Electrically Small Huygens CP Rectenna With a Driven Loop Element Maximizes Its Wireless Power Transfer Efficiency

An electrically small Huygens circularly polarized (HCP) rectenna whose antenna is directly matched to its rectifying circuit is developed and experimentally validated in the ISM band at 915 MHz. The HCP antenna is a near-field resonant parasitic (NFRP) design consisting of a driven element and a crossed pair of balanced electric and magnetic NFRP dipole elements. The rectifier circuit is highly capacitive. It is a full-wave design based on two HSMS286C Schottky diodes. Two innovative driven element designs are explored that facilitate the effective conjoin of the antenna to different classes of external circuits. An HCP antenna with a driven spiral line element is demonstrated. Its input impedance is capacitive and, hence, the system is ideal for matching directly to an inductive external circuit. An HCP antenna with a driven loop element is investigated in a similar manner. The system has an inductive input impedance that is subsequently optimized to be conjugately match to the capacitive rectifier directly. The measured HCP rectenna prototype has broad-angle capture capacity, avoids any polarization mismatch issues, and has exceptional ac to dc conversion efficiency, the maximum reaching 90.6%. It is an ideal candidate for wireless power transfer (WPT)-enabled Internet-of-Things (IoT) applications.