Energy Harvesting for Battery-Free Wireless Sensors Network Embedded in a Reinforced Concrete Beam

This paper deals with the design and the characterization of a compact and efficient rectenna, as well as the practical implementation of a wireless sensors network embedded in a reinforced concrete beam. The wireless sensors are powered by using a far field wireless power transmission technique through a rectenna operating in the European ISM 868 MHz frequency band. The proposed rectenna is based on a compact 3D T-match dipole antenna with a size of 56 mm x 32 mm x 8 mm (0.00035·λ
³
) placed at 60 mm relative to the ground plane of the sensing node used as a reflector (140 mm x 90 mm); and an optimized rectifier. This rectenna provides a maximum dc voltage of 2.8 V and a RF-to-dc efficiency of 67 % across a 10 kΩ load with an illuminating power density of 3.9 μW/cm
²
at 868 MHz. Thus, once embedded in reinforced concrete, the rectenna exhibits a dc power of 212.83 μW when a +32 dBm EIRP power source located at 170 cm (156 cm in air and 14 cm in reinforced concrete) is used. Experimental results validate the sensing nodes supply by wireless power transmission in reinforced concrete, which allows the collect and the LoRaWAN transmission of physical parameters (temperature and humidity).