Hubregt J. Visser

Energy Harvesting for Autonomous Wireless Sensor Networks

Wireless sensor nodes (WSNs) are employed today in many different application areas, ranging from health and lifestyle to automotive, smart building, predictive maintenance (e.g., of machines and infrastructure), and active RFID tags. Currently these devices have limited lifetimes, however, since they require significant operating power. The typical power requirements of some current portable devices, including a body sensor network, are shown in Figure 1.

RF Energy Harvesting and Transport for Wireless Sensor Network Applications: Principles and Requirements

This paper presents an overview of principles and requirements for powering wireless sensors by radio-frequency (RF) energy harvesting or transport. The feasibility of harvesting is discussed, leading to the conclusion that RF energy transport is preferred for powering small sized sensors. These sensors are foreseen in future Smart Buildings. Transmitting in the ISM frequency bands, respecting the transmit power limits ensures that the International Commission on Non-Ionizing Radiation Protection (ICNIRP) exposure limits are not exceeded. With the transmit side limitations being explored, the propagation channel is next discussed, leading to the observation that a better than free-space attenuation may be achieved in indoors line-of-sight environments. Then, the components of the rectifying antenna (rectenna) are being discussed: rectifier, dc-dc boost converter, and antenna. The power efficiencies of all these rectenna subcomponents are being analyzed and finally some examples are shown. To make RF energy transport a feasible powering technology for low-power sensors, a number of precautions need to be taken. The propagation channel characteristics need to be taken into account by creating an appropriate transmit antenna radiation pattern. All subcomponents of the rectenna need to be impedance matched, and the power transfer efficiencies of the rectifier and the boost converter need to be optimized.

Ambient RF Energy Scavenging: GSM and WLAN Power Density Measurements

To assess the feasibility of ambient RF energy scavenging, a survey of expected power density levels distant from GSM-900 and GSM-1800 base stations has been conducted and power density measurements have been performed in a WLAN environment. It appears that for distances ranging from 25 m to 100 m from a GSM base station, power density levels ranging from 0.1 mW/m2 to 3.0 mW/m2 may be expected. First measurements in a WLAN environment indicate even lower power density values, making GSM and WLAN unlikely to produce enough ambient RF energy for wirelessly powering miniature sensors. A single GSM telephone however has proven to deliver enough energy for wirelessly powering small applications on moderate distances.

Co-Design of a CMOS Rectifier and Small Loop Antenna for Highly Sensitive RF Energy Harvesters

In this paper, a design method for the co-design and integration of a CMOS rectifier and small loop antenna is described. In order to improve the sensitivity, the antenna-rectifier interface is analyzed as it plays a crucial role in the co-design optimization. Subsequently, a 5-stage cross-connected differential rectifier with a 7-bit binary-weighted capacitor bank is designed and fabricated in standard 90 nm CMOS technology. The rectifier is brought at resonance with a high-Q loop antenna by means of a control loop that compensates for any variation at the antenna-rectifier interface and passively boosts the antenna voltage to enhance the sensitivity. A complementary MOS diode is proposed to improve the harvesters ability to store and hold energy over a long period of time during which there is insufficient power for rectification. The chip is ESD protected and integrated on a compact loop antenna. Measurements in an anechoic chamber at 868 MHz demonstrate a -27 dBm sensitivity for 1 V output across a capacitive load and 27 meter range for a 1.78 W RF source in an office corridor. The end-to-end power conversion efficiency equals 40% at -17 dBm.

Approximate Antenna Analysis for CAD

Contents. Preface. References. Acknowledgments. Acronyms. 1 Introduction. 1.1 The History of Antennas and Antenna Analysis. 1.2 Antenna Synthesis. 1.3 Approximate Antenna Modeling. 1.4 Organisation of the Book. 1.5 Summary. References. 2 Intravascular MR Antennas - Loops and Solenoids. 2.1 Introduction. 2.2 MRI. 2.3 Intravascular MR Antennas. 2.4 MR Antenna Model. 2.5 Antenna Evaluation. 2.6 In Vitro Testing. 2.7 Antenna Synthesis. 2.8 Safety Aspects. 2.9 Conclusions. References. Appendix A. Biot and Savart Law for Quasi-Static Situation. 3 PCB Antennas - Printed Monopoles. 3.1 Introduction. 3.2 Printed UWB Antenna. 3.3 Printed Strip Monopole Antenna. 3.4 Conclusions. References. 4 RFID Antennas - Folded Dipoles. 4.1 Introduction. 4.2 Wire Folded Dipole Antenna. 4.3 Impedance Control. 4.4 Asymmetric Coplanar Strip Folded Dipole Antenna on a Dielectric Slab. 4.5 Folded Dipole Array Antenna. 4.6 Conclusions. 5 Rectennas - Microstrip Patch Antennas. 5.1 Introduction. 5.2 Rectenna Design Improvements. 5.3 Analytical Models. 5.4 Model Verication. 5.5 Wireless Battery. 5.6 Power and Data Transfer. 5.7 RF Energy Scavenging. 5.8 Conclusions. 6 Large Array Antennas - Open-Ended Rectangular Waveguide Radiators. 6.1 Introduction. 6.2 Waveguide Fields. 6.3 Unit Cell Fields. 6.4 Rectangular Waveguide Cross-Sectional Step. 6.5 Rectangular Waveguide Unit Cell Junction. 6.6 Dielectric Step in the Unit Cell. 6.7 Finite Length Transmission Line. 6.8 Overall GSM of a Cascaded Rectangular Waveguide Structure. 6.9 Validation. 6.10 Conclusions. References. Appendix A. Waveguide Mode Orthogonality and Normalisation Functions. Appendix B. Mode Coupling Integrals for Waveguide to Waveguide Junction. Appendix C. Unit Cell Mode Orthogonality and Normalisation Functions. Appendix D. Mode Coupling Integrals for Rectangular Waveguide to Unit Cell Junction. 7 Summary and Conclusions. 7.1 Full-Wave and Approximate Antenna Analysis. 7.2 Intravascular MR Antennas - Loops and Solenoids. 7.3 PCB Antennas - Printed Monopoles. 7.4 RFID Antennas - Folded Dipoles. 7.5 Rectennas - Microstrip Patch Antennas. 7.6 Large Array Antennas - Open-Ended Rectangular. Waveguide Radiators. References. Index.

Efficient, Compact, Wireless Battery Design

Wireless batteries or rectennas- rectifying antennas - are conceived for converting wireless RF power into DC power. Although power conversion efficiencies exceeding 80% have been reported for high (20 dBm) rectenna input power levels, wireless batteries will be most beneficial at large distances from sources that will radiate at power levels limited by national and international regulations. Therefore, the challenge is in maximizing the power conversion efficiency of wireless batteries for low input power levels, say 0 dBm and below. By directly conjugate matching a rectifying circuit to a microstrip patch antenna, the need for a matching network between the two no longer exists. Thus the efficiency of the wireless battery will improve. Moreover, this matching technique automatically suppresses the reradiation of harmonics by the microstrip patch antenna since the harmonics will be mismatched. Thus, the impedance matching and filtering network encountered in traditional wireless battery designs has become obsolete. With the aid of analytical models developed for antenna and rectifier, single-layer, internally matched and filtered PCB rectennas have been designed for low input power levels. An efficiency of 52% for OdBm input power has been realized at 2.45 GHz for a wireless battery realized on FR4, showing an improvement - next to the size and complexity reduction - of more than 10% over a traditional rectenna design. A series connection of these wireless batteries is shown to be able to power a standard household wall clock.

Ambient RF energy harvesting from DTV stations

In the framework of wireless power transmission and RF energy harvesting, the main objective is to design a harvester that collects ambient Radio Frequencies (RF) broadcasted from DTV (Digital TV) stations. This paper summarizes the main challenges experienced, when designing such a harvester. The distance and the free space path loss between the transmitting station and the harvesting location are calculated. Using Friis equation, the available power at the harvesting location is predicted. A novel broad-band Yagi-Uda antenna that covers the DTV broadcasting frequencies (470 MHz-810 MHz) is presented. The antenna design is based on integrating a wide-band strip dipole into a Yagi-Uda antenna. Moreover, The rectifier part, which converts the harvested RF power into DC is discussed, the simulated and measured input impedance and the output voltage of different commercial rectifiers are shown. A voltage multiplier is used to maximize the output voltage, and a matching network is presented to match the impedance of the multiplier to that of the antenna.

Human++: wireless autonomous sensor technology for body area networks

Recent advances in ultra-low-power circuits and energy harvesters are making self-powered body wireless autonomous transducer solutions (WATS) a reality. Power optimization at the system and application level is crucial in achieving ultra-low-power consumption for the entire system. This paper deals with innovative WATS modeling techniques, and illustrates their impact on the case of autonomous wireless ElectroCardioGram monitoring. The results show the effectiveness of our power optimization approach for improving the WATS autonomy.

Analytical Equations for the Analysis of Folded Dipole Array Antennas

An accurate analytical model has been derived for a linear array of wire folded dipole antennas. The model combines closed form analytical equations for the folded dipole antenna, the re-entrant folded dipole antenna, the two-wire transmission line, the mutual coupling between two folded dipole antennas and the mutual coupling between two thin dipole antennas.

Time efficient method for automated antenna design for wireless energy harvesting

The rectifier circuit in a rectenna (rectifying antenna) is analyzed employing a fast, efficient time-marching algorithm. The thus found complex input impedance dictates the antenna design. To maximize RF-to-DC conversion efficiency we do not want to employ an impedance matching and filtering network. Instead, we require the input impedance of the antenna to be equal to the complex conjugate value of the input impedance of the rectifier circuit. One antenna type feasible of supplying the required complex input impedance is the wire folded dipole array antenna. For this antenna type, an efficient, analytic model has been developed. The fast calculation times when implemented in software allow for an automated antenna design employing a Genetic Algorithm optimization. Thus, antenna designs can be generated within minutes employing standard office computing equipment. The design of a complete rectenna can be accomplished within hours. The direct complex conjugate matching ensures that the design is power-efficient and physically compact.