High Performance Electrically Small Huygens Rectennas Enable Wirelessly Powered Internet of Things Sensing Applications: A Review

Far-field wireless power transfer (WPT) is a major breakthrough technology that will enable the many anticipated ubiquitous Internet of Things (IoT) applications associated with fifth generation (5G), sixth generation (6G), and beyond wireless ecosystems. Rectennas, which are the combination of rectifying circuits and antennas, are the most critical components in far-field WPT systems. However, compact application devices require even smaller integrated rectennas that simultaneously have large electromagnetic wave capture capabilities, high alternating current (AC)-to-direct current (DC) (AC-to-DC) conversion efficiencies, and facilitate a multifunctional wireless performance. This paper reviews various rectenna miniaturization techniques such as meandered planar inverted-F antenna (PIFA) rectennas; miniaturized monopole- and dipole-based rectennas; fractal loop and patch rectennas; dielectric-loaded rectennas; and electrically small near-field resonant parasitic rectennas. Their performance characteristics are summarized and then compared with our previously developed electrically small Huygens rectennas that are proven to be more suitable for IoT applications. They have been tailored, for example, to achieve battery-free IoT sensors as is demonstrated in this paper. Battery-free, wirelessly powered devices are smaller and lighter in weight in comparison to battery-powered devices. Moreover, they are environmentally friendly and, hence, have a significant societal benefit. A series of high-performance electrically small Huygens rectennas are presented including Huygens linearly-polarized (HLP) and circularly-polarized (HCP) rectennas; wirelessly powered IoT sensors based on these designs; and a dual-functional HLP rectenna and antenna system. Finally, two linear uniform HLP rectenna array systems are considered for significantly larger wireless power capture. Example arrays illustrate how they can be integrated advantageously with DC or radio frequency (RF) power-combining schemes for practical IoT applications.