Frequency 'sweet spot' helps signals travel in tunnels

Underground tunnels, usually a difficult environment for radios, can have a frequency 'sweet spot' at which signals may travel several times further than at other frequencies, according to researchers at the National Institute of Standards and Technology (NIST), in the US.

As part of a project to improve wireless communications for emergency responders, the researchers used new data to confirm models developed in the 1970s. This research may lead to enhanced rescue communications systems in subways and mines.

The optimal frequency depends on the dimensions of the tunnel, but for a typical subway-sized tunnel, the sweet spot is found in the frequency range 400 MHz to 1 GHz. The NIST researchers were surprised by how much further signals at the optimal frequency travelled in above-ground building corridors, as well as underground.

Radio signals can be channelled by tunnels because they act like giant waveguides, say the researchers.

The waveguide effect depends on a tunnel’s width, height, surface material and the flatness of the floor as well as the signal frequency.

The researchers found agreement between the measured data and theoretical models, leading to the conclusion that the waveguide effect plays a significant role in radio transmissions in tunnels.

The effect is described in a NIST publication. Lead author Kate Remley noted 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 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 said.

The reports are part of a NIST series contributing to a 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 designing optimal systems, especially emergency responders.