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Infinite periodic arrays of antennas that can be individually described by means of spherical modes are analyzed starting from the generalized scattering matrix (GSM) of an isolated element. After computing the GSM of an isolated element with the finite-element method (FEM), a fast postprocessing can be carried out to calculate the response of the element in an infinite array environment by using addition theorems for spherical modes. For this purpose, an efficient computation of lattice sums of spherical harmonics is used. The main advantage of this method is that the antenna is analyzed only once whatever the array lattice or scan angle. In addition, fast frequency analysis can be performed since the starting point is the computation of the isolated antenna with the FEM, which is suitable for fast frequency sweep. The active reflection coefficient and the embedded radiation pattern of the infinite periodic array are calculated for several examples to show the capabilities of the proposed method. [ABSTRACT FROM AUTHOR]

Multiple scattering of parallel cylinders with arbitrary cross section is computed using the T-matrix of each single scatterer and the general translational matrix for cylindrical waves. Usually, the recommended golden rule to compute the translational matrix is Graf’s addition theorem. However, this approach cannot be properly implemented for some geometries, such as in a two-cylinder case when the center of one of them falls within the minimum circular cylinder that circumscribes the other one. In order to overcome this limitation, a transformation between cylindrical waves and plane waves, followed by propagation of the latter, is proposed. The new approach succeeds due to an adequate truncation of the evanescent plane wave spectrum. This strategy is demonstrated by studying the scattering of three infinite elliptic metallic cylinders for different electrical sizes and observing the convergence of the results as a function of the truncated spectrum. Finally, to conclusively show the interest and applicability of the approach, two more complex problems are treated: a group of infinite elliptic metallic cylinders where two different sizes are combined and a practical real-life filter in substrate integrated waveguide (SIW) technology, including several groups of rectangular dielectric cylinders. [ABSTRACT FROM AUTHOR]

Mutual coupling in finite arrays of antennas with strongly overlapping minimum spheres is quickly calculated by computing the general translation matrix between spherical modes. This matrix is obtained by using the transformation properties of spherical and plane vector waves. Although this approach is less efficient than the classical one, which is based on addition theorems, it allows to overcome the well-known limitation of addition theorems that requires nonintersecting minimum spheres. Symmetry relations are provided for the translation coefficients that greatly increase the speed of computation of the general translation matrix. By computing the reflection and the transmission submatrices of the generalized scattering matrix of a finite antenna array, accurate results are obtained for the S-parameters and the radiation patterns of arrays, in comparison with commercial software or a purely numerical in-house full-wave method. For this purpose, different types of antennas with strongly overlapping hemispheres in an array environment on a ground plane are used, such as apertures, monopoles, cavity-backed patch antennas, or dielectric resonator antennas. [ABSTRACT FROM AUTHOR]

Nowadays, a great part of Internet content is not reachable from search engines. Studying the nature of these contents from a cyber security perspective is of high interest, as they could be part of many malware distribution processes, child pornography or copyrighted material exchange, botnet command and control messages, and so on. Although the research community has put a lot of effort into this challenge, most of the existing works are focused on contents that are hidden in websites. However, there are other relevant services that are used to keep and transmit hidden resources, such as P2P protocols. In the present work, we suggest the concept of Deep Torrent to refer to those torrents available in BitTorrent that cannot be found by means of public websites or search engines. We present an implementation of a complete system to crawl the Deep Torrent and evaluate its existence and size. We describe a basic experiment crawling the Deep Torrent for 39 days, in which an initial estimation of its size is 67 percent of the total number of resources shared in the BitTorrent network. [ABSTRACT FROM PUBLISHER]

The inclusion of thin lossy material layers, such as carbon-based composites, is essential for many practical applications for modeling the propagation of electromagnetic energy through composite structures such as those found in vehicles and electronic equipment enclosures. Many existing schemes suffer problems of late time instability, inaccuracy at low frequency (LF), and/or large computational costs. This paper presents a novel technique for the modeling of thin-layer lossy materials in finite-difference time domain (FDTD) schemes, which overcomes the instability problem at low computational cost. For this, a 1-D subgrid is used for the spatial discretization of the thin-layer material. To overcome the additional time-step constraint posed by the reduction in the spatial cell size, a Crank–Nicolson time-integration scheme is used locally in the subgridded zone, and hybridized with the usual 3-D Yee-FDTD method, which is used for the rest of the computational domain. Several numerical experiments demonstrating the accuracy of this approach are shown and discussed. Results comparing the proposed technique with classical alternatives based on impedance boundary condition approaches are also presented. The new technique is shown to have better accuracy at LFs and late time stability than existing techniques with low computational cost. [ABSTRACT FROM AUTHOR]

In this letter, the Orthogonal Matching Pursuit algorithm is introduced to model the radiation of antennas in terms of elementary sources. This algorithm, combined with the application of the Translational Addition Theorems of spherical waves, provides an accurate sparse approximation of the transmitting coefficients of spherical waves by placing a reduced set of infinitesimal dipoles in the antenna location. The efficiency of the proposed method is demonstrated by showing its capability to deal with electrically large antennas, the ease of implementation, and the computational efficiency. Numerical results are also presented for an array of cavity-backed patch antennas and a pyramidal horn antenna . [ABSTRACT FROM PUBLISHER]

This work is focused on the array thinning problem in shaped beam far-field synthesis. The orthogonal matching pursuit algorithm, which allows finding sparse solutions in linear systems, is combined with a fast full-wave analysis method for antenna arrays based on a spherical wave expansion, thereby taking mutual coupling between real antennas into account in the thinning process. To this aim, the original iterative algorithm is modified so that the current residual is obtained for the selected real coupled antennas in each step of the algorithm. In this way, the remaining nonselected elements are effectively removed instead of turned off. Results for arrays made up of microstrip patch antennas and dielectric resonator antennas arranged in triangular and rectangular lattices are presented. [ABSTRACT FROM PUBLISHER]

We study the properties of finite-length all-pass side-coupled microring delay lines optimized using a genetic algorithm. The average bandwidth-group delay product is found to follow a linear growth with the number of cascaded stages N. Both the magnitude and the slope of the figure of merit vary with the specific optimization criteria but, for all the cases addressed, they are below the unit slope linear dependence with N of a theoretical upper bound. [ABSTRACT FROM PUBLISHER]

Designing a gradient coil for magnetic resonance imaging (MRI) is an electromagnetic inverse problem often formulated as a constrained optimization, which has been successfully solved by inverse boundary element methods. The constant search for new coil features and improved performance has highlighted the need of employing more versatile optimization techniques capable of dealing with the new requirements. In this paper, the solution of linear and nonlinear optimization problems using particle-swarm optimization (PSO) algorithms is presented. Examples of coil designed using this heuristic method are shown, including a comparison to solutions provided by conventional optimization approaches. Numerical experiments reveal that the application of PSO for the solution of inverse boundary element problems for coil design is a computationally efficient algorithm that is capable of handling nonlinear problems and that offers fast convergence, especially for those symmetric coil geometries where the computational effort can be drastically reduced by using suitable dimensionality-reduction techniques. [ABSTRACT FROM AUTHOR]