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A novel image-reconstruction method is proposed for the processing of data acquired at random spatial positions. The images are reconstructed and updated in real time concurrently with the measurements to produce an evolving image, the quality of which is continuously improving and converging as the number of data points increases with the stream of additional measurements. It is shown that the images converge to those obtained with data acquired on a uniformly sampled surface, where the sampling density satisfies the Nyquist limit. The image reconstruction employs a new formulation of the method of scattered power mapping (SPM), which first maps the data into a three-dimensional (3D) preliminary image of the target on a uniform spatial grid, followed by fast Fourier space image deconvolution that provides the high-quality 3D image. [ABSTRACT FROM AUTHOR]

We present a novel algorithm for adjoint sensitivity analysis of time-varying responses that is based on the transmission line modeling (TLM) method. We show that by using at most one extra electromagnetic (EM) simulation, the sensitivities of the complete time domain EM response can be obtained with respect to all designable parameters regardless of their number and regardless of the number of time steps. The complete time domain signature of a certain structure can thus be efficiently and accurately predicted for perturbed structures. Our sensitivities match those obtained using the accurate and time intensive central finite differences (CFD). The adjoint sensitivities are used in the solution of inverse problems through a number of approaches exploiting surrogate model optimization and direct optimization. Our technique enables efficient identification of object position, dimensions and material properties. Index Terms--Inverse problems, optimization, sensitivity analysis, transmission line modeling (TLM) method.

We propose a sensitivity analysis method that computes the S-parameter Jacobian from the volume E-field solutions in the frequency domain. The field solutions may be provided by any valid electromagnetic analysis. The computation is a post-process, which is independent from the simulator's grid, system equations, and discretization method. The sensitivity solver uses its own finite-difference grid and a sensitivity formula based on the finite-difference frequency-domain vector Helmholtz equation for the electric field. Its computational overhead is negligible in comparison with the simulation. We use the sensitivity solver in the gradient-based optimization of microwave structures. Significant reduction of the time required by the overall optimization process is achieved. In all design examples, the sensitivities are computed from the field solutions provided by a commercial finite-element simulator. Index Terms--Computer-aided design (CAD), finite-difference method, finite-element method (FEM), gradient-based optimization, response Jacobians, sensitivity analysis.

Sensitivity analysis is crucial in microwave imaging and design procedures. Recently, we proposed a time-domain self-adjoint method for the computation of response Jacobians. The responses and their derivatives are computed with a single time-domain analysis. The overhead of the Jacobian computation is negligible compared to the time required by the simulation even when the number of optimizable parameters exceeds thousands. However, two drawbacks have become obvious: 1) memory requirements may become excessive when the number of perturbation grid points is large and the simulation time is long and 2) Jacobian accuracy may degrade due to the intrinsic inaccuracy of the local numerical field solution at dielectric interfaces of high contrast. Here, we propose an improved method for the self-adjoint computation of the Jacobian. It drastically reduces the memory requirements by implementing a novel spectral sensitivity formula, which operates on the spectral components of the E-field rather than on its time waveforms. It significantly improves the accuracy of the Jacobian by departing from the conventional finite-difference Yee cell and employing its own independent central-node finite-difference grid. The proposed approach is validated by 2-D and 3-D examples with lossy dielectric inhomogeneous structures. This study aims at the acceleration of wideband microwave image reconstruction via efficient Jacobian calculation. Index Terms--Adjoint-variable method, Jacobian, microwave imaging, sensitivity analysis, time-domain analysis.

We present an inversion methodology for defect characterization using the data from magnetie flux leakage (MFL) measurements. We use a single tangential component of the leakage field as the MFL response. The inversion procedure employs the space mapping methodology. Space mapping is an efficient technique that shifts the optimization burden from a computationally expensive accurate (fine model to a less accurate (coarse) but fast model. Here the fine model is a finite-element method (FEM) simulation, while the coarse model is based on analytical formulas. We achieve good estimation of the defect parameters using just a few FEM simulations, which leads to ubstantial savings in computational cost as compared to other optimization approaches. We examine the efficiency of the proposed inversion technique in estimating the shape parameters of rectangular and cylindrical defects in steel pipes. Our results show good agreement between the actual and estimated defect parameters. Index Terms--Crack sizing, magnetic flux leakage (MFL), magnetic inverse problems, space mapping optimization.

We propose a procedure for full characterization of rectangular surface-breaking cracks based on measurements of only one tangential component of the magnetic field with the magnetic flux leakage (MFL) technique. The parameters of interest include orientation, length, and depth of the cracks. We assume that the length and the depth of the investigated cracks are much larger than the crack width, so that the variation of the MFL response with respect to the width is negligible. Our procedure employs fast direct methods that provide reliable estimation of the crack parameters in three separate consecutive steps. We propose denoising and correction techniques as well. We confirmed the accuracy of the methods by simulations based on the finite-element method (FEM) as well as by experimental MFL observations. Index Terms--Crack sizing, magnetic flux leakage technique, nondestructive testing, parameter estimation.

A compact ultrawideband (UWB) antenna has been designed for UWB cancer detection systems. The numerical analysis and experimental characterization of a printed monopole antenna fed by a 50-[ohm] coplanar waveguide (CPW) and operating in a lossy coupling medium is presented. A detailed 3D model of the antenna and the biological environment has been constructed and simulated in XFDTD and HFSS. The near-field pattern of the antenna in the proximity of a multilayer human body model and the effect of the human body on the input matching of the antenna are studied. Experimental results show that the return loss is below -9.6 dB from 3.4 GHz to 9.9 GHz. Index Terms--Coplanar waveguide, ultrawideband (UWB) antenna, ultrawideband (UWB) imaging.

We apply space mapping to antenna design for the first time. We exploit a coarse-mesh method of moments (MoM) solver as the coarse model and align it with the fine-mesh MoM solution through space mapping. We employ two plans: (I) implicit and output space mapping, and (II) input and output space mapping. We propose a local meshing method which avoids inconsistencies in the coarse model. The proposed techniques are implemented through our user-friendly space mapping framework (SMF) system. In a double annular ring antenna example, the S-parameter is optimized. The finite ground size effect for the MoM is efficiently solved by space mapping plan I and the design specification is satisfied after only three iterations. In a patch antenna example, we optimize the impedance using both plans in separate optimization processes. Comparisons are made. Coarseness in the coarse model and its effect on the space mapping performance are also discussed. Index Terms--Antenna design, CAD, EM optimization, method of moments (MoM), space mapping (SM).

We propose an adjoint-variable method for design sensitivity analysis of printed circuits and antennas where allowable perturbations in the design parameters are of a discrete type. We extend previous work […]

Semianalytical electromagnetic models of grounding via-holes and shorted resonators of different shapes are proposed and experimentally verified. Our models are based on the cavity approach previously used for circular-pad via-holes. For the first time, a model of a square-pad via-hole is developed. By a perturbation approach, an approximate model for slightly rectangular shorted patches is also derived. All our via-hole models are verified experimentally. The influence of the technologically caused eccentricity of the via-hole shape is also modeled and studied by measurements. Additional verification is done by full-wave simulations with Agilent Momentum (2002) and IE3D-10.1. Our models compute the modal resonant frequencies, fields, and equivalent-circuit components of the via-holes. They allow for the study of the frequency limitations of grounding via-holes for high-speed applications, as well as the modes of shorted resonators for printed antennas and filters. The developed models are fast and accurate. Index Terms--Printed shorted antennas, printed shorted resonators, via-holes.

We implement off-grid boundary conditions within Yee's finite-difference time-domain (FDTD) method without disturbing the existing uniform mesh or changing the standard FDTD code. Both perfect electric conductor and perfect magnetic conductor walls are considered. Examples of straight, slanted, and curved walls are shown, the latter two being represented by an enhanced staircase approximation. We show that: 1) offsets comparable to the spatial step size lead to instabilities and 2) this issue is easily resolved by stepping into the neighboring cell. The method enhances the flexibility of the FDTD method with respect to complex geometrical domains without reducing the spatial step. Index Terms-Finite-difference time-domain (FDTD) methods, off-grid boundary conditions (BCs), slanted and curved boundaries.

The dynamics of wave propagation and interactions in general media is described either by the system of Maxwell's equations, or by the wave equation. This paper focuses on problems modeled by the scalar wave equation, with one or more boundaries at infinity. The computational domain is truncated by a perfectly matched layer (PML) absorbing boundary condition (ABC) modified specifically for wave-equation applications. A problem independent approach is used to enhance the PML performance within the whole frequency band of excitation, in the presence of both evanescent and propagating fields. Numerical reflections below 0.1% are achieved with PML thickness of only six to eight cells, in both open and guided-wave problems. Index Terms--Absorbing boundary conditions (ABC), finite-difference time-domain (FDTD) methods, perfectly matched layer (PML), wave equation.

We demonstrate, for the first time, the integration of adjoint variable method (AVM) sensitivities and the method of moments (MoM) within an optimization framework, where the S-parameters are used to define the cost function. The AVM efficiently estimates the sensitivities of the S-parameters using only one MoM simulation with some additional overhead. Two possible approaches are used: the adjoint finite-difference approach and the adjoint Broyden-update approach. The AVM S-parameter sensitivities are successfully utilized to optimize a band-stop filter, a band-pass filter, and an electromagnetically coupled microstrip Yagi antenna. The results show that the optimizers supported by AVM sensitivities are capable of reaching feasible designs satisfying the specifications. Index Terms--Adjoint variable method (AVM), method of moments (MoM), microstrip structures, minimax optimization, sensitivities.

We present a comprehensive microwave design framework for implementing the original, aggressive, implicit, and response residual space-mapping (SM) approaches through widely available software. General steps and tools for possible SM implementations are elaborated. Our presentation is a reference guide for microwave designers using the SM technique. An instructive 'multiple cheese-cutting' example demonstrates the SM approach to engineering design and some possible pitfalls. For the first time, an ADS framework implements the SM steps interactively. A three-section transformer example illustrates the approach, step by step. A six-section H-plane waveguide filter design emerges after four iterations, using the implicit SM and the response-residual space-mapping (RRSM) optimization entirely within the design framework. An RRSM surrogate is developed to match the fine (HFSS) model. We use sparse frequency sweeps and do not require Jacobians of the fine model. Index Terms--Computer-aided design (CAD), engineering optimization, filter design, parameter extraction (PE), space mapping (SM), surrogate modeling.

An electromagnetic feasible adjoint sensitivity technique (EM-FAST) has been proposed recently for use with frequency-domain solvers. It makes the implementation of the adjoint variable approach to design sensitivity analysis straightforward while preserving the accuracy at a level comparable to that of the exact sensitivities. The overhead computations associated with the estimation of the sensitivities in addition to the system analysis are due largely to the calculation of the derivatives of the system matrix. Here, we describe the integration of the EM-FAST with two methods for accelerated estimation of these derivatives: the boundary-layer concept and the Broyden update. We show that the Broyden update approach (Broyden-FAST) leads to an algorithm whose efficiency is problem independent and allows the computation of the response and its gradient through a single system analysis with practically no overhead. Both approaches are illustrated through the design of simple antennas using method of moments solvers. Index Terms--Adjoint sensitivities, antenna design, Broyden update, design methodology, method of moments (MoM), optimization, sensitivity.

A line-of-sight (LoS) approximation to the equivalence principle used in far-field computations is proposed. In the LoS equivalence, only the surface currents at the virtual surface and the edge currents at its contour, which are on the LoS with the observation point, are used in the calculation of the radiated field. It is a far more efficient alternative to the standard equivalence principle, which requires integration over the entire closed virtual surface. We present the theory associated with the LoS equivalence approach, examine the error due to the approximation, and demonstrate its accuracy via the radiation of dipole antennas. Index Terms--Antenna radiation patterns, electromagnetic diffraction, electromagnetic scattering, finite-difference time-doman (FDTD) methods.

In this paper, novel radial basis function-neural network (RBF-NN) models are presented for the efficient filling of the coupling matrix of the method of moments (MoM). Two RBF-NNs are trained to calculate the majority of elements in the coupling matrix. The rest of elements are calculated using the conventional MoM, hence the technique is referred to as neural network-method of moments (NN-MoM). The proposed NN-MoM is applied to the analysis of a number of microstrip patch antenna arrays. The results show that NN-MoM is both accurate and fast. The proposed technique is general and it is convenient to integrate with MoM planar solvers. Index Terms--Method of moments (MoM), neural networks (NNs), patch antenna arrays, radial basis functions (RBFs).

Recently, we proposed a novel technique for design sensitivity analysis of high-frequency structures with respect to localized perturbations in conductive parameters. Here, we generalize the technique to include shape and material variations and utilize the response sensitivities in gradient-based optimization. Our technique belongs to the class of adjoint-variable methods. Thus, it computes the response and its gradient with only two electromagnetic (EM) simulations--of the original and the adjoint problems--regardless of the number of design parameters. For the first time, adjoint sensitivities with respect to conductive, dielectric-magnetic material and shape perturbations are computed via EM solvers on structured grids. Our approximate sensitivity analysis does not require analytical derivatives of the system matrix. This makes the technique versatile and easy to implement. The technique defaults to exact sensitivities with analytical system matrix derivatives when global design parameters are being perturbed. We discuss the accuracy of the approximate sensitivities, as well as the practicality of the exact sensitivities in specific design problems. We also discuss implementations in gradient-based optimization and illustrate them through simulation and design with the frequency domain transmission line method (FD-TLM). Index Terms--Design methodology, frequency domain TLM, sensitivity.

We propose an adjoint-variable approach to design-sensitivity analysis with time-domain methods based on structured grids. Unlike conventional adjoint-based methods, it does not require analytical derivatives of the system matrices. It is simple to implement with existing computational algorithms such as the finite-difference time-domain (FDTD) technique. The resulting FDTD algorithm produces the response and its gradient in the design parameter space with two simulations regardless of the number of design parameters. The proposed method is validated by the adjoint-based FDTD analysis of waveguide structures with metallic boundaries. Index Terms--Adjoint variable methods, design-sensitivity analysis, finite-difference time-domain (FDTD) methods.

We present a novel algorithm for efficient estimation of objective function sensitivities for time-domain transmission-line modeling (TLM) with dispersive Johns matrix boundaries. The original electromagnetic structure is simulated using TLM. An adjoint TLM simulation that runs backward in time is derived and solved. The sensitivities of the objective function with respect to all designable parameters are estimated using only the original and adjoint simulations. We show that the Johns matrix of the adjoint problem at each time step is the transpose of the corresponding matrix for the original structure. Our approach is illustrated through the estimation of the sensitivities of objective functions with respect to the dimensions of waveguide discontinuities. A very good match is obtained between our sensitivity estimates and those obtained using central difference approximation. Index Terms--Computer-aided design (CAD), electromagnetic (EM) modeling, field-based synthesis, sensitivity analysis, transmission-fine modeling (TLM) method.

Recently, an adjoint variable method (AVM) for sensitivity calculations has been proposed for use with the method of moments solvers. In this paper, we extend this method to be suitable for application to multiport devices. The target objective function is usually represented in terms of the device's S-parameters. Our AVM obtains the sensitivities of the S-parameters with respect to all design variables using only one full simulation with additional overhead. This overhead is usually less than the computation time of a full simulation. An analytical expression for the adjoint excitation is derived, leading to stable sensitivities. The potential of the proposed technique is demonstrated by analyzing low-pass and bandpass filters. The results show very good agreement between the proposed AVM and the conventional finite-difference approach (FDA). Moreover, the AVM is always faster than the FDA. The speed-up factor increases as the size of the problem increases. Index Terms--Adjoint techniques, deembedding, method of moments (MoM), microstrip filters, sensitivity.

We present a novel algorithm for efficient estimation of objective function sensitivities for time-domain transmission-line modeling (TLM) with nondispersive boundaries. The original electromagnetic structure is simulated using TLM. An adjoint TLM simulation that runs backward in time is derived and solved. The sensitivities of the objective function with respect to all designable parameters are estimated using only the original and adjoint simulations. Our approach is illustrated through the estimation of the sensitivities of objective functions with respect to the dimensions of waveguide discontinuities. A very good match is obtained between our sensitivity estimates and those obtained through the accurate and time-intensive central difference approximation. Index Terms--Computer-aided design (CAD), electromagnetic modeling, field-based synthesis, transmission-line modeling (TLM) method.

We introduce the idea of implicit space mapping (ISM) and show how it relates to the well-established (explicit) space mapping between coarse and fine device models. Through comparison, a general space mapping concept is proposed. A simple algorithm based on the novel ISM concept is implemented. It is illustrated on a contrived 'cheese-cutting problem' and is applied to electromagnetics-based microwave modeling and design. An auxiliary set of parameters (selected preassigned parameters) is extracted to match the coarse model with the fine model. The calibrated coarse model (the surrogate) is then (re)optimized to predict a better fine model solution. This is an easy space mapping technique to implement since the mapping itself is embedded in the calibrated coarse model and updated automatically in the procedure of parameter extraction. We illustrate our approach through optimization of a high-temperature superconducting filter using Agilent ADS with Momentum and Agilent ADS with Sonnet's em. Index Terms--Circuit design, computer-aided design (CAD), electromagnetics, microwave modeling, optimization, space mapping (SM), surrogate modeling.