PhD Defense of VANDELLE Erika

Wireless energy harvesting (WEH) of ambient or intentional electromagnetic power sources of frequency ranging from 100 MHz to 10 GHz, has appeared as a promising solution to develop self-powered electronics in the past decades. However, the low power densities available, usually lower than 1 µW.cm-2 (-30 dBm.cm-2), result in a limited RF-to-DC conversion efficiency and sensitivity of the energy harvesting system (rectenna) and ambient signal diversities (unknown and time-varying direction of arrival, polarization) prohibit the use of directive antennas.
In this thesis, the power combination techniques of Radio Frequency (RF) or Direct Current (DC) power in multi-antenna WEH systems, together with original structures, are investigated
to address those challenges. Besides, a new Figure-of-Merit (harvesting capability) for rectennas operating in ambient scenarios is derived with probabilistic terms representing frequency, polarization and spatial diversities of ambient signals.
The first part of this thesis focuses on the design of efficient antenna and rectenna elements. Eco-responsible and low-cost prototypes are proposed by using a paper substrate along with an original strategy for the reduction of the losses.
In the second part of this work, the rectification efficiency of a WEH system is enhanced through the combination of the RF power prior to the rectification process, without reduction of the spatial coverage. For this, a 3D multidirectional structure of scanning antenna arrays using passive beam-forming networks is designed to obtain a multidirectional high gain aggregate pattern. This radar-inspired solution involving Butler matrices results in a highly efficient RFto-DC power conversion along with an optimal angular coverage, which leads to a harvesting capability higher than the state-of-the-art.
The last part of this work addresses the limited sensitivity of the RF combination technique compared to that obtained with the series DC combination technique thanks to a reconfigurable system. To this end, modular rectenna unit cells are designed to form a scalable and adaptive interferometric beam-forming network, which offers the possibility to achieve a highly efficient and sensitive WEH system. This solution is suitable for low-power energy harvesting, autonomous passive tracking or RFID applications.