Metasurface antenna can change beam's characteristics

The structure and characteristics of antennas cannot traditionally be changed once fabricated. But in a significant breakthrough, researchers from Hong Kong and China have now developed a so-called ‘sideband-free space-time-coding (STC) metasurface antenna’ that allows manipulation of the direction, frequency and amplitude of the antenna’s electromagnetic waves.

Described in the journal Nature Electronics, the team’s research is set to enable greater user flexibility and to play an important role in 6G wireless communications.

The key to the antenna’s powers is that the response of the metasurface — an artificial, thin-sheet material with sub-wavelength thickness and made of several sub-wavelength meta-atoms — can be changed by switching the meta-atoms on its surface between radiating and non-radiating states, like turning switches on and off, by controlling the electric current. This allows the STC metasurface antenna to realise complicated wave manipulation in the space and frequency domains through space-time coding (ie, software control), and to create a desired radiation pattern and a highly directed beam.

Research leader Professor Chan Chi-hou, from City University of Hong Kong (CityU), said the breakthrough relies on the successful combination of two research advances: amplitude-modulated (AM) leaky-wave antennas and space-time coding techniques. Postdoctoral fellow Dr Wu Gengbo first proposed the concept of AM leaky-wave antennas in 2020 during his PhD studies at CityU.

“The concept provides an analytical approach to synthesise antennas with the desired radiation patterns for different specific uses by simply changing the antennas’ shape and structure,” Wu explained.

But as with other antennas, once the AM leaky-wave antenna is fabricated, its radiation characteristics are fixed. At about that time, Dr Dai Junyan from China’s Southeast University, who pioneered STC technologies, joined Chan’s group at CityU.

“Dr Dai’s expertise in space-time coding and digital metasurfaces to dynamically reconfigure antenna performance added a new, important dimension to the antenna research,” said Chan, who is the Director of CityU’s State Key Laboratory of Terahertz and Millimeter Waves (SKLTMW).

The time modulation of electromagnetic waves on metasurfaces usually generates unwanted harmonic frequencies, called sidebands. These sidebands carry part of the radiated electromagnetic wave energy and interfere with the useful communication channels of the antenna, leading to ‘spectrum pollution’. Chan and his team proposed a novel design which makes use of a waveguide (a line for transmitting electromagnetic waves by successive reflection from the inner wall) and successfully suppressed the undesired harmonics, achieving a high-directivity beam and enabling secure communication.

Image credit: City University of Hong Kong.

“With the AM leaky-wave antenna and space-time coding technologies, we achieve the designated radiation characteristics by controlling the on-off sequences and duration of the ‘switches’ on the antenna through software,” Chan said.

“A high-directivity beam can be generated with the new antenna, allowing a wide range of radiation performance without having to redesign the antenna, except for using different STC inputs,” Wu added.

The energy from the radiated beam of the STC metasurface antenna can be focused to a focal point with fixed or varying focal lengths, which can be used for real-time imaging and treated as a type of radar to scan the environment and feedback data. The invention is thus expected to play an important role in the integration of sensing and communications (ISAC) for 6G wireless communications.

“For example, the radiated beam can scan a person and create an image of the person, allowing mobile phone users to talk to each other with 3D hologram imaging,” Chan said. “It also performs better against eavesdropping than the conventional transmitter architecture.

“We hope that the new-generation antenna technology will become more mature in the future and that it can be applied to smaller integrated circuits at lower cost and in a wider range of applications.”

Top image caption: Professor Chan Chi-hou and Dr Wu Gengbo have developed a new-generation antenna together with researchers at Southeast University. Image credit: City University of Hong Kong.