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Tunable Liquid-Metal Antennas Are Perfect for Connecting the Internet of Things

Engineers at North Carolina State University offer a solution to a looming problem.

Engineers at North Carolina State University have devised a new form of tiny, liquid-metal antenna that's capable of tuning into a wide range of radio frequencies, offering a timely solution to a looming and potentially damning problem in networked electronics—namely, the limits of the radio spectrum itself.

If this whole Internet of Things project is going to work, we'll need better and, crucially, more ways to communicate with it. Simply, the IoT means a whole lot of new devices and sensors begging for bandwidth that's already in increasingly short supply with smart-phones alone communicating via wi-fi, GPS, bluetooth, and 4G, each one of those requiring antennas of different shapes and wavelengths. This situation will only get worse.

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Making it all work will require antennas that can serve multiple purposes, e.g. adaptive radio communication enabled by reconfigurable radiofrequency (RF) electronics. Reconfiguring is difficult, however, and requires a bunch of added complexity in the forms of more switches and more components, with the result being a radio device that's reconfigurable, but only within a limited range.

Enter liquid metal-based antennas, which allow for communication across wider sections of the radio spectrum thanks to their ability to change length—physically morphing to acquire different signals. But, as the North Carolina researchers explain in a paper published this week in the Journal of Applied Physics, this presents its own set of problems in the form of still more complexity, e.g. more moving parts.

"In these applications, the liquid conductors are pneumatically actuated via pumps or contact pressure to change RF current paths," the NC State group, led by electrical engineering professor Jacob Adams, write. "While the enhanced control over the conductor length and location provided by a liquid conductor greatly enhances the tuning range of the devices, the introduction of pumps and microfluidic elements adds to system complexity and requires a closed fluid path, limiting the device topology."

Adams and co. solve this by manipulating the liquid metal antenna via electrical potential. It was discovered by another engineer at NC State, Michael Dickey, that by changing the voltage across a liquid metal confined to a tiny tube in the presence of an electrolyte, it's possible to make the material expand and contract and move up and down within the tube (think of an old thermometer). The reason for this is that a positive voltage causes the formation of a layer of metal oxide on top of the liquid metal, which has the effect of changing its surface tension and, thus, how well it can flow up and down the antenna tube.

So: no complicated devices, just electricity and the liquid metal itself—an alloy of gallium and indium.

As Adams tells IEEE Spectrum, the technology could potentially scale way up to the point of supporting large defense communication systems and radar arrays, which cover wavelength bands ranging from a few megahertz to many gigahertz. "While a single tunable element will probably never be able to cover this entire range, they could potentially cut down on the 'antenna farms' found on large defense platforms, such as on ships and planes."

For us civilians, meanwhile, it seems like a technology well suited for the near-future's Internet of Skin.