Design of Practical Rectennas for RF Energy Harvesting

This paper presents the design of a rectenna deployable for Ultra High Frequency (UHF) band applications such as energy scavenging from an indoor Digital Satellite Decoder (DSD), for use as energy source for low-power devices and sensor networks. A monopole-helical antenna, a type of wire antenna, radiating in axial mode is used in the rectenna circuit for indoor capturing of the RF energy. The presentation addresses issues concerning choice of rectifier diodes and filtering capacitors in the rectenna unit. In particular, use is made of Schottky Barrier Diode (SBD) 1N5824 model in the rectenna because of its metallic semiconductor junction suitable for fast-switching applications of interest to energy harvesters. Not only does it have less reverse recovery time (approximately 100 ps) as compared with regular PN junction (order of 100 μs-100 ns); its energy dissipation is low (about 2.1 times less than the regular PN junction). So, SBD is useful in low power and high frequency applications. Second, capacitors used (1 μF and 100 μF) act as filters which eminently resist voltage changes, remove ripples thereby making the rectenna to adequately deliver steady DC output from the SBD diodes. This physical evaluation of SBD in the forward switching mode as well as its low-power dissipation over those of its counterpart, the PN-junction, seem not to have been critically examined when used in some of the works available on RF energy harvesting. Preliminary results of experimentally captured indoor RF energy with the designed rectenna and the corresponding output voltage at some distances from the RF sources are highlighted. The measured output voltage was 18.72 V at a distance of 2 cm from the RF source, with the corresponding Electric Field Strength of 384 μV/ m. This voltage was sufficient enough to power two numbers white, red and green Light Emitting diodes (LEDs), connected in parallel at the output of the designed rectenna circuit with the output voltage dropped and stabilized to 2.5 V, 1.9 V and 2.1 V, respectively for about 30 minutes of test. It was also observed that up to about 4 m distance away from the RF source, the LEDs remained ON with insignificant decrease in the output voltage.