Efficient and Sensitive Electrically Small Rectenna for Ultra-Low Power RF Energy Harvesting

A new electrically small antenna with size k alpha = 0.415 is presented, fabricated and measured in this work. This is intrinsically matched to 50 Omega, has omni-directional and linear-polarized radiation pattern in the horizontal plane with maximum directivity of 1.75 dBi and simulated radiation efficiency of 93%. The antenna in combination with a low-complex and co-planar rectifier with one single diode forms a high efficient and sensitive electrically small rectenna with k alpha = 0.53 at 868 MHz (UHF RFID-band in Europe). The latter has measured efficiency 22.5% for -19 dBm power input and sensitivity of -44 dBm (or equivalently 0.00028 mu W/cm(2) power density), while at 2.25 mu W/cm(2) is able to supply continuously, i.e., without a boost converter or use of any energy tank, a small electrical device with 118 mu W. In order to increase the dc output voltage and the delivered dc power to the load for lower power density levels, rectenna-array configuration is exploited. Application to batteryless, backscatter wireless sensor node powering is discussed. Specifically, for a power density of 0.1237 mu W/cm(2) the RF energy harvesting system delivers 172 mu W at 2.85 V every 22.5 s.