Holst Centre, imec Work to Optimize Far-Field RF Energy Transport for Autonomous Charging of Wireless Sensors

Holst Centre and imec have identified all the subsystems that make up a far-field RF energy transport system. Such a system may be used to (re)charge the batteries of wireless sensors that are used in smart building integration applications. By optimizing both the transmitter radiation pattern and the receiving rectifying antenna, the RF energy transport efficiency can be greatly improved. In particular, it will allow a significant increase of the operating distance between the RF source and the rechargeable batteries.

For smart building integration, hundreds of wireless sensors are needed per building, sensing light, temperature, presence etc. Cabling all these sensors is too costly, as is the replacement and disposal of the prime batteries. An alternative solution is provided by remotely RF charging rechargeable batteries by using a far-field RF energy transport technology.

Such a technology uses a dedicated RF energy source, transmitting RF radiation in the 2446-2454MHz ISM frequency band. But restrictions posed on the effective isotropic radiated power (EIRP) make it challenging to realize DC power levels in the order of 100µW over distances of multiple meters. Holst Centre and imec now propose measures to improve the energy efficiency of the RF energy transport system. This will allow an increase of the operating distance between source and battery.

First, the researchers show that by characterizing the indoor environment and adapting the transmitting radiation pattern to these characteristics, more power may be obtained at larger distances using less transmit power. Holst Centre and imec propose the use of a cost-effective switched array antenna, in which one dipole/monopole element will be driven and parasitic dipoles/monopoles will be switched open or short-circuited/grounded. By modeling, the best switching scheme to illuminate the receiver and the wall positions for in-phase reflected wave contributions can be determined.