Sensitive ambient RF Energy harvesting

Rectification of microwave signals to generate DC (Direct Current) power has become the subject of research since the 1950’s. Radio Frequency (RF) energy harvesting has experienced a rapid development in recent years due to the increasing number of RF transmitter sources producing an abundant ambient microwave energy waste. Furthermore, the development of wireless power transmission (WPT) technologies has triggered impetus for RF energy harvesting. Hence, RF energy scavenging is a promising solution as it has the potential to provide a viable energy source to meet upcoming demands. Efficient ambient RF energy scavenging is a very challenging issue, as it deals with the low RF power levels available in the environment. The scavengeable power levels are generally unknown and can vary unpredictably; therefore sparking research interest to develop highly sensitive RF energy scavengers to capture ambient RF signals over a range of low input power levels. This research focuses on a real life RF energy scavenging approach to generate electrical power in urban environments. It aims to develop highly sensitive and efficient ambient RF energy scavenging system and method to harvest a broad range of very low level ambient RF power. The feasibility of RF energy harvesting through field measurements and maximum available power analysis in metropolitan areas of Melbourne, Australia is investigated. Scavengeable ambient frequency sources with their associated available RF power levels were identified. RF field investigations and analysis identified the scavengeable levels of ambient RF power are lower than previous published works. Available bands vary considerably from location to location which are highly incoherent and are effected by environmental/free-space conditions. Furthermore, it is demonstrated that commercial frequency bands such as FM (88-108 MHz) and TV (470-890 MHz) provide optimal sources for power scavenging due to their suitable level of the ambient power at a variety of locations. Furthermore, cellular and wireless communication systems (800-1000 MHz) are recommended as alternative power scavenging sources. In order to investigate the feasibility of harvesting ambient EM (electromagnetic) energy from multiple sources (broadcasting and cellular systems) simultaneously, a new highly sensitive multi-resonant rectifier is proposed operating over a broad input power range (−40 to −10 dBm). The measurement results demonstrate that a two tone input to the proposed dual-band RF energy harvesting system can generate 3.14 and 7.24 times more DC power than a single tone at 490 and 860 MHz respectively, resulting in a measured effective efficiency of 54.3% for a dual-tone input power of −10 dBm. Real environmental measurements indicate the rectifier generates 39.38 µW by harvesting RF energy from two bands simultaneously. In order to increase the sensitivity and hence the output DC power, harvesting energy over a wider frequency band is investigated. Therefore, the feasibility of harvesting ambient EM energy from FM broadcasting band is examined. A highly sensitive rectenna is proposed which exhibits favourable impedance matching at 89-11 MHz over a broad range of low input power 50 to 10 dBm (0.01 to 100 µW). The proposed FM rectenna with 22% fractional bandwidth delivers a measured power conversion efficiency of 41% with single tone of −10 dBm. An innovative idea that arose from these investigations was an evaluation of the performance of a rectenna system which was embedded into low profile building materials. This enables to harvest ambient RF energy in urban environments, providing a unique way of delivering power to many low energy home or office devices. Based on the real environmental measurements, the embedded rectenna in plaster generates 175 μW of DC power by harvesting EM energy over the FM frequency band. Finally, the effect of multi-tone excitation (with constant total input power) on output DC power of the rectifier is analysed to facilitate the comparison between single tone and multiple tones. Various factors such as; different frequency spacing, low input power levels and random phase (incoherency) arrangements were considered in frequency and time domain analysis and also in measurements. It is demonstrated that the application of multiple tones simultaneously within the matched frequency band and with constant total input power results in a lower total average output power when compared with single-tone case with the same input power