Applied Electromagnetics

The Applied Electromagnetic Group's (AEG) research emphasis is on solving problems of real-world need which involve electromagnetic phenomena. Since we live in a world where electromagnetic fields and waves invade and pervade nearly every aspect of our lives, the AEG's research has an impact on humanity in many ways. At Clemson, the area of applied electromagnetics encompasses electromagnetic theory, sophisticated analytical and/or numerical techniques, and parallel and distributed computers, all brought together for the development of electromagnetic analysis and design tools and the study of electromagnetic phenomena.

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Projects

The faculty of the AEG focus area are involved in a diverse range of projects -- which underscores the far-reaching impact of electromagnetic phenomena on society.

Sponsors of Current Funded Projects

Signal Coupling and Penetration

Integral equation techniques for solving problems in electromagnetics have been under development by AEG faculty for a number of years. Prominent among problems to which such techniques have been applied are those classified as aperture penetration. Penetration through apertures or holes in conducting surfaces leads to applications in a number of interesting areas, and, if the aperture of interest is small or is oriented on the body surface in such a way that the penetration is small, typical solution methods can lead to results that are very inaccurate or are even meaningless. Analytical/numerical techniques to overcome these inaccuracies have been developed for a number of cases.

Working for three different U.S. Army groups, AEG faculty have developed computer tools to predict the field that penetrates a hole in a missile. Also, coupling to a probe or small antenna mounted on a missile has been investigated, including the case of a probe which is entirely or partially in the region exterior to the missile skin and, also, that in which the probe resides completely inside a cavity within the missile. In the latter case, the signal reaches the interior cavity via a small aperture in the missile surface. One actually determines the signal that reaches a load that terminates the probe or small antenna, when an incident wave of known properties illuminates the missile. The load represents the input impedance of an instrument used to measure the signal received by the probe. The ultimate goal of this task is to determine the signal at the measuring instrument that results from a known incident field impinging upon the missile. In addition, experimental models have been fabricated and data have been collected to have a base for corroborating results obtained from theory.

Innovative Communications Antennas

In modern communications there is a need for a rugged and lightweight omnidirectional antenna capable of radiating/receiving spread spectrum signals. A single antenna must serve both the transmit and receive functions and must be able to transmit in the tens of watts of radiated power. Integral equation methods for analyzing and synthesizing such antennas are under investigation by AEG faculty at Clemson. Because of the extreme bandwidth requirements, schemes are being evaluated for electronically modifying the structure to enhance effective bandwidth. Under control of the communication system to ensure synchronization with the frequency hopping, the dimensions of the radiator are altered, open circuits are switched in and out as are tuning elements and modest matching networks. In the case of antenna elements comprising helical wires, length control is realized by selectively shorting turns in the helix by means of high-speed switches and the elements are fed at different points along the helical structure under the control of switches.

Millimeter Wave Circuits and Systems

The next frequency frontier to be exploited in applications comprises those frequencies above 40 GHz -- the so-called millimeter wave frequencies. These frequencies are useful in many applications including communications and imaging. The fog and cloud penetration properties of the band are especially attractive in imaging applications. The 77 GHz band has been specifically defined for automotive radar and imaging applications. The Department of Electrical and Computer Engineering at Clemson University is engaged in ongoing research in millimeter wave electromagnetics. Areas of activity include micromachining of waveguiding structures, antennas, and synthetic crystal structures using anisotropic silicon etching technology, modeling of guiding structures for millimeter wave frequencies and spatial power combining to achieve high radiated powers.

Low Frequency Fields

Recent concerns for the possible health effects of low-frequency electromagnetic fields (ELF) has resulted in funded activities at Clemson to develop accurate modeling techniques for the assessment of the shielding effectiveness of steel pipe on 60 Hz fields. This work involves the development of hybrid finite element techniques to model the non-linear and inhomogeneous characteristics of the cable structures used to transmit electric power. Possible future directions of this work might be:

Improved Method for Computing Zero-Sequence Impedance

Underground pipe-type cables in power transmission systems are widely used in urban areas. To protect such systems, it is necessary for the utilities to be able to accurately calculate fault currents in the system. An accurate calculation of fault currents requires precise knowledge of the impedance of the underground cables. Sponsored by National Science Foundation, a research project is carried out at Clemson to develop an improved method for computing zero-sequence impedance of underground pipe-type cables. The zero-sequence impedance can be computed based on information of the nonuniform current distribution in the phase conductors as well as the electric field intensity and the total current in the steel pipe. In the determination of the nonuniform current distribution in the phase conductors, a Fourier series technique is employed, taking into account the skin effect, the proximity effect, and the influence of the steel pipe. To compute the electric field intensity and the total current in the steel pipe, including the circulating current and the returning zero-sequence current, a set of equations are formulated based on Faraday's law. The equations are solved by an iterative procedure. It is expected that the computational results will be useful to help utility engineers properly size protective devices, such as fuses and breakers. Also, it is hoped that the numerical techniques developed can be extended to solving other problems encountered in power systems.

Robust Finite Element Methods for Electromagnetic Modeling

The finite element method (FEM) has proven to be a powerful tool in numerical modeling of numerous physical regimes. Until the last few years, difficulties posed by practical electromagnetics problems precluded the use of the FEM for electromagnetic modeling. Recent breakthroughs in approximate boundary conditions for radiation problems have opened the FEM realm to electromagnetics applications. Challenges remain in refining edge-based basis elements and in dealing with infinitesimally thin metallic protrusions. The methodology for addressing both the basis elements and formulations for addressing thin protrusions are near to fruition. Clemson University workers are active participants in the development of these computational tools.

Extending The Range of Real-World Problems

Today there are many important problems in areas such as electromagnetic compatibility (EMC) and electromagnetic interference (EMI), wireless communications, and coupling between radiators and biological organisms that cannot be addressed with the leading electromagnetic analysis tools (such as integral equation and finite element methods) simply because the analysis is too time consuming and costly to carry out. Even with the advances made in parallel and distributed computing, the resource requirements of popular electromagnetic analysis techniques exceed the supply of necessary resources. What is needed now are new techniques for analysis that extend the range of real-world problems that can be addressed with computer resources that exist today. Members of Clemson's AEG focus area are currently investigating applications of spatial decomposition techniques for efficient electromagnetic analysis of electrically large complex structures, systems, and environments. Spatial decomposition is an analysis technique whereby a large complex structure or environment is subdivided into several smaller structures such that the ultimate union of the smaller structures is equivalent to the original large structure. The smaller structures individually require much less memory and computer time for analysis than does the large one. Spatial decomposition techniques are being applied to volumetric integral equation and partial differential equation based formulations at Clemson. Spatial decomposition applied within these formulations has the potential to extend the analysis to larger, more complex structures and systems using the fixed computer resources available today.

Laboratory Facilities

The AEG maintains a laboratory to support several ongoing research projects. The laboratory houses several high-end PCs which are used primarily to develop codes which implement the numerical theory-based formulations developed by the group.

For computationally intensive projects and studies involving distributed processors in AEG research, a four-processor DEC Alpha system is available to the AEG at Clemson University that was obtained through an NSF CISE Research Instrumentation grant. This multiprocessor system has available 512 megabytes of memory for each processor along with a high-speed switched DEC GIGAswitch for high-speed communication between processors.

In addition, members of the AEG have available on a shared basis six DEC Alpha 21064 processors running at 275 MHz, two DEC Alpha 21164 processors running at 300 MHz, eight DEC Alpha processors running at 500MHz, and four DEC Alpha processors running 667MHz. The processors are interconnected via the DEC Gigaswitch which achieves communications speeds up to 100 Mbits/sec between any two processors. Student researchers in the AEG develop codes which exploit the power of this distributed multi-processor system.

To corroborate the numerical results, the measurement facility at Clemson University presently includes an antenna ground plane laboratory (AGPL), a roof-top antenna platform, a 70 ft telescoping antenna tower, a HF Yagi-Uda antenna designed for operation down to 7 MHz, an HP 8510B vector network analyzer, an HP 89441A vector signal analyzer, oscilloscopes and asorted multimeters.



Also, the lab contains an anechoic chamber for high frequency measurements and a micro-milling machine for the construction of microwave ciruits.

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We have two Kenwood HF Transceivers (a TS-950SDX, and a TS-850S) and two Yaesu VHF/UHF Transceivers (FT-726R). The TS-850S and the FT-726R are portable units.

The Department has recently been awarded two grants by the DURIP program (Defense University Research Instrumentation Program). One provided a millimeter receiving system and a Cascade Probe station that is modified to perform nearfield scanning and holographic imaging of radiating apertures.

The other DURIP grant provided a network analyzer capable of 10 Hz-150 MHz band and will provide measurement capabilities for wireless communications systems in the HF band. This grant will also provide an arbitrary waveform generator for studying and constructing spread-spectrum waveforms for use with antennas and electronic switching schemes.

The AGPL is an 18 ft. high by 26 ft. wide by 12 ft. deep room with five of six walls lined with 12 inch microwave absorber and with the remaining wall being a 18 ft high by 21 ft wide by 0.5 inch thick aluminum sheet called a ground plane. The purpose of the microwave absorber is to reduce reflections from the walls and other objects so that a free-space environment is simulated in the room. The large aluminum ground plane serves to isolate the operation region (the interior of the absorber-lined room) from the equipment region where equipment such the HP 8510B and the experimenter are located. Near the center of the aluminum ground plane is a tapered circular opening. In this opening fixtures are mounted which allow one to perform measurements on model antennas and other devices. The mounting fixtures also allow access for cables which connect to instrumentation. Using this facility, it is possible to perform accurate characterization of antenna driving-point admittance down to a low-frequency limit of around 300 MHz using full-scale antenna models. Hence, by use of 1/3-size scale models, it should be feasible to simulate measurement at 100 MHz for all metal antennas types.

The antenna tower that is located on the roof of the EIB allows point-to-point measurements to demonstrate the ability of the antennas designed at Clemson to link radios displaced from each other by several miles in hilly, mountainous, or moderately flat terrain. Antennas developed at Clemson are mounted on the boom of the HF Yagi-Uda antenna which places them nearly 150 ft above ground level. Several members of the Clemson FRI team now hold an amateur radio license which allows them to carry out transmit/receive experiments in the area surrounding Clemson. The Blue Ridge Mountains as well as areas of moderately flat terrain, all highly vegetated, are within twenty miles of Clemson, so rigorous performance tests can be conducted conveniently. The portable HF and UHF/VHF transceivers mentioned previously will be used to perform these tests.

Professors and Their Interests

Chalmers M. Butler: integral equation methods in electromagnetics with applications to penetration through apertures, scattering and diffraction, guided waves, antennas, microwave circuits, high-speed circuits, and interaction of fields with objects in inhomogeneous spaces.

Anthony Q. Martin: integral equations methods, numerical techniques for the analysis of electrically large structures, very wire band antennas, signal coupling and penetration, electromagnetic theory, and hybrid analysis techniques in electromagnetics, finite elements methods.

L. Wilson Pearson: electromagnetic scattering, finite element method modeling of electromagnetic systems, asymptotic methods, millimeter wave sources and systems.

Anh-Vu H. Pham: MMIC design, wireless transceiver architectures, microwave/millimeter wave integration and packaging, and on-wafer characterization techniques for devices, MMICs, and interconnects to 110 GHz.

Xiao-Bang Xu: integral equation methods, numerical analysis, and electromagnetic theory, development of hybrid surface/volume integral equation method for scattering problems, application of Fourier series expansion technique and hybrid finite element/boundary integral equation method for the analysis of low frequency fields, treatment of nonlinear problems.