Remembering 9/11, we can virtually all recall that fire and police teams had problems communicating within The World Trade Center buildings in the zone now known as 'Ground Zero.' Fortunately, researchers at the National Institute of Standards and Technology (NIST) have been engaged in a project to improve wireless communications for emergency responders. They have confirmed that underground tunnels — generally a difficult setting for radio wave propagation — can have a frequency "sweet spot" at which signals may travel several times farther than at other frequencies.

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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 to 1000 MHz. This effect is described in the new NIST Technical Note NIST TN1545. A second new NIST Technical Note, NIST TN1546, 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.

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 pipe-like 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.