Maximizing Wireless Charging Performance Using Smaller Antennas

Originally posted by Michael Luciano, Associate Editor, Wireless Design Magazine

Wireless charging is a technology that’s finally prospering, after being in existence for decades. Last month, I wrote about the Cota wireless charging platform, and its benefits to military personnel. At the time it was published, Cota only supported 2.4 GHz, but aimed to achieve support frequencies up to 5.8 GHz later this year. That milestone was achieved sooner than we thought, with the Cota platform increasing its power up to 50 percent, while still only using one transmitter. To break down how this key advancement was made, it’s important to become familiar with the platform’s actual process.

“When increasing the Cota device’s power by up to 50 percent, it’s worth mentioning there was no change in hardware or amount of power emitted, just an improvement in the system’s total efficiency. Whatever power was being received previously could be obtained in the same amount without radiating more energy,” says Ossia Founder and Chief Technology Officer Hatem Zeine during a recent interview. “The additional energy that gets connected comes from enhanced accuracy.”

Whenever you start the Cota system, it goes through a software-based calibration cycle. During this brief process, all the platform’s timing mechanisms are aligned within the system, which is one way researchers were able to improve the software. Having said that, Zeine noted this alignment process is prone to interference from factors like WiFi and similar devices, even the temperature of the vicinity or surrounding environment.

“We took note of these details, and were able to create a new mechanism for the calibration that takes more aspects of the environment into consideration when enhancing the accuracy of beacon detection,” says Zeine. “The system is based on determining the signal back from the beacon. Now what they’ve done is enhance that accuracy. That improvement increases the amount of power landing on the receiver device by 50 percent.”

The result enabled researchers to delve in the 5.8 GHz space, a breakthrough that required some notable physical adaptations to the platform’s equipment—like smaller antennas.

“The shorter wavelengths in this space means, by the same token that antennas are smaller. So if you have a receiver today needing to cater 2.4 GHz, this particular wavelength is about 5 inches,” says Zeine when describing how smaller antennas improved the Cota platform. “You typically need about half that distance for a good performing antenna. If you go from a 2 ½ inch antenna to 5.8 GHz (which is now a two-inch wavelength or one-inch antenna) this means you can now have smaller devices receiving more power, which really is key to this enhancement.”

Zeine notes how going to higher frequencies of 5.8 GHz means you need smaller antennas, which would actually impact an electronic device at a less visible change. As a result, you could fit more power inside the device and not have to worry. Another benefit for a smaller space that Zeine points out is how putting a higher-efficiency antenna in your device could help receive more power at 5.8 GHz than adding the same kind of space at 2.4 GHz. The 5.8 GHz range also has more unused space, so the system can have a strong enough impact on communication systems and its surrounding environment, without worrying much about issues like interference. On the flip side, when the Cota system is used on the 2.4 GHz system, its impact is equivalent to adding another WiFi hub.

“If you already have one WiFi hub performing in the 2.4 GHz space but add another one in the same environment, you actually lose a bit of performance quality from that first one because the second hub has to share the same wavelength,” says Zeine. “Because 5.8 GHz WiFi doesn’t use the whole spectrum, it means there are spaces you can use without impacting the WiFi, so it’s actually better than adding other hubs, and has minimal or no impact to another WiFi network. Simply put, greater range is possible from the same (platform) transfer. We’re able to power more devices because we have more power and range to capture larger spaces.”

Reaching the 5.8 GHz space means the acquisition of smaller antennas translates to providing power in smaller devices—namely wearables that soldiers could be carrying.

“Today you hear about how enemies can follow troops through the desert using their disposed battery trails. If they run out of batteries or forget to charge their devices, this could obviously become a huge problem,” explains Zeine. “Wireless power means when a soldier goes back to base camp, all these devices would start charging without time lost. The devices would immediately charge by the time soldiers are back out in the field. All equipment would be charged without having to unpack and replace device’s batteries or throw on any unnecessary physical additions to prolong battery life, which a subtle design move as using smaller antennas helps achieve.”