Metamaterial antenna harvests energy from radio waves

The technology could potentially provide wireless power to sensors, LEDs and other devices with low energy requirements.

“By eliminating wired connections and batteries, these antennas could help reduce costs, improve reliability and make some electrical systems more efficient,” said research team leader Jiangfeng Zhou from the University of South Florida. “This would be useful for powering smart home sensors such as those used for temperature, lighting and motion or sensors used to monitor the structure of buildings or bridges, where replacing a battery might be difficult or impossible.”

In Optical Materials Express, the researchers report that lab tests of their new antenna showed that it can harvest 100µW of power from low power radio waves. This was possible because the metamaterial used to make the antenna exhibits perfect absorption of radio waves and was designed to work with low intensities.

“Although more work is needed to miniaturise the antenna, our device crosses a key threshold of 100µW of harvested power with high efficiency using ambient power levels found in the real world,” said Clayton Fowler, who fabricated the sample and performed the measurements. “The technology could also be adapted so that a radio wave source could be provided to power or charge devices around a room.”

Scientists have previously attempted to capture energy from radio waves, but it has been difficult to obtain enough energy to be useful. According to the team in Florida, this is changing due to the development of metamaterials and ambient sources of radio frequency energy available, such as cell phone networks, Wi-Fi, GPS, and Bluetooth signals.

“With the huge explosion in radio wave-based technologies, there will be a lot of waste electromagnetic emissions that could be collected,” Zhou said in a statement. “This, combined with advancements in metamaterials, has created a ripe environment for new devices and applications that could benefit from collecting this waste energy and putting it to use.”

Metamaterials use small, carefully designed structures to interact with light and radio waves in ways that naturally occurring materials do not. To make the energy-harvesting antenna, the researchers used a metamaterial designed for high absorption of radio waves and that allows a higher voltage to flow across the device’s diode. This is said to have improved its efficiency at turning radio waves into power, particularly at low intensity.

For lab tests of the 16cm-by-16cm device, the researchers measured the amount of power harvested while changing the power and frequency of a radio source between 0.7GHz and 2.0GHz. They demonstrated the ability to harvest 100µW of power from radio waves with an intensity of 0.4µW/cm², approximately the level of intensity of the radio waves 100m from a cell phone tower.

“We also placed a cell phone very close to the antenna during a phone call, and it captured enough energy to power an LED during the call,” said Zhou. “Although it would be more practical to harvest energy from cell phone towers, this demonstrated the power capturing abilities of the antenna.”

The researchers are working to make their solution smaller, and they would like to make a version that could collect energy from multiple types of radio waves simultaneously.