Radio communications and tunnels don’t mix, or do they?

Underground tunnels and wireless communications have generally been mutually exclusive entities, much to the chagrin of emergency responders (or anyone who has had their favorite tunes interrupted by tunnel travel for that matter).But serious help may be on the way. 

Researchers at the National Institute of Standards and Technology (NIST) today said underground tunnels can have a frequency “sweet spot” at which signals may travel several times farther than at other frequencies, a development that may enhance rescue communications in subways, car tunnels and mines.

The NIST data will support the development of open standards for design of optimal systems, especially for emergency responders.This optimal frequency depends on the dimensions of the tunnel, NIST said. For a typical subway-sized tunnel, the sweet spot is found in the frequency range 400 megahertz (MHz) to 1 gigahertz (GHz).

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 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.NIST’s 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, NIST said.

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