Design of a Ultra-Compact Low-Power Rectenna in Paper Substrate for Energy Harvesting in the Wi-Fi Band V. Palazzi 1 , C. Kalialakis 2 , F. Alimenti 1 , P. Mezzanotte 1 , L. Roselli 1 , A. Collado 2 , A. Georgiadis 2 1 Department of Engineering, University of Perugia, Perugia, Italy 2 Centre Tecnolgic de Telecomunicacions de Catalunya (CTTC), Castelldefels, Spain valentina.palazzi@studenti.unipg.it Abstract—This paper presents the design of a novel rectenna based on a tapered annular slot and a single-diode rectifier, optimized to operate in the Wi-Fi band and in presence of low input power (Pavs =-15 dBm). In order to obtain a compact layout a double-layer architecture has been considered, according to which the interior metal surface of the annular slot is employed as ground plane for the rectifying circuit placed on the other side of the substrate. As a result, a rectenna with an active area of only 40 × 33 mm 2 and an efficiency included between 26.5 and 28 % has been obtained. The antenna has been fabricated in paper substrate and tested in the anechoic chamber. Then, the rectifier has been designed and optimized within the Advanced Design System suite. Finally, the rectenna performance has been discussed and compared to the State of the Art (SoA). Index Terms—green electronics, rectennas, Internet of Things, circuits on paper, copper laminate technology, rectifiers, slot antennas, wireless power transfer, energy harvesting I. I NTRODUCTION Wireless Power Transfer (WPT) and Energy Harvesting (EH) are becoming ever more important research fields with the emergence of paradigms like the Internet of Things (IoT), smart skin, smart cities, and so forth. Indeed, the exponential growth of the number of electronic apparatuses in the environ- ment, needed to connect all the physical items to the Internet, will impose tremendous challenges in terms of power supply [1] [2]. Firstly, since these sensors and circuits are supposed to be actually embedded in the items, their maintenance might be impeded by the fact that they could be difficult to reach or even to localize. Secondly, these devices should be conformal to the hosting objects both in terms of shape and cost and, finally, the environmental impact of these distributed and ubiquitus electronics should be minimized in order not to increase the pollution risks and cause a worsening of the quality of life. For these reasons, it is important to avoid the traditional battery and to find new efficient and reliable ways to power the circuits from RF, both exploiting already available resources (EH) and enabling the intentional wireless transferring of power (WPT), so as to make them robust and autonomous. Additionally, the exploitation of lightweight, low-cost, re- cyclable and flexible substrates, together with the introduction of compact layouts, are key factors as well in order to ease the integration of the circuits with a wide range of items with different characteristics. In this paper a novel ultra-compact rectenna is presented. The antenna has been designed and fabricated in paper sub- strate, whereas the rectifier has been optimized for low input power, so as to make the circuit suitable for EH applications, in particular in the Wi-Fi band (2.4-2.5 GHz). In Sec. II the rectenna design is reported, with particular focus on the antenna performance, whereas in Sec. III the rectenna performance are discussed as a whole and compared to the State of the Art (SoA). II. RECTENNA DESIGN The aim of this work is to demonstrate the potential of the integration of the slotline (for the antenna) and the microstrip technology (for the circuitry) to obtain a compact, lightweight and efficient rectenna. The layout of the proposed rectenna is reported in Fig. 1. The idea is to employ the interior metal surface of an annular slot antenna as ground plane for the rectifying circuit. In this way, the active area of the rectenna corresponds to the bare antenna area, resulting in a very compact design. In the following subsections both the antenna and the rectifier are described in detail. A. Tapered Annular Slot Antenna The proposed antenna is a double-layer tapered annular slot, firstly introduced in [3]. The slot, resulting from the intersection of an ellipse and a circle, is coupled to the feeding line placed on the other side of the substrate. The line terminates in a circular stub introduced for matching purposes in order to match the antenna to a 50 Ω impedance. The antenna has been designed by means of CST Studio Suite and has been fabricated in paper substrate (Mitsubishi photo paper) by means of the copper laminate technology [4].