Kenneth F. Gudan

Feasibility of wireless sensors using ambient 2.4GHz RF energy

We present a new system for measuring ambient RF energy in the 2.4GHz ISM band. This apparatus is intended to establish the feasibility of harvesting ambient RF energy to power emerging ultra-low-power sensors and microcontrollers. We simultaneously acquire RF measurements from a spatial and polarization diversity antenna system, with both a spectrum analyzer (frequency-selective but slow), and a log amp (wideband but fast), explain key tradeoffs in the measurement configuration, and present a post-processing algorithm which provides a reliable characterization of the RF energy available in the 2.4GHz ISM band. Preliminary results suggest enough energy is available to support a low duty cycle wireless sensor node system. An average RF power of 11nW is observed 10m away from a typical Wi-Fi access point in an office environment, suggesting the possibility of low duty cycle, wirelessly powered sensing and communication using a Bluetooth Low Energy (BLE) or another ultra low power uplink.

Ultra-low power 2.4GHz RF energy harvesting and storage system with −25dBm sensitivity

An RF energy harvesting and storage system is described that trickle charges a battery from incident power levels as low as -25 dBm referred to the feedpoint of an 8 dBi patch antenna. The circuit is optimized for the indoor ambient power range typically observed in the 2.4 GHz ISM band so that we can harvest the energy provided by nearby Wi-Fi, Bluetooth and other devices. In this incident power regime, rectified voltages are low, so power management circuit operation in the 100mV regime is critical. We present several improvements to our prior work that significantly improve its performance, including a novel wideband multi-element antenna array, an improved boost converter, and a redesigned battery charger. At -25dBm RF input power, the new harvesting system sources 150μJ into a rechargeable battery after 1 hour. We believe that this work represents the lowest reported startup power yet achieved in battery-storage RF energy harvesting systems.

A 2.4GHz ambient RF energy harvesting system with −20dBm minimum input power and NiMH battery storage

We describe a radio frequency (RF) energy harvester and power management circuit that trickle charges a battery from incident power levels as low as -20dBm. We designed the harvester for the 2.4 GHz RF band to leverage the ubiquity of energy that is produced by Wi-Fi, Bluetooth, and other devices. This paper reports on the design and current status of the harvester and compares our performance to other published results. In this incident power regime, rectified voltages are low, so power management circuit operation in the 100mV regime is critical. This paper describes a novel rectenna design, boost converter, and battery charger for RF energy harvesting specifically tuned to this low-power regime. At -20dBm RF input power, the harvesting system (rectenna, boost converter, and battery charger) sources 5.8μJ into a rechargeable battery after 1 hour.

Ultra-low power autonomous 2.4GHz RF energy harvesting and storage system

We present an autonomous end-to-end 2.4GHz RF energy harvesting and storage system suitable for harvesting energy from WiFi and similar devices. The system is designed to collect ultra-low power ambient RF input energy and automatically store it into a battery. Previous work in this area required a pushbutton switch for full functionality - RF energy is first stored in a capacitor and manually transferred into a battery. In this work, in addition to the main harvester, an auxiliary control system is developed to replace the pushbutton and achieve fully autonomous energy harvesting and storage. Two design approaches are proposed for the control system. The tradeoffs of each technique will be discussed. The designed control systems were integrated into the main harvesting structure. Experimental results indicate that the system autonomously stores 241μJ into a NiMH battery after 30 minutes with an incident power (at the feedpoint of the antenna) of -21dBm.