Harvesting Ambient RF Energy Efficiently With Optimal Angular Coverage

Ambient electromagnetic and RF energy is an ubiquitous energy resource that is found everywhere but difficult to harvest because of the time-varying orientation of incoming radiations and the low efficiency of RF rectifiers resulting from a low input power density operation. In response to these two challenges that come with the design of rectennas for ambient energy harvesting, this paper discusses a figure of merit to evaluate the rectenna performance that combines the rectification efficiency, the radiation efficiency, and the spherical coverage of the antenna. To illustrate this proposed figure of merit, a rectenna with a strengthen RF-to-dc efficiency and also a full spatial coverage is designed using beam-forming networks. The resulting system is demonstrated for a wireless sensor mounted on a pole. To maintain a reasonable size of the prototype, a miniaturization technique of a 4× 4 Butler matrix (BM) is investigated, which allows for the reduction of the footprint of the BM by 2 compared to its classical microstrip design counterpart at 2.4 GHz. The overall rectification efficiency and dc power patterns are reported for the elevation plane of one multidirectional rectenna at 2.4 GHz for an incident power density of 0.45 μW·cm
^-2
. It is shown that the system can triple the dc output power compared to a three-dipole counterpart occupying the same area. Furthermore, while the maximum harvesting capability at 1 μW · cm
^-2
does not exceed 28π %.steradian in the literature, it reaches 43.6π %.steradian with the proposed demonstrator.