Researchers at TU Wien in Vienna and the University of Rennes in France have developed a technique to create coatings that boost the transmission of radio waves for better WiFi reception.
The method allows the calculation of a tailor-made anti-reflective structure that can be added to a wall that is only partially permeable to a wireless signal so that the entire signal can be channelled through the wall without reflections.
Until now, it was not even clear on a theoretical level that such a thing was possible at all. The research team was able to present a calculation method for this and also tested it successfully in an experiment: microwaves were sent through a complex, disorderly maze of obstacles, then the matching anti-reflective structure was calculated and placed in front of the obstacles in the experiment. The reflection could be made to disappear almost completely: none of the waves returned to the side from which they were injected.
A similar approach has been taken to block sound waves: Cutting noise with a lightweight metamaterial sound screen
“You can think of it as being similar to the anti-reflective coating on your pair of glasses,” says Prof. Stefan Rotter from the Institute of Theoretical Physics at TU Wien. “You add an extra layer to the surface of the glasses, which then causes light waves to pass better to your eyes than before – the reflection is reduced.”
This is more difficult with a disordered medium in which a wave is repeatedly scattered and deflected until it finds its way out of such a maze via complicated paths such as a concrete wall. The waves are scattered at many points so that only a part of them gets through, the rest is reflected or absorbed in the wall.
“First, you simply have to send certain waves through the medium and measure exactly in which way these waves are reflected by the material,” said Michael Horodynski at TU Wien. “We were able to show that this information can be used to calculate a corresponding compensating structure for any medium that scatters waves in a complex way, so that the combination of both media allows waves to pass through completely. The key to this is a mathematical method that we developed to calculate the exact shape of this anti-reflective layer.”
The experiment in Rennes saw microwaves sent through a metallic waveguide in which the waves are scattered by dozens of small objects made of metal and Teflon placed completely randomly and in a disorderly manner. Only about half of the microwave radiation reaches the other side, the rest were reflected.
After precisely measuring the scattering behaviour of this system, it was possible to use the newly developed method to calculate which additional scattering points form a perfect “anti-reflective layer” for exactly this random system.
This provided 100% transmission with negligible reflection for any waveform that hits the anti-reflective structure.
This compensation for wave scattering with additional scattering can be used for other applications such as imaging in biophysics. Wave dynamics and wave scattering will also play a major role in 6G, the next generation of mobile communications after 5G.
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