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A novel wireless sensor network system with compatibility of microwave power transmission (MPT) using a Gallium Nitride (GaN) power amplifier has been fabricated and tested. The sensor node operates using electrical power supplied by the MPT system. Time- and frequency-division operations are proposed for the compatibility. Under the frequency-division operation, receiving signal strength indicator of −85 dBm and packet error rate of <1% were measured when the available DC power of a rectifier with 160 mW output power. Under 120-min measurement, sustainable power balance between radio-frequency–DC conversion and power consumption in stable operation of the sensor node was achieved.
We proposed and examined a microwave power transfer system for electric vehicles (EVs). In this system, electricity is transmitted from a transmitting antenna over an EV to a receiving antenna on the roof of the EV. We used a rectenna to convert the received microwave power to direct current power. The conversion efficiency of a rectenna array is affected by the input power level distribution, and we have to form a flat-topped beam pattern to increase the conversion efficiency. We conducted an experiment to form a flat-topped beam pattern by using a phased array antenna. In this experiment, the output power of each antenna element is uniform and cannot be controlled independently. Hence, we controlled only the output phases of each antenna element and formed a flat-topped beam pattern. The distance between the transmitting antenna and the receiving area is 6.45 m, and the receiving area corresponds to a space in which the azimuth and elevation are in the range of −5°–5°.
This paper proposes a combined harvesting system to improve the efficiency and flexibility of autonomous wireless network nodes, supplied by means of wireless power transfer technique. In particular, a mixed system for electromagnetic (EM) and thermal energy harvesting (EH), conceived for passive nodes of wireless sensor networks and radio frequency (RF) identification tags, is described. The proposed system aims at increasing the effectiveness and the efficiency of the EH system by integrating an antenna and a rectifier with a thermo-electric generator (TEG) able to perform thermal EH. The energy provided by the thermal harvester is exploited twice: to increase the rectifier efficiency by providing a voltage usable to improve the bias condition of the rectifying diode, and to provide additional dc energy, harvested for free. Ultimately, a great efficiency improvement, especially at low incident RF power, has been observed. The design methodology and the EM performance of a quarter-wavelength patch antenna, integrated with the TEG are resumed. Then, a test campaign to evaluate the thermal EH performance has been carried out. Afterward, a rectifier with variable bias voltage, operating at the same frequency of the antenna, has been opportunely designed to exploit the harvested thermal energy to bias the diode. A measurement campaign has been then carried out to test the efficiency increment obtained and to validate the proposed solution.
This paper presents the design procedure of two ultra-high-frequency radio frequency identification reader antennas used in searching tagged items. They consist of microstrip arrays with alternating orthogonal dipoles, which are fed in series by a pair of microstrip lines. The dipoles are designed properly to provide the required bandwidth. The inter-element distance is adjusted to the center frequency, where the elements provide in-phase excitation and create two orthogonal electric-field components that give beams with direction diversity. Simulated results show that the return loss bandwidth (RL > 13 dB) of the first antenna design covers the required frequency band of ETSI (865–868 MHz) standard. In addition, simulated and measured results of the second antenna design indicate that the return loss bandwidth covers both the frequency bands of european telecommunications standards institute (ETSI) and federal communications commission (FCC) (865–928 MHz) standards. Regarding the coverage volume in the vicinity of the antenna, it was deduced that both antennas can read tagged items in a semi-cylindrical volume that extends to a radius of more than 50 cm. Finally, a case study of reading tagged books in front of a library cabinet with six shelves has been presented.
Design steps are outlined for maximizing the RF-to-dc power conversion efficiency (PCE) of a rectenna. It turns out that at a frequency of 868 MHz, a high-ohmic loaded rectifier will lead to a highly sensitive and power conversion efficient rectenna. It is demonstrated that a rectenna thus designed, using a 50 Ω antenna and lumped element matching network gives a superior PCE compared with state of the art also for lower resistive loading. By omitting the matching network and directly, conjugate impedance matching the antenna to the rectifier, the PCE may be further increased and the rectenna size reduced as it is demonstrated with a rectenna prototype measuring only 0.028 squared wavelengths at 868 MHz and demonstrating a PCE of 55% for a −10 dBm RF input power level.
Magneto-inductive waves are a form of propagation which only exists in certain types of magnetic metamaterials formed from inductively coupled resonant circuits. We present an investigation of their potential as contactless power transfer devices capable of carrying power along a surface between suitably prepared terminals while simultaneously offering a broadband data channel. Input impedances and their matching conditions are explored with a view to offering a simple power system design. A device with 75% peak and 40% minimum efficiency is demonstrated and designs with potential for better than 70% mean and 90% peak are reported. The product of planar magnetic coupling and metamaterial cell Q factor is determined to be a key optimization parameter for high efficiency.