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Optimization of Focusing by Strip and Pixel Arrays

Description: Professor Kevin Webb and students at Purdue University have demonstrated the design of conducting strip and pixel arrays for focusing electromagnetic waves [1, 2]. Their key point was to design structures to focus waves in the near field using full wave modeling and optimization methods for design. Their designs included arrays of conducting strips optimized with a downhill search algorithm and arrays of conducting and dielectric pixels optimized with the iterative direct binary search method. They used a finite element code for modeling. This report documents our attempts to duplicate and verify their results. We have modeled 2D conducting strips and both conducting and dielectric pixel arrays with moment method and FDTD codes to compare with Webb's results. New designs for strip arrays were developed with optimization by the downhill simplex method with simulated annealing. Strip arrays were optimized to focus an incident plane wave at a point or at two separated points and to switch between focusing points with a change in frequency. We also tried putting a line current source at the focus point for the plane wave to see how it would work as a directive antenna. We have not tried optimizing the conducting or dielectric pixel arrays, but modeled the structures designed by Webb with the moment method and FDTD to compare with the Purdue results.
Date: June 30, 2005
Creator: Burke, G J; White, D A & Thompson, C A
Partner: UNT Libraries Government Documents Department

Evaluation of the Mathieu Function Series for Diffraction by a Slot

Description: Diffraction of a wave passing through a slot is a fundamental problem that has applications in many situations. A problem of current interest is the propagation of waves through obstacles such as buildings in an urban environment, where an entrance aperture could be approximated as a slot. Solutions for the diffracted fields of a slot have been obtained by exact eigenfunction expansions as well as various approximate and numerical methods. The eigenfunction solution, in terms of Mathieu functions, has been published by Stamnes and Eide [1] for a slot with several types of converging incident waves. Results of the eigenfunction solution are presented in [2] for slots up to 20 wavelengths in width and compared with Kirchho. and Debye approximations. We have attempted to duplicate the eigenfunction series results in [2]. Little information is given in [2] about the numerical issues in the evaluation. We encountered a number of problems in the evaluation of the Mathieu functions for wide slots that required modifications of the Mathieu function evaluation methods described in [3]. Results were obtained matching those for the largest slot of 20{lambda} in [2] and also for slots up to 40{lambda} width. The evaluation of the eigenfunction solution was checked against a 2D moment method solution and was in close agreement up to a slot width of 40{lambda}. The eigenfunction solution published by Stamnes and Eide is summarized below, followed by a discussion of the Mathieu function evaluation and problems encountered with large slot widths. Results are then presented for varying slot width and compared with the 2D moment method solutions. A moment method solution for a diverging wave incident on the gap between two square cylinders is also presented and compared with the fields in a slot representing the front aperture.
Date: June 15, 2005
Creator: Burke, G J & Pao, H Y
Partner: UNT Libraries Government Documents Department

The Numerical Electromagnetics Code (NEC) - A Brief History

Description: The Numerical Electromagnetics Code, NEC as it is commonly known, continues to be one of the more widely used antenna modeling codes in existence. With several versions in use that reflect different levels of capability and availability, there are now 450 copies of NEC4 and 250 copies of NEC3 that have been distributed by Lawrence Livermore National Laboratory to a limited class of qualified recipients, and several hundred copies of NEC2 that had a recorded distribution by LLNL. These numbers do not account for numerous copies (perhaps 1000s) that were acquired through other means capitalizing on the open source code, the absence of distribution controls prior to NEC3 and the availability of versions on the Internet. In this paper we briefly review the history of the code that is concisely displayed in Figure 1. We will show how it capitalized on the research of prominent contributors in the early days of computational electromagnetics, how a combination of events led to the tri-service-supported code development program that ultimately led to NEC and how it evolved to the present day product. The authors apologize that space limitations do not allow us to provide a list of references or to acknowledge the numerous contributors to the code both of which can be found in the code documents.
Date: January 20, 2004
Creator: Burke, G J; Miller, E K & Poggio, A J
Partner: UNT Libraries Government Documents Department

Extension of a thin-wire algorithm for wires moved laterally within a mesh

Description: It was shown that the accuracy of results for wires moved laterally from mesh edges can be greatly improved by taking account of the behavior of the field in the vicinity of the wire. Also, the distance to the end of the wire can be adjusted within a cell by using a general second-order difference form for the derivative. Making the wire location completely independent of the mesh would require the additional ability to tilt the wire with respect to the edges. This seems to be a considerably more difficult problem than moving the wires laterally, since the component of the mesh field parallel to the wire gets mixed with the larger radial electric field due to charge on the wire. Simply averaging the mesh fields did not seem to work well, except in the case where the wire was tilted in one coordinate plane, and the mesh fields above and below the plane of the wire, on edges orthogonal to the wire normal, could be averaged. Further study is needed to develop a more general capability to tilt a wire with respect to the mesh.
Date: November 1, 1998
Creator: Burke, G J & Steich, D J
Partner: UNT Libraries Government Documents Department

Comparison of equations for the EDTD solution in anisotropic and dispersive media

Description: The recursive-convolution solution for anisotropic and dispersive media was seen to yield accurate results for reflection from ferrite slabs up to a frequency limit set by the sampling interval. Depending on the application, the results shown might be considered usable up to about 300 GHz, which corresponds to about 13 cells per wavelength. Results at still higher frequencies might be usable when a time delay or frequency shift due to dispersion can be tolerated. Several different forms of the update equations were considered which can result from different approximations in reducing the continuous equations to discrete form.
Date: January 1, 1997
Creator: Burke, G.J. & Steich, D.J.
Partner: UNT Libraries Government Documents Department

Evaluation of the discrete complex-image method for a NEC-like moment-method solution

Description: The discrete image approximation for the field of a half-space is tested in the NEC antenna modeling program as an alternative to the interpolation method presently used. The accuracy and speed of the discrete image approximation are examined for varying number of images and approximation contour, and the solution for current is obtained on a horizontal wire approaching the interface.
Date: January 5, 1996
Creator: Burke, G.J.
Partner: UNT Libraries Government Documents Department

The determination of the constitutive parameters of a medium with application to a reinforced concrete pad

Description: This report describes the experimental and analytical program performed to determine the constitutive parameters of the reinforced concrete pad in the test facility used during the Low Power On-the-Ground portion of the NASA Boeing 757 HIRF Tests. These tests were conducted during the period September 20 to October 21, 1994 in the LESLI facility at the Phillips Laboratory, Kirtland AFB, NM. The on-the-ground tests were designed to meet several objectives including support of a flight test series and the generation of data for the validation of codes and models that could be used to predict the electromagnetic environment in transport aircraft. To satisfy these objectives, tests were to be executed in a known environment and the data compared to modeling results. A critical feature of this testing was the ``known environment`` which implies knowledge of the parameters which are critical to an effective modeling activity and which could include, among many other things, definitions of the airplane and its physical and electrical configuration, the ground upon which it sits when stationary, the fields impinging on the aircraft, and the radiating or bounding structure in the simulator. The authors would want to specify the electromagnetic characteristics of the entire space that would likely enter into a mathematical modeling effort so that the model can be made as ``close`` to physical reality as desired prior to exercising computational algorithms which might introduce their own uncertainties. Since the authors are evaluating codes used for determination of the electromagnetic environment in aircraft and since the on-the-ground test involved an airplane parked on a pad in the simulator facility, they would require a definition of the constitutive parameters of the pad.
Date: January 15, 1995
Creator: Poggio, A.J.; Burke, G.J. & Pennock, S.T.
Partner: UNT Libraries Government Documents Department

A physical optics model for scattering by irregular terrain at HF

Description: Physical optics models were developed for scattering of HF radiation by irregular terrain when an antenna is located on or near the scattering surface. The primary interest was in skywave radiation patterns for communication links using the ionosphere. Second-order reflections were included in the solution through an image approximation, and the UTD result for an impedance half-plane was used to reduce reflections due to truncating the current distribution on the surface. Arbitrary 2-D or 3-D terrain profiles can be entered into the codes, and the source can be an antenna with currents generated by the NEC method of-moments code or a point source. Results are validated by comparing with 2-D integral equation solutions for actual terrain contours.
Date: March 8, 1994
Creator: Burke, G. J.
Partner: UNT Libraries Government Documents Department

A physical optics model for scattering of HF radiation by irregular terrain

Description: Physical optics models were developed for scattering of HF radiation by irregular terrain when an antenna is located on or near the scattering surface. The primary interest was in skywave radiation patterns for communication links using the ionosphere. Second-order reflections were included in the solution through an image approximation, and the UTD result for an impedance half-plane was used to reduce reflections due to truncating the current distribution on the surface. Arbitrary 2-D or 3-D terrain profiles can be entered into the codes, and the source can be an antenna with currents generated by the NEC method-of-moments code or a point source. Results were validated by comparing with 2-D integral equation solutions for actual terrain contours.
Date: January 1, 1994
Creator: Burke, G. J.
Partner: UNT Libraries Government Documents Department

Extending NEC to model wire objects in infinite chiral media

Description: The development of a moment-method model for wire objects in an infinite chiral medium is described. In this work, the Numerical Electromagnetics Code (NEC) was extended by including a new integral-equation kernel obtained from the dyadic Green`s function for an infinite chiral medium. The NEC moment-method treatment using point matching and a three-term sinusoidal current expansion was adapted to the case of a chiral medium. Examples of current distributions and radiation patterns for simple antennas are presented, and the validation of the code is discussed. 15 refs.
Date: March 1, 1992
Creator: Burke, G. J.; Miller, E. K. & Bhattachryya, A. K.
Partner: UNT Libraries Government Documents Department

Wavenumbers for currents on infinite- and finite-length wires in a chiral medium

Description: There is increasing interest in determining the electromagnetic properties of material media differing from free space and the effects thereof on the radiation, propagation, and scattering of electromagnetic fields. A material property of special present interest is that of chirality. Chirality manifests itself as a ``handedness`` wherein a chiral medium does not support propagation of a linearly-polarized plane wave, but which instead decomposes into two circularly-polarized waves that propagate at different speeds. Initial work in this area was devoted to developing various analytical solutions to some basic problems such as the Green`s Dyadic for a point current source. Attention is now being increasingly devoted to using this early work for a variety of applications such as analyzing antennas in chiral media scattering from chiral objects; scattering from objects having chiral coatings; and reflection from planar chiral interfaces. The focus of the work described here is determining the wavenumbers (={minus}{alpha}{minus}j{beta}) of the current waves excited on wire antennas located in an infinite chiral medium using two complementary approaches. One is to use an extension of an existing computer model (NEC) that permits modeling of arbitrary wire objects located in an infinite chiral medium. The other is to develop a solution for an infinitely long cylindrical antenna also located in an infinite chiral medium. The latter canonical problem is of interest in its own right as well as providing a means for achieving mutual validation with the NEC model. 9 refs.
Date: March 1, 1992
Creator: Bhattacharyya, A. K.; Burke, G. J. & Miller, E. K.
Partner: UNT Libraries Government Documents Department

Extending NEC to model wire objects in infinite chiral media. [Numerical electromagnetic code (NEC)]

Description: The development of a moment-method model for wire objects in an infinite chiral medium is described. In this work, the Numerical Electromagnetics Code (NEC) was extended by including a new integral-equation kernel obtained from the dyadic Green's function for an infinite chiral medium. The NEC moment-method treatment using point matching and a three-term sinusoidal current expansion was adapted to the case of a chiral medium. Examples of current distributions and radiation patterns for simple antennas are presented, and the validation of the code is discussed. 15 refs.
Date: January 1, 1992
Creator: Burke, G.J. (Lawrence Livermore National Lab., CA (United States)); Miller, E.K. (Los Alamos National Lab., NM (United States)) & Bhattachryya, A.K. (New Mexico State Univ., Las Cruces, NM (United States). Physical Science Lab.)
Partner: UNT Libraries Government Documents Department

Wavenumbers for currents on infinite- and finite-length wires in a chiral medium

Description: There is increasing interest in determining the electromagnetic properties of material media differing from free space and the effects thereof on the radiation, propagation, and scattering of electromagnetic fields. A material property of special present interest is that of chirality. Chirality manifests itself as a handedness'' wherein a chiral medium does not support propagation of a linearly-polarized plane wave, but which instead decomposes into two circularly-polarized waves that propagate at different speeds. Initial work in this area was devoted to developing various analytical solutions to some basic problems such as the Green's Dyadic for a point current source. Attention is now being increasingly devoted to using this early work for a variety of applications such as analyzing antennas in chiral media scattering from chiral objects; scattering from objects having chiral coatings; and reflection from planar chiral interfaces. The focus of the work described here is determining the wavenumbers (={minus}{alpha}{minus}j{beta}) of the current waves excited on wire antennas located in an infinite chiral medium using two complementary approaches. One is to use an extension of an existing computer model (NEC) that permits modeling of arbitrary wire objects located in an infinite chiral medium. The other is to develop a solution for an infinitely long cylindrical antenna also located in an infinite chiral medium. The latter canonical problem is of interest in its own right as well as providing a means for achieving mutual validation with the NEC model. 9 refs.
Date: January 1, 1992
Creator: Bhattacharyya, A.K. (New Mexico State Univ., Las Cruces, NM (United States)); Burke, G.J. (Lawrence Livermore National Lab., CA (United States)) & Miller, E.K. (Los Alamos National Lab., NM (United States))
Partner: UNT Libraries Government Documents Department

Low-frequency computational electromagnetics for antenna analysis

Description: An overview of low-frequency, computational methods for modeling the electromagnetic characteristics of antennas is presented here. The article presents a brief analytical background, and summarizes the essential ingredients of the method of moments, for numerically solving low-frequency antenna problems. Some extensions to the basic models of perfectly conducting objects in free space are also summarized, followed by a consideration of some of the same computational issues that affect model accuracy, efficiency and utility. A variety of representative computations are then presented to illustrate various modeling aspects and capabilities that are currently available. A fairly extensive bibliography is included to suggest further reference material to the reader. 90 refs., 27 figs.
Date: January 1, 1991
Creator: Miller, E.K. (Los Alamos National Lab., NM (USA)) & Burke, G.J. (Lawrence Livermore National Lab., CA (USA))
Partner: UNT Libraries Government Documents Department

Numerical modeling of HF skywave radiation from antennas in irregular terrain

Description: The problem of computing the radiation pattern of an HF antenna sited in irregular terrain was investigated. The primary interest is in antennas for skywave communication, however ionospheric models were not considered. Several methods for modeling terrain effects are briefly reviewed. A geometrical optics model for arbitrary terrain is developed and results are compared with published results from solution of a Volterra integral equation for scattering by a Gaussian ridge. This report covers work on the first phase of a project for the US Navy to develop and apply models for terrain effects in HF communications involving skywave. 45 refs., 16 figs.
Date: November 1, 1990
Creator: Burke, G.J.
Partner: UNT Libraries Government Documents Department

Present capabilities and new developments in antenna modeling with the numerical electromagnetics code NEC

Description: Computer modeling of antennas, since its start in the late 1960's, has become a powerful and widely used tool for antenna design. Computer codes have been developed based on the Method-of-Moments, Geometrical Theory of Diffraction, or integration of Maxwell's equations. Of such tools, the Numerical Electromagnetics Code-Method of Moments (NEC) has become one of the most widely used codes for modeling resonant sized antennas. There are several reasons for this including the systematic updating and extension of its capabilities, extensive user-oriented documentation and accessibility of its developers for user assistance. The result is that there are estimated to be several hundred users of various versions of NEC world wide. 23 refs., 10 figs.
Date: April 8, 1988
Creator: Burke, G.J.
Partner: UNT Libraries Government Documents Department

ELF-VLF Communications through the Earth Project report for calendar year 1984. Revision 1

Description: We use computer models and experiments to explore the feasibility of communication between points underground and on the Earth's surface. Emphasis is placed on ELF-VLF electromagnetic propagation through the Earth; nominally, we investigated propagation in the 200 Hz-30 kHz frequency range. The computer modeling included calculations of the fields of a point electric or magnetic source in a homogeneous half space or a stratified earth. Initial results for an insulated antenna of finite length are also considered. The experiments involved through-the-earth transmissions at two locations in Pennsylvania, both of which had large formations of limestone. Initial results indicate that information rates as high as kbits/s may be possible for subsurface depths of 300 m or less. Accuracy of these estimates depends on the electromagnetic propagation constant of the rock, the noise characteristics, and the modulation scheme. Although a nuisance for evaluating through-the-Earth propagation, the existence of subsurface metal conductors can improve the transmission character of the site.
Date: August 1, 1985
Creator: Buettner, H.M.; Burke, G.J.; Didwall, E.M.; Holladay, G. & Lytle, R.J.
Partner: UNT Libraries Government Documents Department

Preliminary assessment of the electromagnetic environment in the immediate vicinity of the ETA accelerator

Description: The electromagnetic fields in the immediate vicinity of the Experimental Test Accelerator (ETA) at the Lawrence Livermore Laboratory have been characterized. Various EM sensors that cover the frequency band from the very low frequencies up into the GHz region have been used. The report describes in detail the probes, the test set-up and the data processing techniques.
Date: April 1, 1980
Creator: Cabayan, H.S.; Bogdan, E.; Zicker, J.; Wythe, D. & Burke, G.J.
Partner: UNT Libraries Government Documents Department