. By applying a temporal testing procedure, we get a matrix equation that is solved in a marching-on in degree technique as the degree of the temporal basis functions is increased. Numerical results computed using the proposed formulation are presented and compared with the solutions of the conventional time domain finite difference method (TD-FDM) and analytic solutions.
In this paper, the optimization of both sum and difference patterns of linear monopulse antennas with low side lobe levels, high directivity and also narrow main beam width are efficiently solved by Continuous Ant Colony Optimization (ACO) Method. The synthesis problem is optimized by defining a suitable cost function which is based on limitation of the side lobe level. In this work, three different parameters are considered to be optimized separately which are the excitation amplitude of each element, the excitation phase of each element and finally the element-to-element spacing. Numerical results of each step, sum and difference patterns, are illustrated in each related part. Finally, we investigate placing some nulls in specific directions to suppress the jamming signals in both sum and difference patterns.
In order to simulate the electromagnetic scattering of targets with thin-coating accurately, a conformal finite-difference timedomain (CFDTD) method based on effective constitutive parameters is presented in this paper. Two kinds of coating problems are considered. For a coated target with medium backing material, the CFDTD formulations on conformal cells are the same as those of the conventional FDTD, but the parameters in FDTD formulations are replaced by effective constitutive parameters to include the curved coating message of target. For a coated target with perfectly conducting (PEC) backing material, the contour-path integral is used to exclude the curved PEC part, and effective constitutive parameters are then introduced to include the coating message. The bistatic radar cross section (RCS) of coated spheres with medium backing material and with PEC backing material are computed, respectively, to validate the presented CFDTD scheme. The backscattering of a composite airfoil, which is made of radar absorbing material (RAM) and metal framework, and coated by fiberglass-reinforced plastics, is also analyzed to demonstrate the feasibility of presented scheme.
In this paper, we report, for the first time to the best of our knowledge, the detailed modeling and performance of Raman amplification and superluminal propagation of weak ultrafast femtosecond optical pulses in nonlinear loop single mode silicon-oninsulator anomalously dispersive optical waveguides. Using the device, theoretical results for 100-fs signal optical pulse show that when the launch peak power of signal pulse is fixed at −10 dBm, the gain value up to 30 dB can be achieved, and the delay time of superluminal propagation can also be adjusted by changing the system parameters, including initial chirp and peak power of pump pulse, initial delay time between pump and signal pulses, and waveguide length, etc.
The adaptive beamformers often suffer severe performance degradation when there exist uncertainties in the steering vector of interest. In this paper, we develop a new approach to robust adaptive beamforming in the presence of an unknown signal steering vector. Based on the observed data, we try to estimate an equivalent directionof- arrival (DOA) for each sensor, in which all factors causing the steering vector uncertainties are ascribed to the DOA uncertainty only. The equivalent DOA of each sensor can be estimated one by one with the assumption that the elements of the steering vector are uncorrelated with each other. Using a Bayesian approach, the equivalent DOA estimator of each sensor is a weighted sum of a set of candidate DOA's, which are combined according to the value of the a posteriori probability for each pointing direction. In this way, the signal steering vector and the diagonal loading sample matrix inversion (DL-SMI) version adaptive beamformer can be obtained. Numerical simulations illustrate the robustness of the proposed beamforming algorithm.
A rectangular patch antenna with two circular apertures, a defected ground structure (DGS), and a shorting post is proposed in this paper. The novel structure can reduce the radar cross section (RCS) of the antenna at its operating frequency. At the same time, the return loss of the proposed antenna is maintained, and the RCSs of the patch antenna at the frequencies outside the operating band are also reduced. The proposed antenna is simulated by using high frequency electromagnetic simulation software. The peak of RCSs is reduced about 7 dB and the broadband RCSs are below −30 dB from 2 to 8 GHz. This result is useful for low-RCS antenna application.
Monopole antennas are usually used for measurement of the shielding effectiveness of metallic enclosures. This paper presents an accurate numerical modeling for monopole antennas attached in shielded enclosures with apertures. The electric field integral equation (EFIE) is formulated for the induced currents on both the monopoles and enclosure. The precorrected-FFT accelerated method of moments (pFFT-MoM) is used to solve the integral equation and the incomplete LU (ILU) preconditioner is applied to speed up the convergence of the equation. Compared with existing methods, the model presented in this paper considers the mutual coupling between the monopoles and shielded enclosure. Therefore, it is a better approximation to the actual measurement geometry.
Detection andestimation of depth of shallow buried landmines using microwave remote sensing is a complex and computationally intensive task. Despite a lot of research to correctly locate and identify the buried landmines, and to estimate its depth using microwave remote sensing data which is essential for demining with minimal risk, a lot of uncertainties still exist. Therefore in this paper, an extensive study using a groundbasedX-bandscatterometer for detection of shallow buried landmine and estimation of its depth has been carriedout. An experimental setup consisting of a ground basedscatterometer operating in microwave X-band(10 GHz, 3 cm) has been usedto generate backscatter data in a gridof 24×24 and a series of experiments under laboratory conditions conducted using dummy landmines (without explosives) buried to different depths up to 10 cm in dry smooth sand. It is difficult to detect the buried landmine by visual inspection of the raw data; therefore a novel approach by fusion of image processing techniques with electromagnetic (EM) analysis has been evolvedfor detection andestimation of depth of the landmine. The raw data generated through the experiments was processedthrough a series of image processing steps and a region of interest segmentedusing Otsu andmaxim um entropy based thresholding methods for further processing. A detection figure test has been proposedat this stage to reduce false alarms. Genetic algorithm (GA) with an electromagnetic (EM) model fusion has been proposedto estimate the depth after segmenting a suspectedregion containing the mine. The main advantage of the proposed model is that it does not have any requirement of separate training and test data set to train the optimizer and validate the results. Analyses of the results indicate that it is possible to segment suspected region of interest containing the landmines in data obtained in microwave Xband using either of the two thresholding methods. The depth of buriedland mines estimatedusing the proposed GA optimized EM model was also foundto be in goodagreemen t with the actual depth. The proposedanalysis is expectedto be extremely useful in future in detection and estimation of the depth of landmines using satellite data in microwave X-band.
Extraction of vegetation water content and soil moisture from microwave observations requires development of a high fidelity scattering model. A number of factors associated with the vegetation canopy and with the underlying bare soil should be taken into account. In this paper,w e propose an electromagnetic scattering model for a soybean canopy which includes the coherent effect due to the soybean structure and takes advantage of recently advanced scattering models for rough surface. We also take care of some other issues, such as including curvature effect in studying the ground bounce scattering mechanisms,and using array theory with perturbation for characterizing the inter-plant structure to account for the prevailing agriculture practice of soybean. Good agreements are obtained between the model results and measurement data.
In this paper, a set of formulas to analyze the scattering from open-ended rectangular cavity is presented on the basis of Shooting and Bouncing Ray (SBR) method. By analyzing the ray paths inside the cavity, the Physical-Optics (PO) integration on the aperture is carried out in a close form. Using closed-form solution, the Radar Cross Section (RCS) of cavity in high frequency can be studied sententiously and accurately. All the peaks and nulls in the RCS plot of cavity are predicted successfully with the formulas deduced in the paper, and a 3-D scattering pattern of rectangular cavity is simulated by the proposed method.
In this paper an approach for stability analysis of microwave oscillators is proposed. Using the perturbation theory and averaging method, a theorem which relates the oscillation stability to the stability of the periodic average of the circuit's Jacobian is mentioned. Using this theorem, a criterion for oscillation stability is devised. The proposed criterion is applied to the stability analysis of a negative resistance diode oscillators and a Colpitts oscillator. This method is readily applicable to microwave CAD routines.
In this paper we describe an effective and inherently parallel approximate inverse preconditioner based on Frobenius-norm minimization that can be easily combined with the fast multipole method. We show the numerical and parallel scalability of the preconditioner for solving large-scale dense linear systems of equations arising from the discretization of boundary integral equations in electromagnetism. We introduce simple deflating strategies based on low-rank matrix updates that can enhance the robustness of the approximate inverse on tough problems. Finally, we illustrate how to improve the locality of the preconditioner by using nested iterative schemes with different levels of accuracy for the matrix-vector products. Experiments on a set of model problems representative of realistic scattering simulations in industry illustrate the potential of the proposed techniques for solving large-scale applications in electromagnetism.
In this paper a novel numerical optimization technique for antenna configurations is introduced. This algorithm is inspired from colonizing weeds, which is shown to be very robust and adaptive to changes in the environment. Thus, capturing their properties would lead to a powerful optimization algorithm. The feasibility, efficiency and effectiveness of the proposed algorithm for optimization of antenna problems are examined by a set of antenna configurations. The obtained results are compared with a particle swarm optimization technique which is widely used in antenna optimization. Numerical results show that there is a good agreement between the corresponding results.
In this paper, analytical relation for pulse width evolution and broadening in fiber systems using the Volterra series transfer function (VSTF) in linear and nonlinear cases are derived. This evaluation is done for traditional and optimum dispersion compensated fibers. Effects of group velocity dispersion (GVD) and self-phase modulation (SPM) are taken into account. It is shown that the analytical formulation can be applied to design and analysis the long hauls practical systems, and is helpful in understanding the pulse distortion caused by the interaction between SPM and GVD. The proposed relations are extracted analytically and for the first time pulse broadening factor in general case is derived.
Computation of coherent electromagnetic wave propagation through rain medium with a realistic raindrop shape is the subject of this work. T-Matrix approach towards the computation of forward scattering amplitude of raindrops is considered numerically exact. The results of Total Cross Section and forward scattering amplitude due to raindrops with MPP model shape are calculated by T-Matrix method. Both horizontal and vertical polarization of incident wave are considered where specific attenuation, phase shift and cross polarization discrimination (XPD) for terrestrial coherent electromagnetic wave propagation in the frequency range of 3-300 GHz are presented. Furthermore the effect of temperature on specific attenuation vs. frequency is investigated.
A new numerical method is proposed for the analysis of electromagnetic scattering from conducting surfaces. The method involves Monte Carlo integration technique in the Method of Moments solution of the Electric Field Integral Equation for determining the unknown induced current distribution on the surface of the scatterers. The unknown current distribution is represented in terms of a modified entire domain polynomial basis functions satisfying the appropriate edge conditions and symmetry conditions of the problem. This leads to very small order of the Method of Moments matrix as compared to the conventional sub-domain basis functions. The accuracy and the effectiveness of the method are demonstrated in three cases of scattering from conducting circular disks and results are compared with the solutions using conventional sub-domain basis functions. While the sub domain analysis is incapable of handling large domain problems, the proposed method overcomes this limitation. It is also observed that the proposed method is superior to conventional sub-domain method in dealing with singularity problem of the integral equation easily and efficiently.
Desired far-field radiation patterns of 5 × 11 conformal antenna array are synthesized using a hybrid genetic algorithm (HGA), which combines the simplified quadratic interpolation (SQI) method and the real-coded genetic algorithm (RCGA). This hybrid genetic algorithm is shown to outperform standard genetic algorithm (GA) when used to synthesize amplitude weights of the elements to satisfy specified deep notches, nulls and average sidelobe level constraints. The HGA procedure appears to be a high effective means to compensate the mutual coupling effects on the individual element patterns for the conformal antenna array.
Using eigen-modes of a one-dimensional array of slits together with a mode matching technique, we investigate the extraordinary transmission through a subwavelength grating. The analysis serves to determine the contribution of various transmission mechanisms to the overall transmission. It is shown that surface plasmon polaritons excited on the input interface of the grating at certain wavelengths can absorb the incident power and thus reduce the total transmitted power. We also examine the characteristics of the different types of modes involved in the transmission through a metallic grating.
Analytical TM scattering from semi-elliptic channel loaded with confocal elliptic cylindrical impedance core is investigated. Fields in every regions are expressed appropriately in terms of Mathieu functions. Applying boundary conditions at the impedance core and across different regions of channels and using orthogonality of angular Mathieu functions result in two simultaneous set of equations which would be solved numerically.
A novel design of dual circularly polarized antenna is proposed. By etching cross slots on the patch, circular polarization (CP) is achieved. The patch is fed by two L-strips which provide wide impedance bandwidth and high isolation level. Polarization diversity between left-hand circular polarization (LHCP) and right-hand circular polarization (RHCP) is provided by switching the two ports and both LHCP and RHCP signals can be received simultaneously. The experimental results show that the 3 dB axial ratio CP bandwidth of the proposed design is increased two times, as compared to a referential CP antenna fed by L-strip. The details of experimental results for the proposed design are presented and discussed.
This paper describes a novel design of a dual-polarized ultra wideband horn antenna. Based on a VSWR ≤ 2.6, the bandwidth of the designed UWB horn antenna is from 8-18 GHz, most suitable for radar systems. A newcoaxial line to quadruple-ridged waveguide transition and a newtec hnique for tapering the flared section of the horn is introduced to improve the return loss and matching of the impedance, respectively. Results of simulation for VSWR, isolation, gain and radiation pattern of designed horn antenna are presented and discussed.
A tiny metallic cylinder placed into a planar dielectric waveguide scatters the field developed by a current-carrying skew strip centralized at the middle of the slab. Due to the small size of the scatterer, the induced surface current is taken independent of the azimuthal angle. The Green's function of the problem is expressed in closed form and it is inserted to the scattering integral after the polar equation of the strip has been determined. The behavior of nearfield quantities in the slab, with respect to geometrical and material parameters, is observed and examined.