Efficient RF harvesting for low-power input with low-cost lossy substrate rectenna grid

An efficient, low-cost and low-complexity rectenna-grid is analyzed, fabricated and measured, for low-power RF input and RF density. The design consists of two single series rectifier circuits that add the currents from each diode, in order to increase the offered power at the load. Despite the fact that a lossy and low-cost FR-4 substrate was used, the rectenna-grid offers 200 μW at load for 1 μW=cm² ambient power density, while for 0.01 μW=cm² the power at load reaches 0.8 μW. Furthermore, RF-to-DC rectification efficiency of 20.5% and 35.3% is achieved at -20 and -10 dBm power input, respectively. Measurements of the proposed RF harvester agree well with simulations. Finally, the rectenna was connected to a DC-to-DC converter and the open voltage was increased from 298 mV to 1.4 V, charging a 1 mF capacitor at 37 min. The stored energy could supply a sensor in a wireless network, with consumption 20 mW, rate 1 kbps and word-length of 20 bits.