Here are a couple of items from the NIST Tech Beat, an e-mail newsletter of the National Institute of Science and Technology. The first could really be important in an emergency situation. The second is still more in the realm of research, but the speed at which some of these things become products, I wouldn’t be surprised if we soon see products based on this technology.
Emergency Links: NIST Identifies ‘Sweet Spot’ for Radios in Tunnels
As part of a project to improve wireless communications for emergency responders, researchers at the National Institute of Standards and Technology (NIST) have confirmed that underground tunnels—generally a difficult setting for radios—can have a frequency “sweet spot†at which signals may travel several times farther than at other frequencies. The finding, which uses extensive new data to confirm models developed in the 1970s, may point to strategies for enhancing rescue communications in subways and mines.
The optimal frequency depends on the dimensions of the tunnel. For a typical subway-sized tunnel, the sweet spot is found in the frequency range 400 megahertz (MHz) to 1 gigahertz (GHz). This effect is described in one of two new NIST publications.* The reports are part of a NIST series contributing to the first comprehensive public data collection on radio transmissions in large buildings and structures. Historically, companies have designed radios based on proprietary tests. The NIST data will support the development of open standards for design of optimal systems, especially for emergency responders.
NIST researchers were surprised by how much farther signals at the optimal frequency traveled in above-ground building corridors, as well as underground. Tunnels can channel radio signals in the right frequency range because they act like giant waveguides, the pipelike channels that confine and direct microwaves on integrated circuit wafers, and in antenna feed systems and optical fibers. The channel shape reduces the losses caused when signals are absorbed or scattered by structural features. The waveguide effect depends on a tunnel’s width, height, surface material and roughness, and the flatness of the floor as well as the signal frequency. NIST authors found good agreement between their measured data and theoretical models, leading to the conclusion that the waveguide effect plays a significant role in radio transmissions in tunnels.
Lead author Kate Remley notes that the results may help design wireless systems that improve control of, for example, search and rescue robots in subways. Some handheld radios used by emergency responders for voice communications already operate within the optimal range for a typical subway, between around 400 MHz and 800 MHz. To provide the broadband data transfer capability desired for search and rescue with video (a bandwidth of at least 1 MHz), a regulatory change would be needed, Remley says.
The tunnel studies were performed in 2007 at Black Diamond Mines Regional Park near Antioch, Calif., an old complex used in the early 1900s to extract pure sand for glass production.
The second new NIST report** describes mapping of radio signals in 12 large building structures including an apartment complex, a hotel, office buildings, a sports stadium and a shopping mall.
The research is supported in part by the U.S. Department of Justice and the Department of Homeland Security. Both reports will be available on NIST’s Metrology for Wireless Systems Web page.
* K. A. Remley, G. Koepke, C. L. Holloway, C. Grosvenor, D.G. Camell, J. Ladbury, R.T. Johnk, D. Novotny, W.F. Young, G. Hough, M.D. McKinley, Y. Becquet and J. Korsnes. “Measurements to Support Modulated-Signal Radio Transmissions for the Public-Safety Sectorâ€. NIST Technical Note 1546, April, 2008.
** C. L. Holloway, W.F. Young, G. H. Koepke, K. A. Remley, D. G. Camell and Y. Becquet. “Attenuation of Radio Wave Signals Into Twelve Large Building Structuresâ€. NIST Technical Note 1545.
Disorder Enables Extreme Sensitivity in Piezoelectric MaterialsA research team working at the National Institute of Standards and Technology (NIST) has found an explanation for the extreme sensitivity to mechanical pressure or voltage of a special class of solid materials called relaxors.* The ability to control and tailor this sensitivity would allow industry to enhance a range of devices used in medical ultrasound imaging, loudspeakers, sonar and computer hard drives.
Relaxors are piezoelectrics—they change shape when a battery is connected across opposite ends of the material, or they produce a voltage when squeezed. “Relaxors are roughly 10 times more sensitive than any other known piezoelectric,†explains NIST researcher Peter Gehring. They are extremely useful for device applications because they can convert between electrical and mechanical forms of energy with little energy loss.
A team of scientists from Brookhaven National Laboratory, Stony Brook University, Johns Hopkins University and NIST used the neutron scattering facilities at the NIST Center for Neutron Research (NCNR) to study how the atomic “acoustic vibrations,†which are essentially sound waves, inside relaxors respond to an applied voltage. They found that an intrinsic disorder in the chemical structure of the relaxor crystal apparently is responsible for its special properties.
Atoms in solids are usually arranged in a perfect crystal lattice, and they vibrate about these positions and propagate energy in the form of sound waves. In typical piezoelectric materials, these acoustic vibrations persist for a long time much like the ripples in a pond of water long after a pebble has been thrown in.
Not so with relaxors: these vibrations quickly die out. The research team led by Brookhaven’s Guangyong Xu, compared how the sound waves propagated in different directions, and observed a large asymmetry in the response of the relaxor lattice when subjected to an applied voltage.
“We learned that the lattice’s intrinsic chemical disorder affects the basic behavior and organization of the materials,†says Gehring. The disorder that breaks up the acoustic vibrations makes the material structurally unstable and very sensitive to applied pressure or an applied voltage.
That disorder occurs because the well-defined lattice of atoms alternates randomly between one of three of its elements—zinc, niobium and titanium—each of which carries a different electrical charge.
The research was funded by the Office of Basic Energy Sciences within the U.S. Department of Energy’s Office of Science and the Natural Science and Research Council of Canada.
* G. Xu, J. Wen, C. Stock and P.M. Gehring. Phase instability induced by polar nanoregions in a relaxor ferroelectric system. Nature Materials. Published online May 11, 2008.
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