Current wireless charging pads mainly use short-range induction and are very efficient, but they can only charge close to the device being charged. Now new research shows that by harnessing the radiation suppression capabilities of loop antennas, it is possible to charge devices not only at greater distances with over 80% efficiency, but also in a variety of directions, paving the way for a new era of wireless power transfer suitable for devices ranging from mobile gadgets to biomedical implants.
Considering radiation losses is crucial for efficient long-distance wireless power transmission. Engineers at Aalto University have developed an improved method of wireless charging over long distances. By enhancing the interaction between transmitting and receiving antennas and exploiting the phenomenon of "radiation suppression," they deepen our theoretical understanding of wireless power transfer beyond traditional inductive methods, a major advance in the field.
Short-distance charging, such as charging through an induction pad, which uses a near-field magnetic field to transmit electrical energy, is very efficient, but when the distance is longer, the efficiency drops sharply. New research shows that this high efficiency can be maintained over long distances by suppressing the radiation impedance of the loop antenna that sends and receives electrical energy.
Two loop antennas (radius: 3.6 cm) can transmit power to each other 18 cm apart. Image source: NamHa-Van/AaltoUniversity
Previously, the lab developed an omnidirectional wireless charging system that can charge devices in any direction. Now, they have extended this work with a new theory of wireless charging dynamics that looks more closely at distances and conditions at near (non-radiative) and long range (radiative). In particular, they showed that using optimal frequencies in the hundreds of megahertz range, high transmission efficiencies of over 80% can be achieved at distances approximately five times the size of the antenna.
"We wanted to balance the effective transmission power with radiation losses, which always occur over longer distances," said first author Nam Ha-Van, a postdoctoral researcher at Aalto University. "It turns out that when the currents in the loop antenna have equal amplitudes and opposite phases, we can cancel out the radiation losses, thereby increasing efficiency."
Researchers have created a method that allows mathematical or experimental analysis of any wireless power transmission system. This allows for a more complete assessment of power transmission efficiency at short and long distances that has not been possible before. They then tested how charging works between two loop antennas (see figure) that are quite far apart relative to their size, identifying radiation suppression as the mechanism that helps improve transmission efficiency.
"It's all about finding the best setup for wireless power transfer, both close and long distances," said Ha-Van. "With our method, we can now extend the transmission range beyond traditional wireless charging systems while maintaining high efficiency. Wireless power transfer is not only important for phones and gadgets, but biomedical implants with limited battery capacity can also benefit from it. The research by Ha-Van and colleagues can also take into account obstacles such as human tissue that may hinder charging."