> REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 1 Abstract — This paper describes the design and experimental characterization of a battery-less bi-directional 2.45-GHz circuit operating in oscillator mode as a wireless power transmitter or in rectifier mode as an energy harvester, with a measured efficiency greater than 50% in both operating states. The DC voltage harvested in rectifier mode provides the drain bias for the oscillator. The FET-gate self-bias mechanism is exploited in both functionalities, to get rid of gate external bias. Bi-directionality is based on the time-reversal properties of a harmonic-suppression oscillator. Energy autonomy is possible at received RF power as low as -4 dBm. This is obtained by means of a bias-assisting feedback loop, consisting of a single matched low-power diode in shunt configuration. A hybrid prototype is demonstrated with the ability to operate as an energy-autonomous power relay node by switching between the transmit and receive power modes. Index Terms — Microwave oscillator, wireless power transfer, energy harvesting, rectifier, autonomous circuit, battery-less. I. INTRODUCTION Miniaturized low-power densely distributed wireless systems are becoming an enabling technology in various applications, from environmental monitoring, e-health, home and industrial automation [1]. Their main advantages are fast and economical deployment, absence of wiring infrastructure, and seamless dynamic repositioning of devices. Several solutions have been pursued to reduce energy consumption [2] as well as to develop high-efficiency harvesting capabilities in order to eliminate the need for battery replacement thus This work was supported by the EU COST Action IC1301 “Wireless Power Transmission for Sustainable Electronics” (WIPE). The work of M. Del Prete, A. Costanzo and D. Masotti was partly funded by the Italian Ministry of the Instruction, University and Research (MIUR), within the framework of the national project GRETA. The work of A. Georgiadis and A. Collado was funded by the Spanish MEC and FEDER funds through project TEC2012-39143 and the Generalitat de Catalunya under grant 2014 SGR 1551. This paper is an expanded paper from the IEEE MTT-S International Microwave Symposium, Phoenix, AZ, USA, May 18–22, 2015. Massimo Del Prete, Alessandra Costanzo and Diego Masotti are with the Department of Electrical, Electronic and Information Engineering, “Guglielmo Marconi,” University of Bologna, Italy (email: massimo.delprete3@unibo.it, alessandra.costanzo@unibo.it, diego.masotti @unibo.it). Apostolos Georgiadis, Ana Collado are with Centre Tecnologic de Telecomunicacions de Catalunya (CTTC), Castelldefels, Spain (ageorgiadis@cttc.es, acollado@cttc.es). Zoya Popovic is with University of Colorado, Boulder, United States (zoya@Colorado.edu). reducing maintenance cost [3], [4]. In such scenarios, strategically located dedicated RF sources can help increase battery lifetime through wireless power delivery to sensors distributed in the source coverage area. One of the issues in RF energy harvesting (EH) solutions is the unknown and often variable RF link (rectenna location and polarization), which can threaten the effective use of energy-autonomous wireless systems. A useful figure of merit for wireless power transfer (WPT) links is the total efficiency from the input of the transmitting antenna to the output of a DC-DC converter [4]: (1) It consists of the product of three contributions: the radio- link, the rectenna and the DC-DC converter efficiencies. Only if intentional RF sources are available η RF-RF can be kept sufficiently high and maximized by design of both transmit and receive parts of the WPT system. In view of a seamless reconfiguration of architectures based on wireless devices, it is of great interest to rely on devices integrating energy harvesting and wireless power transfer capabilities, thus acting on demand either as a user or as a wireless power provider. If such devices operate bi- directionally to exploit themselves the harvested power or to act as power relay nodes, with the highest possible conversion efficiency, they can be used to provide energy to randomly but closely located wireless devices. In this way such wireless devices can dynamically relocated counting on these power relay nodes. The added value of this two-step power transmission can be put into evidence by simply referring to Friis equation. The radio-link efficiency η RF-RF between the RF source and the device (tag) can now be split into two terms: the efficiency of the link between the source and the relay node: 2 4         ⋅ ⋅ ⋅ = = - - RELAY TX RELAY RX TX TX RELAY RX RELAY TX d G G P P π λ η (2) and the efficiency between the relay node and the tag: 2 4         ⋅ ⋅ ⋅ = = - - TAG RELAY TAG RELAY TX RELAY TX TAG RX TAG RELAY d G G P P π λ η (3) DC ST RX DC TX RX DC DC DC RF RF RF TOT P P P P P P ⋅ ⋅ = ⋅ ⋅ = - - - η η η η A 2.45 GHz Energy-autonomous Wireless Power Relay Node Massimo Del Prete Student, IEEE, Alessandra Costanzo, Senior member, IEEE, Apostolos Georgiadis, Senior member, IEEE, Ana Collado, Senior member, IEEE, Diego Masotti, Member, IEEE, and Zoya Popovic Fellow, IEEE