Octave and Decade Printed UWB Rectifiers Based on Nonuniform Transmission Lines for Energy Harvesting

Ambient RF energy harvesting is a potential energy source for low-power and battery-less wireless sensors, enabling a range of applications from monitoring to security as part of the Internet-of-Things (IoT) scenario. One of the main challenges of ambient RF energy harvesting is the requirement of operation over a multitude of frequency bands of low ambient power densities resulting in a very wide aggregate operating bandwidth. In this paper, design examples of novel ultra-wideband energy harvesters are demonstrated with octave and decade bandwidths in the UHF and low microwave spectrum. The RF-dc conversion efficiency is maximized by tailoring the dimensions of a nonuniform transmission line used to provide broadband impedance matching. The design challenges in terms of impedance matching based on the Bode-Fano theoretical limit, losses and miniaturization are highlighted. Two prototypes are presented and their performance is evaluated. The octave band rectifier showed a measured RF-dc conversion efficiency of more than 60% over a frequency band of 470 to 860 MHz at 10-dBm input power. The decade band rectifier fabricated on Kapton substrate using inkjet printing featured a higher than 33% efficiency over a frequency band from 250 MHz to 3 GHz at 10-dBm input power.