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 findings may point to strategies to enhance rescue communications in subways and mines.

The engineers found that for a typical subway-sized tunnel, the sweet spot is found at the 400 megahertz (MHz) to 1 gigahertz (GHz) frequency range. Tunnels can channel radio signals in the right frequency range, because they act like giant waveguides 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 scientists performed the tunnel studies at Black Diamond Mines Regional Park near Antioch, CA, an old complex used in the early 1900s to extract pure sand for glass production. NIST researchers 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. The NIST data will support development of open standards to optimize system design, especially for emergency responders.

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