Machine Design

Finding a Tunnel’s Sweet Spot

Security forces and law enforcement agencies depend as much on wireless communications as teenagers and plugged-in executives.

So what will they do when an emergency strikes in a tunnel or mine, and their communications start breaking down? (Radio signals have a hard time traveling through deep tunnels and even long corridors in some buildings.) Researchers at NIST are trying to find a solution, a sweet spot in the frequency range at which signals travel several times farther than at other frequencies. For a typical subway tunnel, researchers determined the sweet spot is between 400 MHz and 1 GHz. They also believe tunnels act like giant waveguides, with the tunnel dimensions, materials, and flatness of the floor determining their effects on RF signals. And at the sweet spot, the shape of the tunnel reduces losses caused by RF signals being absorbed or scattered by structures. NIST personnel hope their work will lead to wireless communication devices for search and rescue robots used in tunnels and mines.

National Institute of Science and Technology Gaithersburg, MD


An electronics engineer aligns antennas in an abandoned California silica mine during a NIST study looking for frequencies that best transmit radio signals.

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