Complex Fields
Goals
The Complex Fields Project develops and maintains measurement methods to quantify fields in complex environments, such as electrically large cavities and highly nonuniform boundaries. Applications include the reverberation chamber, the statistics of electromagnetic fields in rooms and buildings, the communications needs of first responders to emergencies, measurements of shielding effectiveness of advanced composites, coupling to large-scale systems and components, coupling to biological objects, advanced numerical methods, and metamaterials. These efforts support industry and government agencies, national and international standards, health care, homeland security, and nanotechnology.
Customer Needs
Large, complex systems located in complex field environments need to be tested for electromagnetic compatibility (EMC). Electromagnetic interference (EMI) affects U.S. competitiveness (through trade restrictions and regulations), national security, health, and safety. EMC regulations and requirements constitute 1 to 10 percent of the total U.S. product costs and can cause delays to market. We are providing research and support to address a number of critical areas:
Reverberation Chambers — Reverberation chambers are used to test large complex systems at high frequencies. Users of reverberation chambers need better models of statistical parameters, improved chamber and stirrer designs, test object models, and guidance for incorporating reverberation chamber technology into national and international standards.
Shielding of Advanced Composites — Shielding effectiveness of advanced composites cannot be readily tested with existing methods, such as the American Society for Testing and Materials (ASTM) coaxial fixture. New methods are needed, such as the use of nested reverberation chambers.
Coupling to Biological Objects — The increased use of wireless devices in scattering-rich environments is creating questions about the effects of such electromagnetic field exposure on humans and animals. These conditions need to be replicated in controlled experiments.
First Responder Communications — First responders to emergency situations encounter difficult communications scenarios. There is a need to better understand propagation in a wide variety of environments, including collapsed buildings.
Complex Boundaries — The electromagnetic interface between complex media is a difficult modeling and measurement problem. Advances at both the macro- and micro-scale are needed.
Technical Strategy
Our goal is to develop and evaluate reliable and cost-effective standards, test methods, and measurement services related to complex electromagnetic fields for EMC of electronic devices and other applications in health, defense, and homeland security. This includes investigating new applications for existing test facilities as well as improving methods for evaluating the critical characteristics of support hardware, such as antennas, cables, connectors, enclosures, and absorbing material.
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Reverberation chambers are increasingly a key tool for EMC testing in the gigahertz frequency range. The recent publication of IEC61000-4-21 on reverberation chamber test methods will increase usage. We have played a leading role in developing reverberation chamber technology. We continue to develop models for the statistical behavior of the fields in the test volume and near the boundaries. While the typical target for a reverberation chamber is a Rayleigh field distribution, multiple-input/ multiple-output (MIMO) system testing requires a Rician field distribution. We will investigate methods to accurately control the ratio between direct and indirect coupling for MIMO and other test applications. Probes are traditionally calibrated in highly controlled reference fields. We will investigate whether the large volume of a reverberation chamber can be used to simultaneously calibrate a large number of field probes. We are developing analytical models of test-object directivity. We will continue to experimentally test these models and transfer results to committees developing standards for reverberation chambers.
Advanced composites offer weight and performance advantages over metals and are increasingly being used in aerospace and other applications. Plastics inherently provide no significant shielding to electromagnetic fields. Plastics can be “metalized” byDivision 29 introducing conducting fibers; however, this may affect mechanical performance. There is a need to reliably measure the EM shielding properties of advanced composites so that manufacturers can find the right balance between electrical and mechanical performance for a particular application. We are investigating the use of nested reverberation chambers for this purpose. We are investigating better statistical descriptors for the shielded fields to more accurately define shielding effectiveness in complex coupling environments.
With the proliferation of wireless devices in recent years, there is a growing need to test the operation and functionality of these various devices in different multipath environments, ranging from line-of-sight environment to a pure Rayleigh environment. We have recently shown how a reverberation chamber can be used to generate a variable K-factor propagation environment for the testing of wireless communications devices. It was shown that by judiciously changing the characteristics of the reverberation chamber and/or the antenna configurations in the chamber, any desired Rician K-factor (that is, any desirable Rician multipath propagation environment) could be achieved. We have developed two different approaches, the first based on one transmitting antenna, and the second based on two transmitting antennas. Reverberation chambers represent reliable and repeatable test facilities that have the capability of simulating any Rician multipath environment for the testing of wireless communications devices. Such a test facility will be useful in the testing of the operation and functionality of the new emerging wireless devices in the future. Our paper on this topic was published in November 2006 issue of IEEE Transactions on Antennas and Propagation. In it, we laid out the framework, presented a few simple expressions, and presented experimental evidence to support the possibility of using reverberation chambers for testing of wireless devices in different Rician multipath environments. Not all multipath environments can be characterized by a Rician distribution. An example would be an environment where both scattering components and multiple direct components are present. In such an environment, the direct coupling components have different amplitudes and phases (or different Doppler shifts). Such environments could in principle be simulated in reverberation chambers in which several transmitting antennas are used. The use of reverberation chambers in these other multipath environments is the topic of future work.
First responders need reliable communications in emergency scenarios. Disaster scenarios and terrorist attacks may result in scenarios where responders or citizens are trapped in collapsed or blocked buildings. The propagation of signals in the bands used by first-responder radios and cellular telephones needs to be investigated. We have performed unique experiments to define communications links in buildings prior, during, and after demolition. These data will give invaluable insight into the communications problems faced by first responders.
Metamaterials (that is, engineered or man-made materials) have generated considerable interest in recent years. Metamaterials are commonly engineered by arranging a set of scatterers throughout a region of space in a specifi c pattern so as to achieve some desirable bulk behavior. Examples of electromagnetic metamaterials are artificial dielectrics, photonic bandgap structures, and frequency-selective surfaces. Recently there have been studies on the properties and potential applications of double-negative (DNG) materials. We are investigating a composite medium consisting of insulating magneto-dielectric spherical particles embedded in a background matrix to achieve DNG behavior. We have shown that the effective permeability and permittivity of the mixture can be simultaneously negative for wavelengths where the spherical inclusions are resonant.
We work closely with national and international standards bodies to transfer experimental and theoretical results and to improve test methods for large, complex systems. We plan to continue participation in various IEC, CISPR, ANSI, SAE and IEEE standards committees related to EMC test methods.
Accomplishments
- Phantom Study — The electric field statistics in a heavily loaded reverberation chamber were evaluated and compared to the empty-chamber statistics. The load consisted of phantoms (water bottles) filled with tissue-simulating liquid. Wholebody average specific absorption rate (SAR) in the phantoms was also determined via direct measurements of temperature increase and indirect measurements of insertion loss. The results support the use of a reverberation chamber for animal exposure studies.
- Propagation in Collapsed Buildings — Three experimental studies on signal propagation in buildings prior, during, and after demolition were completed. These experiments provide invaluable data on first-responder communication challenges.
- Effective Boundary Conditions — Effective boundary conditions for thin films with applications to active materials such as frequency-tunable surfaces have been derived and published.
- Equivalent-Layer Models — Three equivalent- layer models were developed for the electromagnetic characterization of carbon fiber composites used on aircraft.
Publications