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In this study, the authors present a set of compact and no-overflow formulations to calculate the response of logging tools with coaxial coil antennas in a concentric cylindrically multilayered formation. It can be applied to fast forward computation and the inverse problem for electromagnetic well-logging. The tool mandrel can be cylindrically multilayered, and the electrical parameters of each layer are arbitrary. In contrast with previous formulations, they adopt the novel reflection and transmission coefficients in the authors' formulations so that the notorious overflow problem for numerical computation can be obviated. Numerical results in comparison with those from other approaches have demonstrated the validity and stability of their new formulations. [ABSTRACT FROM AUTHOR]

Antenna arrays with shaped patterns have drawn significant attention for their wide applications in radar, sonar and communication systems. The combination of genetic algorithm (GA) and fast Fourier transform (FFT) has been used to synthesise shaped pattern of antenna array's factor without considering coupling effects. In this work, the GA-FFT method is generalised by integrating with a new virtual active element pattern (AEP) expansion method which approximates each AEP as the radiation pattern by exciting several equally spaced virtual elements surrounding the real element position. The generalised GA-FFT can be applied for the shaped pattern synthesis of unequally spaced linear arrays including mutual coupling and platform effect. Several synthesis examples with different pattern shapes and different antenna structures are given to demonstrate the effectiveness, accuracy and robustness of the proposed method. [ABSTRACT FROM AUTHOR]

A novel short- and open-stub loaded spiral resonator (SOSL-SR) is presented and analysed by the odd- and evenmode method to obtain three controllable resonant modes. Therefore, a triple-mode bandpass filter (BPF) has been designed with good frequency selectivity based on the proposed SOSL-SR. Then, by dividing these three resonant modes into two passbands or stopbands, a dual-band BPF and a dual-band bandstop filter using the proposed SOSL-SR have been demonstrated. Because of the spiral structure, the sizes of these three designed microstrip filters are reduced significantly. Good agreement between simulated and measured results validates the design concept. [ABSTRACT FROM AUTHOR]

In this study, the authors consider the adaptive detection with multiple-input multiple-output radar in compound-Gaussian clutter. The covariance matrices of the primary and the secondary data share a common structure, but different power levels (textures). A Bayesian framework is exploited where both the textures and the structure are assumed to be random. Precisely, the textures follow Gamma distribution or inverse Gamma distribution and the structure is drawn from an inverse complex Wishart distribution. In this framework, two generalised likelihood ratio tests are derived. Finally, they evaluate the capabilities of the proposed detectors against compound-Gaussian clutter as well as their superiority with respect to some existing techniques. [ABSTRACT FROM AUTHOR]

To design three-dimensional (3D) conformal conical arrays with desired 3D co-polarisation and crosspolarisation radiation patterns, a pattern synthesis method based on equivalent principle and trust region method is presented. Dynamic range ratio (DRR) of element excitations is taken into consideration. An analytical function is utilised to constrain DRR and avoid discontinuity of objective function. Owing to the optimisation is based on very good initial currents distribution, the optimisation procedure is very fast. The analytical gradient and Hessian matrix expressions of the objective function are derived. This also improves the optimisation efficiency compared with using the finite difference methods to solve gradient and Hessian matrix. To demonstrate the effectiveness of this proposed method, a conformal conical array with 454 elements has been designed. The amplitude and phase excitations were optimised by using the proposed method. [ABSTRACT FROM AUTHOR]

A printed monopole ultra-wideband antenna with reduced ground plane effect is presented. The ground plane dependence of the antenna is reduced by cutting a slot in the radiator, and then introducing a strip in the slot, to reduce the current distribution on the ground plane at a desired frequency. At this frequency, the impedance performance of the antenna is mainly affected by the monopole radiator, thus reducing the effect of the ground plane on the antenna. Measurement results confirm that this approach works well and also show good far-field properties of the antenna. The antenna is printed on a small FR4 substrate with an area of 25 mm2; it achieves an impedance bandwidth of 3.6 to over 11 GHz, and broadband ground plane effect suppression around the design frequency of 5 GHz. [ABSTRACT FROM AUTHOR]

New dual-mode microstrip bandpass filters (BPFs) using pie-section truncated semi-circle and/or quarter-circle resonators are presented. The even-mode and odd-mode frequency responses of these two resonators are investigated. Compared to the four-parameter optimisation of the conventional stub-loaded dual-mode resonator, only three parameters need to be adjusted and optimised for the proposed resonators. Based on these two novel resonators, three BPF examples are designed, fabricated and tested. In addition, the technique of source-load cross coupling to improve performance of the filters is adopted. The good agreement between simulated and measured results validates the design concept. [ABSTRACT FROM AUTHOR]

In this study, a planar printed double-dipole antenna with broadband performance and high directivity is presented. The proposed antenna consists of two dipole elements, two pairs of parallel striplines and a microstrip balun. The main features of the designed antenna are the two dipole elements designed with different lengths and excited in the left and right parts with a pair of antiphase signals. The two dipole elements are designed with different lengths for achieving broadband performance. High directivity is obtained by feeding the two dipole elements with the two antiphase signals. In order to illustrate the effectiveness of the proposed design, a prototype of the proposed antenna is fabricated and measured. Experimental results of the fabricated antenna have verified the effectiveness of the proposed design. Measured results show that the fabricated antenna provides a 53.7% impedance bandwidth ranging from 4.5 to 7.8 GHz and a measured gain better than 4 dBi. Good unidirectional radiation patterns with a high front-to-back ratio greater than 17 dB are achieved. [ABSTRACT FROM AUTHOR]

In this study, a printed stepped open-slot antenna with unidirectional radiation characteristics is presented. Using a stepped feeding slot and a parasitic metal strip, the impedance matching is improved and a wide bandwidth is achieved. Additionally, by modifying the ground plane with a pair of L-shaped slots, the radiation directivity of the proposed antenna is significantly enhanced and good unidirectional radiation characteristics are obtained. To demonstrate the effectiveness of the design, a prototype antenna is fabricated and tested. Experimental results show that the antenna provides a fractional bandwidth of 101%, ranging from 3.65 to 11.1 GHz with a compact size (30 mm × 30 mm). A moderate gain, which is better than 4 dBi, is also obtained. In addition, a stable radiation directivity, which is fixed in the endfire direction (y-axis direction) is achieved. The measured front-to-back ratio is larger than 9.5 dB within the effective bandwidth. [ABSTRACT FROM AUTHOR]

Sparse linear array has some advantages over uniform linear array in some applications. However, it is well known that sparse array always suffers from manifold ambiguity, which has significant influence on direction-of-arrival (DOA) estimation and resolution. In this study, a new method based on the multiple signal classification (MUSIC) for resolving manifold ambiguities of uncorrelated sources by using semi-circular substrate is proposed for DOA estimation of sparse array. Spurious MUSIC peak spectrums are generated because of a linear combination of the steering vectors of true DOAs. The main idea of the proposed method is to setup one or several semi-circular medium substrates at the front of some elements. The phases of these elements are changed through the refractive index and the radii of those semi-circular substrates. This treatment breaks down the previous linearity of the steering vectors for the array without adding the substrates. Thus, true DOA peaks can be discriminated according to this feature. Simulation results show good performance of the proposed method. Trivial and nontrivial ambiguities are efficiently resolved. [ABSTRACT FROM AUTHOR]

To solve large-scale electromagnetic scattering problems for bodies of revolution, a fast inhomogeneous plane wave algorithm (FIPWA) is developed. By using the Weyl identity and designing a proper integral path, the aggregation and disaggregation factors can be computed efficiently in FIPWA. Both the memory and CPU time requirements are significantly reduced for large scale BOR problems over the method of moments. Numerical results are given to demonstrate the validity and the efficiency of the FIPWA method to perfect conductors, but the method can be extended to dielectric BORs. © 2010 Wiley Periodicals, Inc. Microwave Opt Technol Lett 52: 1915–1922, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.25319 [ABSTRACT FROM AUTHOR]

Electrical impedance tomography (EIT) is a developing imaging modality for early detection of breast cancer. In an EIT system, a low-frequency current is applied sequentially between different electrode pairs while voltage measurements are made at other electrodes to arrive at the electrical impedance values. The set of impedance measurement data is then computed to produce a 3D electrical conductivity map of the volume to be imaged. In this work, the design, measurements, and inversion of a full 3D EIT system are discussed. Experimentally determined EIT images of phantom objects are presented. © 2008 Wiley Periodicals, Inc. Microwave Opt Technol Lett 50: 3261–3271, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.23932 [ABSTRACT FROM AUTHOR]

A thermoacoustic tomography (TAT) forward solver based on the spectral element method (SEM) with perfectly matched layer absorbing boundary condition has been developed. The TAT forward solver is intended to model acoustically inhomogeneous media with high accuracy in the frequency domain. The high-order basis functions used in the SEM are Gauss-Lobatto-Legendre (GLL) polynomials. Due to the orthogonality of the GLL basis functions and GLL quadrature integration, the mass matrix is diagonal and the stiffness matrix is sparse. Thus, the proposed method greatly reduces the memory requirement and computational time in comparison with the conventional finite element method (FEM). Numerical results show that the high-order SEM is able to achieve the same accuracy as the FEM but with a much smaller number of unknowns. Therefore, the TAT forward solver based on SEM is able to simulate a large-scale and realistic TAT problem. [ABSTRACT FROM AUTHOR]

A 2D electrical impedance tomography (EIT) system has been developed at Duke University as an experimental system to test the forward and inverse algorithms for EIT application. The forward model is based on the 2nd-order finite element method (FEM), while the image reconstruction is based on the distorted Born iterative method (DBIM). The major contributions of this work are the application of the higher-order FEM as a forward solver, and the DBIM as an inverse solver to the integrated EIT system. The forward model has been validated with the measured data to within 0.5% accuracy. Excellent images have been reconstructed with these collected EIT data sets. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 2989–2998, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.22938 [ABSTRACT FROM AUTHOR]

This paper presents a novel radiation boundary condition (RBC) with the spectral integral method (SIM) to truncate the computational domain in the finite-element method (FEM). Because of the spectral accuracy of the SIM, the sampling density on the radiation boundary requires less than four points per wavelength to achieve a high accuracy (1%). As a result, the introduction of the SIM as an RBC actually can decrease the total number of unknowns in the system equation. Numerical results illustrate the usefulness of this novel RBC for both homogeneous and inhomogeneous objects. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 1995–2002, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.22608 [ABSTRACT FROM AUTHOR]

Non-Cartesian sampling is widely used for fast magnetic resonance imaging (MRI). Accurate and fast image reconstruction from non-Cartesian k-space data becomes a challenge and gains a lot of attention. Images provided by conventional direct reconstruction methods usually bear ringing, streaking, and other leakage artifacts caused by discontinuous structures. In this paper, we tackle these problems by analyzing the principal point spread function (PSF) of non-Cartesian reconstruction and propose a leakage reduction reconstruction scheme based on discontinuity subtraction. Data fidelity in k-space is enforced during each iteration. Multidimensional nonuniform fast Fourier transform (NUFFT) algorithms are utilized to simulate the k-space samples as well as to reconstruct images. The proposed method is compared to the direct reconstruction method on computer-simulated phantoms and physical scans. Non-Cartesian sampling trajectories including 2D spiral, 2D and 3D radial trajectories are studied. The proposed method is found useful on reducing artifacts due to high image discontinuities. It also improves the quality of images reconstructed from undersampled data. [ABSTRACT FROM AUTHOR]

In this paper we review the recent development in pseudospectral time-domain (PSTD) methods for broadband electromagnetics. Starting from the basic ideas for high-order approximation of spatial derivatives in periodic and non-periodic domains, we will discuss the pseudospectral time-domain methods for Maxwell's equations. Furthermore, some recent time integration schemes for the PSTD methods will be also presented. We will demonstrate these efficient methods with some new results for large scale problems. Copyright © 2004 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

In this work, 2.5-dimensional finite-difference time-domain (FDTD) and pseudospectral time-domain (PSTD) algorithms are developed for electromagnetic waves in media that are invariant in one spatial dimension. Both algorithms use Berenger's perfect matched layer (PML) as an absorbing boundary condition. The FDTD method is an extension of Yee's 3D algorithm to 2.5D. The PSTD algorithm uses the fast Fourier transform to approximate spatial derivatives, and gives a much higher accuracy than the FDTD method. Numerical results have been validated by comparing them with analytical solutions. The methods have been applied to ground-penetrating-radar and dielectric-waveguide problems. © 2003 Wiley Periodicals, Inc. Microwave Opt Technol Lett 36: 430–436, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.10783 [ABSTRACT FROM AUTHOR]

A novel method combining the conjugate-gradient and nonuniform fast Fourier transform (CG–NUFFT) algorithms for solving integral equations in electromagnetics is presented. The CG–NUFFT method has been applied to one- and two-dimensional inhomogeneous media. Numerical results show the advantages of the CG–NUFFT method over the CG–FFT method for problems with both electrically large and small regions. © 2000 John Wiley & Sons, Inc. Microwave Opt Technol Lett 24: 385–389, 2000. [ABSTRACT FROM AUTHOR]

A generalized k-space (GkS) method is developed for transient electromagnetic modeling. This method consists of the solution of two equations: (1) the scattering equation in the spectral-time (k - 1) domain, and (2) the constitutive equation in the spatial-time (r - t) domain. Both are derived as local algebraic equations, and therefore can be solved with O(N) operations. The connection between the r - t domain and k - t domain is obtained by the spatial FFT algorithm. Therefore, in each time step, the number of complex multiply-add operations is O N log2, N, and the storage requirement is O(N). Because it treats the spatial derivatives by Fourier transform, the k-space method, compared to the finite-difference method, provides a high-order accuracy for the same discretization, it is shown that with the same accuracy requirement, the GkS method requires much fewer unknowns than the conventional finite-difference method. [ABSTRACT FROM AUTHOR]

We propose a new technique to solve the integral equation for wave scattering by an axisymmetric inhomogeneity using the conjugate gradient (CG) method together with the fast Fourier-Hankel transforms (FFHT). The method uses fast Hankel transform (FHT) and fast Fourier transform (FFT) requiring only O(N log N) floating-point operations per iteration and O(N) storage. [ABSTRACT FROM AUTHOR]

The pseudospectral time-domain (PSTD) algorithm is extended to model the electromagnetic scattering from both 2-D and 3-D objects. The plane-wave excitation and the time-domain near–far field transforms in the stretched coordinates are efficiently incorporated into the scattered-field PSTD algorithm. Numerical results demonstrate the effectiveness of the algorithm in wave-scattering problems. © 2001 John Wiley & Sons, Inc. Microwave Opt Technol Lett 29: 123–125, 2001. [ABSTRACT FROM AUTHOR]

In the conjugate gradient–fast Fourier transform (CGFFT) method, the FFT is used to evaluate the convolution integrals. When the function to be transformed has discontinuities, the accuracy of the FFT results, and thus the CGFFT results, will degrade. In this letter, an efficient FFT algorithm is developed for discontinuous functions with both uniform and nonuniform sampled data, with O(Np+N log N) complexity, where N is the number of sampling points and p is the interpolation order. The algorithm is incorporated into the CGFFT method. Numerical results for slabs demonstrate the efficiency and accuracy of the new FFT and CGFFT algorithms. © 2001 John Wiley & Sons, Inc. Microwave Opt Technol Lett 29: 47–49, 2001. [ABSTRACT FROM AUTHOR]

The problem of synthesising a sparse linear array with multiple patterns is formulated as an extended reweighted l1-norm minimisation with multiple convex constraints. Synthesis results show that the proposed method can find the common sparse element positions with optimised excitations for multiple-pattern requirements, and the mainlobe, sidelobe as well as nulling region for each pattern can be accurately controlled. [ABSTRACT FROM AUTHOR]

A novel extension of the cubic phase function (CPF), known as the parameterised centroid frequency-chirp rate distribution (PCFCRD), is proposed for multicomponent linear frequency modulated signals analysis. The PCFCRD is coherent and introduces a constant delay to reduce the noise correlation. Mathematic analysis and numerical simulations demonstrate that, compared with the coherent integrated CPF, the PCFCRD can obtain a higher anti-noise performance. [ABSTRACT FROM AUTHOR]

On the basis of a single multimode resonator (MMR), a miniaturised microstrip sext-band bandpass filter (BPF) with high band-to-band isolations has been presented. The proposed MMR is an stepped impedance resonator (SIR) with one short-circuited stub loaded in the middle and two identical open-circuited stubs located at two sides. Transmission poles (TPs) and transmission zeros (TZs) can be excited when the electrical lengths of two open stubs increase. The TZs introduced by open stubs separate the TPs into multiple groups; therefore, the design of a sext-band BPF becomes available. Moreover, two input/output tapped branches and radical stub-loaded short-circuited lines are treated as perturbation structures in order to increase design freedoms for improving filter performance. Good agreement between simulated and measured results validates the design method. [ABSTRACT FROM AUTHOR]

A unidirectional planar monopole ultra-wideband (UWB) antenna is presented. To realise unidirectional radiation, a new feed structure is implemented without modifications in the ground plane or the monopole. By using a wrench-shaped feeding structure, unidirectional radiation of the planar monopole antenna can be realised, especially at high frequencies. The antenna is built on a 20 x 30 mm FR4 substrate to cover a frequency bandwidth from 3.1 to over 14 GHz for a voltage standing wave ratio ≤2. [ABSTRACT FROM AUTHOR]