Power-Efficient UHF Rectification for Long-Distance Wireless Power Transfer in Sensor Nodes

In the last decade, the development and the implementation of smart and ubiquitous sensors in various fields of interest has opened several issues about the life cycle of these devices and the disposal or replacement of their batteries. One potential solution can be found in wireless power transfer and in radio-frequency (RF) Energy Harvesting, which enables devices to be powered up to several meters from a RF source. This study proposes the optimization of an UHF rectifier operating at 868 MHz, and investigates the effects of the main parasitic elements of its implementation, like printed circuit board (PCB) metal traces and the package of lumped elements. Furthermore, an optimization method for matching network design, composed of two specific harmonic balance simulations, is described: in the first one, ideal matching network components are used in order to improve simulation time; in the second, S-parameters provided by manufacturers are used. Finally, a PCB prototype is characterized, producing 4.56
$\mu \mathrm{W}$
of rectified power, with 36.3% RF-to-DC conversion efficiency and 278 mV in optimum load conditions, with a -19 dBm harmonic input signal at 868 MHz. An estimate of output power over distance is reported in order to evaluate the maximum operative distance of potential commercial devices, like micro-power management integrated circuits.