, shady keyrouz 2 and a. b. smolders 2 Design steps are outlined for maximizing the RF-to-dc power conversion efficiency (PCE) of a rectenna. It turns out that at a frequency of 868 MHz, a high-ohmic loaded rectifier will lead to a highly sensitive and power conversion efficient rectenna. It is demonstrated that a rectenna thus designed, using a 50 V antenna and lumped element matching network gives a superior PCE compared with state of the art also for lower resistive loading. By omitting the matching network and directly, conjugate impedance matching the antenna to the rectifier, the PCE may be further increased and the rectenna size reduced as it is demonstrated with a rectenna prototype measuring only 0.028 squared wavelengths at 868 MHz and demonstrating a PCE of 55% for a 210 dBm RF input power level.Keywords: Antenna, Rectenna, Impedance, Efficiency . While inductive, resonant coupling based WPT has reached a state of commercial exploitation, far-field WPT still needs to catch up. In this contribution, we describe a necessary step for making a commercial use of far-field WPT feasible. We describe a design strategy to increase the power conversion efficiency (PCE) of a rectifying antenna or rectenna. A far-field WPT system is schematically shown in Fig. 1 The radiated fields of a transmitter are picked up in the farfield region of the transmit antenna by a receive antenna. This signal is rectified, processed (voltage boost converted), and stored in a capacitor or battery. From this capacitor or battery an application (load) is being powered. The dashed box on the receiving side is the rectenna. When the transmitter is unintentional (e.g. a digital television broadcasting station or a base station for the global system of mobile communication), we specify the WPT as RF power harvesting.In applying far-field radiative WPT, we want to realize power over a large distance. We will be using frequencies in the license-free bands for Industry, Science, and Medicine (ISM) that are limited in allowed transmit powers. Due to this transmit power limitation and the spherical spreading of the transmitted waves, the power density at the receiving antenna will be very low. To keep the PCE, i.e. the ratio of dc output power delivered to the load to the RF input power at the clamps of the receive antenna, as high as possible, all subcomponents of the rectenna (see Fig. 1), must be well matched and means must be devised to maximize the voltage at the input of the rectifier, that is, maximize the rectifier's sensitivity.
I I . R E C T I F I E RBefore discussing the rectifier in detail, we will first look at the impedance matching of the rectifier to the receiving antenna, and then look at the sensitivity of the rectifier.
A) Impedance matching and sensitivityWe regard the rectifier as complex impedance that is matched to the antenna by means of a lossless L-network (see Fig. 2) [2]. The antenna is represented by an equivalent source V s with an internal resistance R s .For a maximum power transfer, the antenna is impedance ma...