DSpace Collection:
http://hdl.handle.net/2117/658
20141128T01:51:24Z

On the convergence of the ACA algorithm for radiation and scattering problems
http://hdl.handle.net/2117/24654
Title: On the convergence of the ACA algorithm for radiation and scattering problems
Authors: Heldring, Alexander; Úbeda Farré, Eduard; Rius Casals, Juan Manuel
Abstract: The adaptive cross approximation (ACA) algorithm, when used to accelerate the numerical solution of integral equations for radiation and scattering problems, sometimes suffers from inaccuracies. These inaccuracies occur when the ACA convergence criterion, which is based on an approximation of the residual relative error, is prematurely satisfied. This paper identifies the two sources of this problem and proposes adaptations of the algorithm that remedy them.
20141110T17:44:02Z

A quadband antenna for public safety applications
http://hdl.handle.net/2117/24294
Title: A quadband antenna for public safety applications
Authors: Mopidevi, Hema Swaroop; Damgaci, T; Rodrigo López, Daniel; Jofre Roca, Lluís; Cetiner, Bedri Artug
Abstract: The design, fabrication, and characterization of a frequencyreconfigurable antenna for the United States Public Safety (PS) wireless communication applications are presented. This antenna is quadband operating in the PS bands220, 470, 800, and 4960 MHz. It is an electrically small antenna with calculated ka similar to 0.55 at 220 MHz. The antenna has two reconfigurable modes of operations. In mode 1, 220, 470, and 4960MHz bands are excited. Mode 2 provides operation over 800 and 4960MHz bands. This dynamic frequency reconfiguration is accomplished by two radio frequency microelectromechanical systems switches strategically located within the antenna architecture. The measured and simulated results for impedance and radiation characteristics agree well, where similar to 3%, 4%, 21%, and 17% fractional bandwidths have been measured in the four bands, respectively, while maintaining integrity of radiation pattern.
20141007T13:24:32Z

Nonconforming discretization of the electricfield integral equation for closed perfectly conducting objects
http://hdl.handle.net/2117/24163
Title: Nonconforming discretization of the electricfield integral equation for closed perfectly conducting objects
Authors: Úbeda Farré, Eduard; Rius Casals, Juan Manuel; Heldring, Alexander
Abstract: Galerkin implementations of the method of moments (MoM) of the electricfield integral equation (EFIE) have been traditionally carried out with divergenceconforming sets. The normalcontinuity constraint across edges gives rise to cumbersome implementations around junctions for composite objects and to less accurate implementations of the combined field integral equation (CFIE) for closed sharpedged conductors. We present a new MoMdiscretization of the EFIE for closed conductors based on the nonconforming monopolarRWG set, with no continuity across edges. This new approach, which we call
20140925T14:15:46Z

New graphical processing technique for fast shadowing computation in PO surface integral
http://hdl.handle.net/2117/23208
Title: New graphical processing technique for fast shadowing computation in PO surface integral
Authors: Rius Casals, Juan Manuel; Carbó Meseguer, Alexis; Bjerkemo, Jakob; Úbeda Farré, Eduard; Heldring, Alexander; Mallorquí Franquet, Jordi Joan; Broquetas Ibars, Antoni
Abstract: This paper presents a new graphical processing technique for fast computation of PO surface integral. In contrast with the original graphical processing approach introduced by the authors in 1993, the new one combines a novel shadowing computation algorithm together with the conventional facetbased Gordon's formula, instead of the pixelbased Asvestas' approximation. The resulting hybrid approach needs more CPU power for very complex radar targets, but is free from the pixel discretization noise inherent to graphical processing. It has the same accuracy as conventional Physical Optics computation, but shadowed facets detection is more than 10 times faster than with the most efficient alternative algorithms of O(N log N) computational cost. © 2014 IEEE.
20140612T13:33:08Z

Unit cell for frequencytunable beamscanning reflectarrays
http://hdl.handle.net/2117/23126
Title: Unit cell for frequencytunable beamscanning reflectarrays
Authors: Rodrigo López, Daniel; Jofre Roca, Lluís; Perruisseau Carrier, Julien
Abstract: A reflectarray cell able to dynamically control the reflection phase at a variable frequency is presented. This capability enables beamscanning reflectarrays with frequency reconfigurability, which is a novel capability with applications in frequencyhopping systems, cognitive radio and satellite communications. The proposed cell combines switching and variable impedance loading techniques to maximize the frequency range over which a large dynamic phase range can be obtained. Analytical and numerical approaches are used to design and optimize the reflecting cell, which uses two semiconductor RFswitches and one varactor. An analog phase range above 270 degrees is achieved over a 50% frequency range, from 1.88 GHz to 3.07 GHz, with flat losses of 0.8 dB. For an analog phase range of 180 degrees the cell achieves a 1: 2 frequency reconfiguration range. It is also verified that the cell preserves good performance, and in particular low crosspolarization, under oblique incidence as well. A fully operational cell was fabricated and measured, demonstrating good agreement with simulation results.
20140602T15:12:00Z

Reconfigurable pixellayer isolator for frequencytunable onfrequency repeaters
http://hdl.handle.net/2117/22788
Title: Reconfigurable pixellayer isolator for frequencytunable onfrequency repeaters
Authors: Rodrigo López, Daniel; Díaz Tapia, Edgar; Jofre Roca, Lluís
Abstract: The maximum gain in onfrequency repeaters is limited by the isolation level between their two antennas due to closeloop stability conditions. In this letter, a novel pixellayer isolator for tunable onfrequency repeaters is presented. The scattered field of the reconfigurable pixel layer is used to dynamically minimize the mutual coupling between the antennas. Insight is given on the linkage between coupling and reflection coefficients, and a useful design procedure is proposed. The optimized pixel repeater can tune the operating frequency over a 40% range, with tunable instantaneous bandwidth (1%3%) and achieving a reliable isolation of 50 dB. The repeater has a radiation efficiency of 93% and a constant radiation pattern for the different configurations. Good agreement is obtained between simulations and measurements.
20140430T12:30:33Z

Optical signal processor for millimeterwave interferometric radiometry
http://hdl.handle.net/2117/22617
Title: Optical signal processor for millimeterwave interferometric radiometry
Authors: Nova Lavado, Enrique; Romeu Robert, Jordi; Capdevila Cascante, Santiago; Torres Torres, Francisco; Jofre Roca, Lluís
Abstract: In interferometric radiometry, the correlations between all pairs of radiofrequency (RF) receivers must be performed in order to obtain the scene visibilities. This represents a cumbersome problem in passive imaging where large signal bandwidths are typically required to achieve fastacquisition times and improve the radiometric resolution of the image. In this case, the signal distribution and the correlation at intermediate frequencies require very fast signal acquisition and processing subsystems. This paper presents a technique to perform correlations of millimeterwave signals produced by thermal emission in the optical domain as a solution to the aforementioned problem. The proposed method is based on converting the RF signal to the optical domain by modulating a laser beam with the RF signal using a {\rm LiNbO}3 phase modulator. This conversion allows to perform the signal distribution in the optical domain to obtain the combination of the receiver pairs. The correlation is obtained by measuring the power of the photocurrent produced by photodetecting the combined signal. The results of an experimental validation consisting on the acquisition of passive images using a linear interferometric array are presented to support the feasibility of the method. In addition, performance considerations of the system have been developed and validated by calculating the standard deviation of a visibility measurement.
20140411T12:01:45Z

Fast computation of an infinite, longitudinallyvarying and harmonic strip load acting on a viscoelastic halfspace
http://hdl.handle.net/2117/22264
Title: Fast computation of an infinite, longitudinallyvarying and harmonic strip load acting on a viscoelastic halfspace
Authors: Arcos Villamarín, Robert; Clot Razquin, Arnau; Romeu Garbí, Jordi; Martín Román, SaraRegina
Abstract: In the context of the developing of a numerically efficient model of railway induced ground vibrations, the problem of an infinite, longitudinallyvarying and harmonic strip load acting on the surface of a viscoelastic halfspace is reformulated in order to improve its numerical evaluation. On one hand, the static integrands of this specific problem are modified and introduced into the original integrands to reduce their spectral content at high wavenumbers, saving computational time needed for the numerical integration. On the other hand, a change of variable is applied on the displacements integral solutions resulting on frequency independent integrands. These formulae allow to obtain the displacement solution in the complete xyω field easier and, mostly, faster
20140318T16:01:01Z

Radiometric and spatial resolution constraints in millimetermave closerange passive screener systems
http://hdl.handle.net/2117/21939
Title: Radiometric and spatial resolution constraints in millimetermave closerange passive screener systems
Authors: Nova, Enrique; Romeu Robert, Jordi; Torres Torres, Francisco; Pablos Hernández, Miriam; Riera, Jose Manuel; Broquetas Ibars, Antoni; Jofre Roca, Lluís
Abstract: This paper presents a comparative study of the radiometric sensitivity and spatial resolution of three nearfield (NF) passive screener systems: real aperture, 1D synthetic aperture (SA), and 2D SA radiometers are compared. The analytical expressions for the radiometric resolution, the number of required antennas, and the number of pixels in the image are derived taking into account the distortion produced by the NF geometry at nonboresight directions where the distortion is dominant. Based on the theoretical results, a performance comparison among the studied systems is carried out to show the advantages and drawbacks when using the radiometers in a closerange screening application. Moreover, the screener performance in a closerange environment is discussed from the results obtained in the aforementioned comparison.
20140307T15:04:06Z

Silicon micromachined lens antenna for THz integrated heterodyne arrays
http://hdl.handle.net/2117/21678
Title: Silicon micromachined lens antenna for THz integrated heterodyne arrays
Authors: Llombart Juan, Nuria; Lee, Choonsup; Alonso del Pino, María; Chattopadhyay, Goutam; Jung Kubiak, Cecile; Jofre Roca, Lluís; Mehdi, Imran
Abstract: In this paper, we present the design, fabrication, and measurements of a lens THz antenna that can be fabricated using conventional photolithography and deep reactive etching processes. The antenna is composed of an extended hemispherical silicon lens and a leaky wave waveguide feed. Both elements are fabricated using silicon micromachining techniques, enabling the fabrication of future large antenna arrays with a parallel process. To show the concept, a first antenna prototype has been fabricated using this fabrication process. Measurements obtained at 550 GHz are presented.
20140220T16:40:37Z

Stable discretization of the electricmagnetic field integral equation with the taylororthogonal basis functions
http://hdl.handle.net/2117/20089
Title: Stable discretization of the electricmagnetic field integral equation with the taylororthogonal basis functions
Authors: Úbeda Farré, Eduard; Tamayo Palau, José María; Rius Casals, Juan Manuel; Heldring, Alexander
Abstract: We present two new facetoriented discretizations in method of
moments (MoM) of the electricmagnetic
field integral equation (EMFIE)
with the recently proposed Taylororthogonal (TO) and divergenceTaylororthogonal (divTO) basis functions. These new schemes, which we call stable, unlike the recently published divergence TO discretization of the
EMFIE, which we call standard, result in impedance matrices with stable condition number in the very low frequency regime. More importantly, we show for sharpedged objects of moderately small dimensions that the
computed RCS with the stable EMFIE schemes show improved accuracy with respect to the standard EMFIE scheme. The computed RCS for the sharpedged objects tested becomes much closer to the RCS computed with the RWG discretization of the electricfield integral equation (EFIE), which is wellknown to provide good RCS accuracy in these cases. To provide best assessment on the relative performance of the several implementations, we have cancelled the main numerical sources of error in the RCS computation: (i) we implement the EMFIE so that the nonnull static quasisolenoidal
current does not contribute in the far
field computation; (ii) we compute with machineprecision the strongly singular Kernelcontributions in the impedance elements with the direct evaluation method.
20130903T10:33:27Z

Optically beamformed wideband array performance
http://hdl.handle.net/2117/20072
Title: Optically beamformed wideband array performance
Authors: Jofre Roca, Lluís; Stoltidou, Chysavgi; Blanch Boris, Sebastián; Mengual, Teresa; Vidal, Borja; Marti, Javier; Mckenzie, Iain; Cura, J.M. del
Abstract: Optical beamforming networks (OBFNs) are an interesting alternative for the design of wideband antenna arrays, mainly due to their low losses and weight, their high parallel processing capabilities and their electromagnetic immunity. Nevertheless, for a practical implementation, different technological and architectural tradeoffs need to be assessed. In this paper, the performance of an OBFN system is analytically studied and experimentally demonstrated. The study forms part of the optical beamforming network project, a multigroup research project supported by the European Space Agency (ESA). Differently from bulky freespace geometries, the architecture is based on fiberoptic delay lines for the time delays generation and on a spatial light modulator for the phase shifts generation. Experimental results for an Xband prototype are provided, showing beam steering accuracy, multibeam capability, amplitude distribution weighting and wideband operation.
Description: IEEE Antennas and Propagation Society
Research Awards for 2009
20130826T08:40:19Z

Accelerated direct solution of the methodofmoments linear system
http://hdl.handle.net/2117/19930
Title: Accelerated direct solution of the methodofmoments linear system
Authors: Heldring, Alexander; Tamayo, J.M.; Úbeda Farré, Eduard; Rius Casals, Juan Manuel
Abstract: This paper addresses the direct (noniterative) solution of the methodofmoments (MoM) linear system, accelerated through blockwise compression of the MoM impedance matrix. Efficient matrix block compression is achieved using the adaptive crossapproximation (ACA) algorithm and the truncated singular value decomposition (SVD) postcompression. Subsequently, a matrix decomposition is applied that preserves the compression and allows for fast solution by backsubstitution. Although not as fast as some iterative methods for very large problems, accelerated direct solution has several desirable features, including: few problemdependent parameters; fixed time solution avoiding convergence problems; and high efficiency for multiple excitation problems [e.g., monostatic radar cross section (RCS)]. Emphasis in this paper is on the multiscale compressed block decomposition (MSCBD) algorithm, introduced by Heldring , which is numerically compared to alternative fast direct methods. A new concise proof is given for the N2 computational complexity of the MSCBD. Some numerical results are presented, in particular, a monostatic RCS computation involving 1 043 577 unknowns and 1000 incident field directions, and an application of the MSCBD to the volume integral equation (VIE) for inhomogeneous dielectrics.
20130711T14:38:23Z

Sparsified adaptive cross approximation algorithm for accelerated method of moments computations
http://hdl.handle.net/2117/19928
Title: Sparsified adaptive cross approximation algorithm for accelerated method of moments computations
Authors: Heldring, Alexander; Tamayo Palau, José María; Simon, C.; Úbeda Farré, Eduard; Rius Casals, Juan Manuel
Abstract: This paper presents a modification of the adaptive cross approximation (ACA) algorithm for accelerated solution of the Method of Moments linear system for electrically large radiation and scattering problems. As with ACA, subblocks of the impedance matrix that represent the interaction between well separated subdomains are substituted by “compressed” approximations allowing for reduced storage and accelerated iterative solution. The modified algorithm approximates the original subblocks with products of sparse matrices, constructed with the aid of the ACA algorithm and of a subsampling of the original basis functions belonging to either subdomain. Because of the sampling, an additional error is introduced with respect to ACA, but this error is controllable. Just like ordinary ACA, sparsified ACA is kernelindependent and needs no problemspecific information, except for the topology of the basis functions. As a numerical example, RCS computations of the NASA almond are presented, showing an important gain in efficiency. Furthermore, the numerical experiment reveals a computational complexity close to N logN for sparsified ACA for a target electrical size of up to 50 wavelengths.
20130711T14:21:54Z

Fractalshaped antennas and their application to GSM 900/1800
http://hdl.handle.net/2117/19687
Title: Fractalshaped antennas and their application to GSM 900/1800
Authors: Puente Baliarda, Carles; Anguera Pros, Jaume; Romeu Robert, Jordi; Borja, C.; Navarro, Mónica; Soler, Jordi
20130626T17:05:35Z