F.J. Villegas

University of California, Los Angeles, Los Angeles, CA, United States

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Publications (15)16.02 Total impact

  • F.J. Villegas
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    ABSTRACT: A parallel genetic algorithm (GA) optimization tool has been developed for the synthesis of arbitrarily shaped beam coverage using planar 2D phased-array antennas. Typically, the synthesis of a contoured beam footprint using a planar 2D array is difficult because of the inherently large number of degrees of freedom involved (in general, the amplitude and phase of each element must be determined). We make use of a parallel GA tool in this study to compensate for this aspect of the design problem. The algorithm essentially compares a desired pattern envelope with that of trial arrays, and quantifies the effectiveness or desirability of each test case via a fitness function. The GA uses this information to rank and select subsequent arrays over a given number of generations via the conventional stochastic operators, i.e., selection, crossover, and mutation. Each fitness evaluation of a trial pattern is done on a node of the aerospace fellowship cluster supercomputer, which increases the speed of the algorithm linearly with the number of nodes. Because of the continuous nature of the parameters for this optimization problem, a real parameter encoding scheme is employed for the GA chromosome in order to avoid the quantization errors associated with a binary representation. A benchmark 10 times 10 (100) element array is employed, and various results of optimized coverage patterns are shown herein to illustrate the effectiveness and validity of the technique.
    IEEE Transactions on Antennas and Propagation 07/2007; · 2.33 Impact Factor
  • Source
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    ABSTRACT: In this paper, we describe an electromagnetic genetic algorithm (GA) optimization (EGO) application developed for the cluster supercomputing platform. A representative patch antenna design example for commercial wireless applications is detailed, which illustrates the versatility and applicability of the method. We show that EGO allows us to combine the accuracy of full-wave EM analysis with the robustness of GA optimization and the speed of a parallel computing algorithm. A representative patch antenna design case study is presented. We illustrate the use of EGO to design a dual-band antenna element for wireless communication (1.9 and 2.4 GHz) applications. The resulting antenna exhibits acceptable dual-band operation (i.e., better than -10 dB return loss with 5.3 and 7% operating bandwidths at 1.9 and 2.4 GHz) while maintaining a cross-pol maximum field level at least 11 dB below the co-pol maximum.
    IEEE Transactions on Antennas and Propagation 10/2004; · 2.33 Impact Factor
  • F.J. Villegas, Y. Rahmat-Samii, D.R. Jackson
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    ABSTRACT: In this paper, we propose a novel hybrid space/spectral-domain method-of-moments (MoM) calculation of canonical plane-wave scattering from finite periodic structures comprised of dissimilar cylindrical cavities in an infinite ground plane, covered by a dielectric superstrate. We take full advantage of analyzing this particular type of problem (comprised of a canonical geometry) by employing a set of entire-domain basis functions (waveguide modes), which truly span the Hilbert space in which the desired solution resides. The convergence behavior of a solution using said functions is quite robust. Results show that typically only a few basis functions are required to adequately represent the aperture fields with fidelity. We have also reformulated the conventional magnetic field integral equation in a hybrid fashion, whereby the exterior (unbounded half space) MoM interaction terms have been cast in the spectral domain and the interior (cavity) interactions cast in the space domain directly. The spectral representation allows us to trivially include a superstrate layer in the geometry and additionally leads to an efficient evaluation of the interaction integrals due to an overall reduction in the spatial dimensionality. The interior interactions are readily handled in the space domain, where we take advantage of the orthogonality property of waveguide modes. Most importantly, the technique is significantly more accurate than purely numerical methods, i.e., those in which a discretization of the geometry is necessary.
    IEEE Transactions on Antennas and Propagation 10/2003; · 2.33 Impact Factor
  • F.J. Villegas, Y. Rahmat-Samii, D.R. Jackson
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    ABSTRACT: In this paper, we investigate the scattering characteristics of finite one-dimensional (1D) and two-dimensional arrays of cylindrical cavities embedded within an infinite perfect electric conducting ground plane. Fundamental radar cross-section (RCS) features of said arrays are illustrated, with the RCS computed with respect to the scattered (diffracted) field component, obtained directly from the aperture currents. The aperture currents on the finite structure are computed using a hybrid space/spectral-domain MoM formulation. The complete set of (TE,TM) cylindrical eigenmodes are used to represent the unknowns. The dependence of the current distribution on the source polarization is shown for the canonical cavity. Three-dimensional plots of the RCS signature of a 1D finite array illustrate the unique radiation characteristics of such structures, e.g., the constant-directivity surfaces form cones about the array axis in 3D space. The effects of a superstrate layer on the radiation behavior of a 2D finite array are also shown. We find that the layer can in fact either enhance or diminish the radiation, depending on the source polarization. It also tends to reduce endfire radiation in all cases. The scattering signature of large finite arrays is computed approximately by the introduction of an active element factor defined specifically for scattering problems. Using the MoM solution of a reduced window array, we obtain the far-field patterns of larger structures by a simple computation of the array factor. An improvement to this approximation makes use of an exterior equivalent source, which models the localized edge effects. Finally, the use of nonuniform arrays for sidelobe suppression and 2D "masked" structures is illustrated.
    IEEE Transactions on Antennas and Propagation 10/2003; · 2.33 Impact Factor
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    ABSTRACT: The radiation field from a leaky mode on a semi-infinite printed-circuit line is investigated. Asymptotic formulas for the radiation field are presented for a mode that is either in the physical region (a fast wave with respect to the substrate mode into which leakage occurs), or in the spectral-gap region (a slow wave). These formulas, as well as a numerically exact representation of the leaky-mode radiation field, are used to examine the nature of the radiation field and to study how it changes as the leaky mode transitions from the physical region to the spectral-gap region. A comparison is made between the field radiated by the leaky-mode current and that radiated by a practical probe feed that launches the semi-infinite leaky mode current. Another comparison is made between the field radiated by the leaky-mode current and the exact field radiated by the total current that is excited by a practical gap voltage source on an infinite line.
    Radio Science - RADIO SCI. 01/2003; 38:17-1.
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    ABSTRACT: In this paper, we make use of genetic algorithm (GA) optimization to arrive at an optimal (perhaps counterintuitive) design for a dual-band patch antenna intended for wireless communication applications (operating at 1.9 and 2.4 GHz). To this end, we combine the accuracy of MoM full-wave solutions with the speed of parallel processing, and the robustness of the evolutionary optimization approach in an electromagnetic GA optimization application (EGO). The design (a descendant of the E-patch antenna), aims at achieving the goal of having an acceptable input VSWR while maintaining a low cross-polarized far field. To this end, an appropriate hybrid fitness function is employed in the GA. Field and S<sub>11</sub> measurements of various GA-optimized patch configurations exhibit good agreement with the MoM calculations.
    Antennas and Propagation Society International Symposium, 2002. IEEE; 02/2002
  • F.J. Villegas, Y. Rahmat-Samii, D.R. Jackson
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    ABSTRACT: We make use of a hybrid method-of-moments formulation for the solution of scattering from finite, nonuniform, one- and two-dimensional arrays of cylindrical cavities in an infinite ground plane. We show that the method is rather versatile, with the capability to handle both small and large finite arrays of dissimilar elements, including a dielectric superstrate layer if desired. A modified AEF definition, specifically developed for scattering type problems, has been employed to make the analysis of very large arrays tenable. Scattering from novel non-periodic topologies is accommodated with the use of a 'masking' procedure. We illustrate the overall versatility, accuracy and efficiency of the method with a survey of various applications of interest to the antenna and microwave engineering communities
    Antennas and Propagation Society International Symposium, 2001. IEEE; 02/2001
  • F.J. Villegas, Y. Rahmat-Samii, D.R. Jackson
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    ABSTRACT: We propose a new hybrid MoM formulation for the analysis of scattering from dielectrically-covered finite arrays of cylindrical cavities in an infinite ground plane, illuminated by a plane-wave source. The new formulation focuses on the use of techniques that maximize both the accuracy and efficiency of the numerical computations. As a result, the algorithm is quite useful for analyzing rather large yet finite structures without resorting to approximate methods. The hybrid approach is both more accurate and efficient than purely numerical solution techniques. Hence, the method has the additional capacity to serve as a useful benchmarking application for other less accurate numerical algorithms
    Antennas and Propagation Society International Symposium, 2001. IEEE; 02/2001
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    ABSTRACT: We examined the scattering characteristics of a canonical rod/hole system. We found that the holes could simply be modeled as sections of cylindrical waveguides with short-circuit terminations. Comparisons with fundamental quantities such as the guided wavelength and cutoff frequency yielded results that agreed quite well. The notion of an effective single-hole efficiency was introduced, implicitly accounting for inter-element coupling. We found that it asymptotically tends to a constant value as the number of holes increases. We also concluded that the proximity of the rod to the grating plays an important role in the coupling phenomenon, a result of the traveling-wave nature of the rod field. Finally, we compared a computed array far-field pattern with one based on a simple array-factor expression that includes an HFSS (a commercial 3D finite-element EM simulator) computed leakage constant. The results lead to the conclusion that use of the effective efficiency as an array synthesis parameter will yield the predicted antenna characteristics
    Antennas and Propagation Society International Symposium, 2000. IEEE; 02/2000
  • F.J. Villegas, Y. Rahmat-Samii
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    ABSTRACT: The radiation characteristics of a periodically-loaded perfect electric conductor (PEC) circular cylinder fed by a proximity-coupled dielectric rod waveguide are investigated. The configuration, which is illustrated, is currently being employed as the core radiating mechanism in a host of millimeter-wave (MMW) antenna systems for imaging and communications applications. Several rows of one dimensional gratings are placed along the cylinder axis, each with at slightly differing period, thus allowing for quasi-continuous scanning of the main beam in the E-plane of the antenna. For typical applications, precise control over the scanning behavior and overall radiation characteristics (e.g. beamwidth) is crucial, and as such an analysis tool capable of predicting the performance of the antenna would prove highly beneficial. To this end, we have developed an accurate, analytical formulation for the total radiated fields of the structure based on an eigenmode expansion. Using a cylindrical modal spectrum representation in which a fictitious axial leaky-wave current is impressed on the cylinder surface allows us to model the surface periodicity without the need for a time-consuming full-wave analysis. Several salient features of the antenna's radiative properties concerning the various geometrical parameters involved are presented
    Antennas and Propagation Society International Symposium, 1999. IEEE; 09/1999
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    ABSTRACT: The leakage fields radiated by planar semi-infinite transmission-line currents are studied for two canonical structures: the air-gap stripline and the planar strip of current in free space. The first structure represents a general class of printed-circuit structures where leakage occurs into a substrate mode. The second structure is representative of structures for which leakage occurs primarily into space. It is found that the transverse behavior of the leakage fields for a semi-infinite line differs in significant ways from that expected from an infinite line, a result that is at first surprising, but is understandable on physical grounds, and which is explained by the analysis presented here. The exact leakage fields are compared with two asymptotic approximations: a stationary-phase [geometrical optics (GO)] evaluation, and a uniform asymptotic expansion (UAE). The UAE solution is the more accurate one, and accounts for diffraction-like effects that arise due to the semi-infinite nature of the line. One conclusion is that the observation distance must be very large for the true leakage fields to closely resemble the GO fields, although a quasi-GO behavior may still be evident at shorter distances, depending on the value of the leakage angle. Another conclusion is that the transverse behavior of the GO leakage field of the printed-circuit structure is an exponentially increasing field within the leakage region, as expected from a simple ray-optics picture, while it is not for the strip in free space
    IEEE Transactions on Microwave Theory and Techniques 05/1999; · 2.23 Impact Factor
  • F.J. Villegas, D.I. Stones, H.A. Hung
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    ABSTRACT: A novel waveguide-to-microstrip transition is developed using a new design methodology based on iris coupling. Key features of the design are a single-layer substrate, new matching topology, and new cavity enclosure. The transition lends itself to a low-cost implementation, while maintaining the enclosure's hermetic integrity. An extensive tolerance study shows that the present design is robust and very stable with respect to manufacturing and assembly variations. Careful consideration has been given to the mechanical aspects of the transition's implementation in order to achieve seamless integration into the overall package manufacturing and assembly process without sacrificing electrical performance. Proof of concept was achieved by implementing a Q-band (f<sub>0</sub>=44.5 GHz) design on alumina, a W-band (f<sub>0</sub>=94 GHz) design on z-cut quartz, and a W-band design on fused silica. All exhibited better than 22 dB return loss at their center frequencies with less than 0.3 dB insertion loss, and at minimum a 10% 15 dB return-loss bandwidth
    IEEE Transactions on Microwave Theory and Techniques 02/1999; · 2.23 Impact Factor
  • H.-B. Lee, D. Koh, T. Itoh, F.J. Villegas, H.A. Hung
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    ABSTRACT: The shapes of matching stubs are designed with an optimizer based on full-wave analysis. By selecting the shape of matching stub as the design parameter, compact circuits for matching could be obtained. FEM is used as a full-wave analyzer, and adjoint variable method is adopted for sensitivity analysis
    Microwave Conference Proceedings, 1997. APMC '97, 1997 Asia-Pacific; 01/1998
  • F.J. Villegas, D.I. Stones, H.A. Hung
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    ABSTRACT: A novel waveguide (w/g)-to-microstrip transition has been developed using a new design methodology based on iris coupling. The current design exhibits reduced sensitivity to w/g backshort position and yields a low-cost, hermetically-sealed transition. A 44 GHz-band design on alumina and a 94 GHz-band design on z-cut quartz were implemented, both exhibiting better than 22 dB return loss at their center frequencies with less than 0.3 dB insertion loss, with a greater than 10% bandwidth
    Microwave Symposium Digest, 1997., IEEE MTT-S International; 07/1997
  • F.J. Villegas, D.R. Jackson, J.T. Williams
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    ABSTRACT: The leakage fields radiated by planar semi-infinite transmission line currents are studied for two canonical structures, the air-gap stripline and a strip in free space. The exact leakage fields are compared with the following two asymptotic approximations: a stationary-phase (geometrical optics or GO) evaluation and a uniform asymptotic expansion
    Microwave Symposium Digest, 1996., IEEE MTT-S International; 07/1996 · 2.23 Impact Factor

Publication Stats

135 Citations
24 Downloads
534 Views
16.02 Total Impact Points

Institutions

  • 1998–2003
    • University of California, Los Angeles
      • Department of Electrical Engineering
      Los Angeles, CA, United States
  • 2000
    • California State University, Los Angeles
      Los Angeles, California, United States
  • 1999
    • University of Houston
      • Department of Electrical & Computer Engineering
      Houston, TX, United States