Power-optimized waveforms (POWs) are the enabling technology for realizing an internet-of-things (IoTs). An IoT will require billions or trillions of sensors, which must rely on passive, backscatter communication to facilitate the wireless transfer of information. Passive, backscatter sensors are uniquely suited for an IoT because of their ease of installation, low-cost, and lack of potentially toxic batteries. POW's primary benefit is that they can greatly improve the energy-harvesting efficiency of passive sensors, which increases their range and reliability. An overview of POWs is presented followed by measured results validated by a theoretical model and computer simulations. These measured results conducted at 5.8 GHz demonstrate the highest reported efficiency of a low-power, microwave energy-harvesting circuit of 26.3% at an input power of −10.2 dBm when using an excitation signal with a peak-to-average-power ratio of 12.

The paper presents a simplified analysis of harmonically terminated rectifier circuit and experimental results of a Schottky diode rectifier with even and odd harmonic terminations. The analysis is based on the Fourier series expansion of the voltage and current across the diode circuit. Harmonic terminations similar to the techniques used for power amplifiers are studied. A maximum efficiency of 84% at 30 dBm is obtained with second- and third-order harmonics terminated. The optimum value of dc load to maximize efficiency is obtained by sweeping the load. An optimal operating range of 28–35 dBm is obtained. The applications of the rectifier in wireless charging and power transfer systems are discussed.

Power transfer efficiency in loosely coupled inductive systems can be enhanced by resonance. Primary and secondary can be tuned to same resonant frequency. In this paper, MOSFET-based Varactors and switchable capacitors are used for re-tuning of such a system at 13.56 MHz. This is achieved either using each cap structure alone or as a hybrid model. These techniques are designed for 13.56 MHz wireless power transfer system.

Resonant Inductive Coupling Wireless Power Transfer (RIC-WPT)is a key technology to provide an efficient wireless power channel to consumer electronics, biomedical implants and wireless sensor networks. Due to its non radiative nature, RIC Wireless Power Transfer has been considered safe for humans when adhered to magnetic health radiation safety regulations (Christ et al., 2013), unveiling a large range of potential applications in which this technology could be used. However, current deployments are limited to point-to-point links and do not explore the capabilities of Multi-Node RIC-WPT Systems. In such a system, the multi-path relaying effect between different nodes could effectively improve the performance of the link in terms of power transferred to the load and power transfer efficiency. However, depending on the impedance and resonant frequency of the nodes that generate the multi-path effect, these nodes could also act as interfering objects, therefore (a) making the transmitter and/or receiver act as a pass-band filter and (b) losing part of the transmitter magnetic field through coupling to the interfering node. In this paper, a circuit-based analytical model that predicts the behavior of a Multi-Node Resonant Inductive Coupling link is proposed and used to perform a design-space exploration of the multi-path relaying effect in RIC Wireless Power Transfer Systems.

This work presents the design of a system for wireless power transmission based on a compact rectenna array able to supply low-power electronic devices such as wireless sensors. The receiving section is realized with an array of 12 rectangular patch antennas. Each elements of the array is connected with a suitable harmonic filter and a rectifying circuit by means of a coaxial feeding point. The transmitting section is realized with a one-dimensional prime focus parabolic reflector antenna, with a linear feeder composed by four dipole antennas. The rectenna array, the harmonic filter, the rectifying circuit of the receivers, and the transmitting section were optimized to reach the maximum operative range and efficiency, in term of power transfer. A system prototype has been designed, optimized, fabricated, and experimentally assessed. In particular, a prototype operating in the S band and able to provide a supply power of about 50 mW serves as proof-of-concept. Moreover, theoretical guidelines for the design of wireless power transmission are provided. The obtained experimental results are quite promising and demonstrated the capabilities of wireless power transmission systems as alternative power supply sources.

Battery-powered wireless sensor networks have been extensively deployed in condition monitoring and structural health monitoring systems, but the performance of wireless sensors are limited by battery capacity and difficulty of application in rotating machines. In this paper, a variety of commercial wireless charging solutions and coil-shaft configurations for magnetic coupling are compared, having in mind of the application of continuously charging wireless sensors on rotating machines. For the co-axial configuration of the transmitter coil and the receiver coil, a Qi standard compliant wireless charging kit and a custom charging circuit are successfully applied to charge wireless sensors on small rotating test rigs. In order to harvest and store intermittent energy input from the wireless power source, a prototype receiver circuit using a supercapacitor and low-dropout regulator is designed and validated. Based on the prototype circuit, the radial configuration of single transmitter coil and multiple receiver coils is demonstrated for wireless power transfer to the sensor nodes on the drivetrain of a small wind turbine test rig.

In this paper, a technology based on thin flexible polyimide substrate (Kapton) to develop antennas for millimeter wave wireless power transmission is presented. Firstly, we characterize the Kapton polyimide (relative permittivity and loss tangent) using a ring resonator method up to V band. A 60 GHz patch antenna is designed, fabricated, and measured to validate our technology. Crossed-dipoles array antennas at Ku band and K band for energy harvesting are also designed, fabricated, and measured. Then a prototype of crossed-slot dipole antenna at V band is proposed. Finally, a resistivity characterization of Au bump used in flip-chip packaging is done, which leads us one step further toward aheterogeneous integration on flexible substrate of different components for Wireless Sensor Network nodes.

The optimal design problem for a wireless power transfer link based on a resonant inductive coupling is addressed in this paper. It is assumed that the magnetic coupling coefficient and the inductor quality factors are known. By employing the conjugate image impedances, the values of the inductances realizing the optimal design with respect to given values of the network input and load impedances are derived. It is demonstrated that there is just one optimal design maximizing both the power delivered to the load and the power transfer efficiency of the link. The four possible schemes corresponding to the use of a parallel or a series arrangement for the two coupled resonators (Parallel-Parallel, Series-Series, Parallel-Series, and Series-Parallel) are considered and discussed. Closed form analytical formulas are derived and validated by circuital simulations.

In this work, a high-efficiency radio-frequency energy-harvesting system that takes use of the Portuguese Digital Television signal (750–758 MHz) to obtain DC power is proposed. To be useful, it is optimized to operate at low-power conditions. For the rectifier, three different solutions are presented: a single-series diode, a single-shunt diode, and a voltage-doubler configuration. The efficiency is similar for the three rectifiers – about 54% with a sine-wave excitation and −10.5 dBm of input power. Field measurements with the voltage-doubler have shown 63% efficiency for the same input power.

Here we demonstrate wireless energy transfer that exploits the conductivity and permittivity of soil to create a potential gradient on the surface around an earthed electrode, distributing electrical energy over the area. This generated surface potential can be amplified using a special standing-wave receiver for harnessing the distributed energy. We have experimentally mapped the surface potential around the electrode and plotted the received energy covering an area of 1200 m2. Key operating parameters are determined with a discussion on optimizing the system efficiency. This technique could address the challenge of distributing electrical energy to many low power devices over large outdoor areas without the use of wires.

In this paper, some theoretical aspects and experimental results are discussed with the aim to provide supplementary dc energy to radio frequency identification (RFID) tags by exploiting the nonlinear nature of rectifier devices. Three nonlinear phenomena are treated: (i) the impedance power dependence, (ii) the harmonic production, and (iii) the dependence on the radio frequency waveform. The novelty of the work relies on proposing a double rectifier composite system in where the nonlinearity of each rectifier is exploited to enhance the global powering performance of the system. Using the passive RFID technology as a beacon for the implementation, the approach considers combining the internal rectifier circuit of a commercial RFID chip operating at 868 MHz with an external rectifier circuit operating at 2.17 GHz. The solution triggers in a composite system RFID tag-harvester integrated in a single-feed dual-band antenna. The experimental validation shows 5 dB of tag sensitivity enhancement when it is empowered by the external harvester. The enhanced sensitivity produces an increase in the theoretical reading range distance from 3.3 to 6.1 m.

Capacitive power transfer (CPT) technology can achieve wireless power transfer based on electric field coupling. However, practical CPT systems often have cross-coupling between coupling plates of the capacitive-coupling interface, which makes accurate system analysis and compensation design tedious and complicated. In this paper, an accurate steady-state equivalent circuit model of the capacitive-coupling interface with cross-coupling is established. The model includes a parallel input capacitor linked with a series output capacitor by an ideal transformer whose turns ratio reflects the extent of cross-coupling between the plates. Effects of coupling variation on the model are analyzed in detail. The model is used for primary and secondary tuning design to achieve the maximum power transfer of a CPT system with cross-coupling. The effectiveness of the proposed model is demonstrated by both simulations and experimental results.

This paper focuses on non-radiative wireless power transfer implemented by means of a resonant magnetic coupling. The case of one transmitter and two receivers is considered and a rigorous analytical procedure is developed demonstrating that maximum power transfer or maximum efficiency can be achieved by appropriately selecting the load values. Both cases of coupled and uncoupled receivers are solved; closed formulas are derived for the optimal loads, which maximize either power or efficiency. It is shown that the resistances that realize maximum power transfer are always greater than the resistances that realize maximum efficiency. According to this observation, an optimal range of operation for the load resistances is also determined. Furthermore, it is demonstrated that in the case where the receivers are coupled the introduction of appropriate compensating reactances allows retrieving the same results corresponding to the uncoupled case both for powers and efficiency. Theoretical data are validated by comparisons with numerical results.