Comparison of Conventional 50-ohm Impedance Matching and Antenna Self-Matching for Energy Harvester : Master of Science Thesis

Abstract

In recent time, the demand of low power wirelessly operated application devices has shown a rapid growth. To energize these low power devices with conventional batteries is a significant problem because these storage devices have small lifetime. Conventional batteries do not provide continuous operation without deteriorating the performance of electronic devices because proper maintenance and replacement are required. Scavenging the energy from ambient sources to energize the low powered wireless operated devices is up-and-coming strategy. Different techniques can be adopted but the most suitable and trending technique, on which my thesis work is focused, is the radio frequency (RF) energy harvesting from ambient electromagnetic (EM) waves in European UHF RFID spectrum from 865.7 MHz to 867.7 MHz. RF energy harvesting technique is useful due to readily availability of radiation signals i.e. cellular networks, TV, radio, Wi-Fi and satellites, which are the sources of EM waves in environment. So, this technique can be considered as an auspicious solution for the replacement of conventional batteries to provide the continuous power to small wirelessly operated electronic devices over significant longer periods.

In this thesis work, the RF energy harvesting system is simulated and fabricated which is divided into two sections; RF rectifier circuit and patch antenna. Patch antenna captures the RF signals from environment and transfer it to the RF rectifier. RF rectifier circuit converts the RF power to DC and store the charge in supercapacitor. If the impedance of RF rectifier circuit and rectifying patch antenna is not complex conjugate of each other, then maximum power transfer (MPT) will not be achieved. Therefore, Impedance matching network is introduced between harvesting patch antenna and RF rectifier circuit for maximum power transfer. Our target frequency in this work is 866 MHz.

RF rectifier circuit is simulated on advance design system (ADS) and patch antenna is simulated in HFSS. RF rectifier circuit and patch antenna are fabricated on FR4 and RT/duroid 5880 substrate respectively. Performance of the whole RF energy harvesting system is showing that, at 28dBm input power level to the reader antenna with gain 12.5dBi, our system has bandwidth of 80 MHZ from 800 to 880 MHz. Output voltage of the system are above 3V, which are enough to wake up the RFID IC and for low power level applications. The maximum read range of this RF energy harvesting system is charging is 94cm. Charging speed of supercapacitor is higher around central frequency, while slower as the operational frequency moves away from central frequency. Bandwidth and charging speed can be increased by increasing the input power level.