Abbas Semnani

Reconstruction of One-Dimensional Dielectric Scatterers Using Differential Evolution and Particle Swarm Optimization

A comparison between differential evolution (DE) and particle swarm optimization (PSO) in solving 1-D small-scale inverse scattering problems is presented. In this comparison, the efficiency of both aforementioned optimization techniques is examined for permittivity and conductivity profile reconstruction problems. The comparison is carried out under the same conditions of initial population of candidate solutions and number of iterations. Numerical results indicate that both optimization methods are reliable tools for inverse scattering applications even when noisy measurements are considered. In the particular case of small-scale problems investigated in this letter, DE outperforms the PSO in terms of reconstruction accuracy. This is considered an indicative result and not generally applicable.

Two-Dimensional Microwave Imaging Based on Hybrid Scatterer Representation and Differential Evolution

A hybrid method for solving two-dimensional inverse scattering problems is proposed. The method utilizes differential evolution as a global optimizer and is based on two alternative representations of the unknown scatterer. Initially, the scatterer properties are represented by means of truncated cosine Fourier series expansion that involves limited number of unknown expansion coefficients. Then, the reconstructed profile obtained is used as an initial estimate and the differential evolution is further applied to a scatterer representation based on pulse function expansion. In this representation, the scatterer region is subdivided by a fine grid and the scatterer properties are considered constant within each cell. When the truncated cosine Fourier expansion representation is adopted, the dimension of the solution space can be reduced and the instabilities caused by the ill-posedness of the problem are suppressed. In the second step of the hybrid method, where the pulse functions representation is considered, the scatterer reconstruction is finer and more accurate due to its quite accurate initial estimate. Numerical results show that the hybrid method results in lower reconstruction error compared to above-mentioned representations. Also, the hybrid method outperforms the other two representations, even in the presence of noisy field measurements.

An Enhanced Method for Inverse Scattering Problems Using Fourier Series Expansion in Conjunction with FDTD and PSO

A new computationally efficient algorithm for reconstruc- tion of lossy and inhomogeneous 1-D media by using inverse scattering method in time domain is proposed. In this algorithm, cosine Fourier series expansion is utilized in conjunction with finite difference time do- main (FDTD) and particle swarm optimization (PSO) methods. The performance of the proposed algorithm is studied for several 1-D per- mittivity and conductivity profile reconstruction cases. Various types of regularization terms are examined and compared with each other in the presented method. It is shown that the number of unknowns in optimization routine is reduced to about 1/3 as compared with conven- tional methods which leads to a considerable reduction in the amount of computations, while the precision of the solutions would not be affected significantly. Another advantage of the proposed expansion method is that, since only a limited number of terms are taken in the expansion, the divergence of the algorithm is far less likely to occur. Sensitivity analysis of the suggested method to the number of expan- sion terms in the algorithm is studied, as well.

Frequency response of atmospheric pressure gas breakdown in micro/nanogaps

In this paper, we study gas breakdown in micro/nanogaps at atmospheric pressure from low RF to high millimeter band. For gaps larger than about 10 μm, the breakdown voltage agrees with macroscale vacuum experiments, exhibiting a sharp decrease at a critical frequency, due to transition between the boundary- and diffusion-controlled regimes, and a gradual increase at very high frequencies as a result of inefficient energy transfer by field. For sub-micron gaps, a much lower breakdown is obtained almost independent of frequency because of the dominance of field emission.

Pre-breakdown evaluation of gas discharge mechanisms in microgaps

The individual contributions of various gas discharge mechanisms to total pre-breakdown current in microgaps are quantified numerically. The variation of contributions of field emission and secondary electron emission with increasing electric field shows contrasting behavior even for a given gap size. The total current near breakdown decreases rapidly with gap size indicating that microscale discharges operate in a high-current, low-voltage regime. This study provides the first such analysis of breakdown mechanisms and aids in the formulation of physics-based theories for microscale breakdown.

TRUNCATED COSINE FOURIER SERIES EXPANSION METHOD FOR SOLVING 2-D INVERSE SCATTERING PROBLEMS

Truncated cosine Fourier series expansion method is applied for reconstruction of lossy and inhomogeneous 2-D media by using inverse scattering method in time domain. In this method, the unknown parameters are expanded in a cosine Fourier series and coefficients of this expansion are optimized in particle swarm optimization (PSO) routine with the aid of finite difference time domain (FDTD) method as an electromagnetic (EM) solver. The performance of the algorithm is studied for several 2-D permittivity and conductivity profile reconstruction cases. It is shown that since only a limited number of terms are retained in the expansion, using the proposed method guarantees the well-posedness of the problem and uniqueness of the solution and various types of regularization may be used to only have more precise reconstruction. It is also shown that the number of unknowns in optimization routine is reduced more than 75 percent as compared with conventional methods which leads to a considerable reduction in the amount of computations with negligible adverse effect on the precision of reconstruction. Sensitivity analysis of the suggested method to the number of expansion terms in the algorithm is studied, as well.

Miniaturized Reflectarray Unit Cell Using Fractal-Shaped Patch-Slot Configuration

This letter introduces a new class of miniaturized reflectarray unit cells with increased phase swing employing Minkowski fractal-shaped patch-slot elements. Square, 1st Minkowski, and 2nd Minkowski fractal patches are designed as a reflectarray unit cell. A slot with variable lengths of 0 <; Ls <; 6 mm is used in the ground plane to perform the phase variation function. The resonant frequency corresponding to the maximum phase swing is reduced from 10.6 GHz for the square patch down to 8.8 and 8.3 GHz for the first- and second-order Minkowski fractal patches, respectively, which is equivalent to 17% and 22% size reduction. Unit cells with different patch type and slot length are fabricated, and close agreement is observed between the measured and simulated results. As it has been proven for conventional phased array antennas, this size reduction can lead to a decrease in mutual coupling in reflectarray antennas. Alternatively, it allows for smaller distance between reflectarray antenna elements, which renders a wider beam-scanning range.

An Enhanced Hybrid Method for Solving Inverse Scattering Problems

A new hybrid method for solving inverse scattering problems in time domain is proposed. In this method, a two-step optimization-based routine is used in which direct and so called expansion methods are utilized simultaneously to obtain more precise and rapid reconstruction. In the first step, a coarse solution is achieved with the help of truncated cosine Fourier series expansion method. Then, with this solution as an initial guess for the second step, direct optimization is used to obtain a much more accurate solution of the problem. In both steps, finite difference time domain and particle swarm optimization are used as an appropriate electromagnetic solver and global optimization routine, respectively. The most important advantage of this method is that because of a suitable initial answer in direct optimization routine, sensitivity of the algorithm to the regularization parameter is decreased and convergence of the results is completely guaranteed. Performance of the proposed method is demonstrated and compared with expansion and conventional direct optimization methods for several 1-D permittivity and conductivity profile reconstruction cases.

Mutual coupling reduction in waveguide-slot-array antennas using electromagnetic bandgap (EBG) structures

The application of an electromagnetic bandgap (EBG) structure as a tool for reduction of mutual coupling in planar waveguide-slot-array antennas (WSAA) is investigated for the first time. First, surface-wave suppression was demonstrated by placing an EBG array over the ground plane of a linear 1 × 4 waveguide-slot-array antenna. Two 1 × 4 waveguide-slot-array antennas were then put next to each other, with the EBG array over the conducting plane separating them, and up to 10 dB reduction in mutual coupling was observed. Based on these achievements, a planar 2 × 4 waveguide-slot-array antenna was designed using the EBG array to reduce mutual coupling. The EBG array compensated for the undesired frequency shift due to mutual coupling through surface-wave suppression. In addition, the radiation patterns of the EBG-loaded antenna remained almost the same as for the unloaded antenna, with a 1 dB increase in antenna gain. Finally, EBG-loaded and simple 2 × 4 waveguide-slot-array antennas were fabricated, and simulation results were compared with measurements, with good agreement. As a key feature of this approach, a planar waveguide-slot-array antenna could be constructed by designing a linear antenna and then forming an array from multiples of the antenna, without the need for any new design, by imposing the EBG structure to reduce the mutual coupling between the adjacent waveguides.

High-power microwave gas discharge in high-Q evanescent-mode cavity resonators and its instantaneous/long-term effects

This paper presents the first experimental and theoretical investigation of high-power RF gas discharge as applied to RF front-end filters with critical air gaps in the 10s of μm. Specifically, a strongly-coupled high-Q evanescent-mode resonant cavity is utilized as a vehicle in this study. This cavity tends to concentrate the resonant electric field in a small volume between its loading post and top wall. Both experiments conducted up to 45.3 dBm at 6.5 GHz and molecular-dynamics-based modeling suggest that, as the input power is increased, gas ionization leads to increasing gas discharge inside this volume. In addition to the measured RF data, surface analysis of the cavity post and top wall lead to the same conclusion. Besides the observed instantaneous effects on the resonators RF performance, the impact of gas discharge in its long-term performance and potential failure is analyzed.