| Wireless power transfer(WPT)is not only a scientific fantasy,but also would bring about tremendous revolutions to the world that is experiencing an energy crisis and desperately needing new energy supply solutions.From super engineerings such as Space Solar Power Stations,to the tiny hundreds of millions of wireless sensors that make up the Internet of Things,wireless power transfer is urgently needed as a disruptive innovation to break the existing technological paradigms.At present,WPT technology based on magnetic coupling resonance has taken a place in the markets of consumer electronics and automotive wireless charging.However,the limited operating distance makes this technology difficult to meet the actual demands of medium-long distance application scenarios(>1 m).Radiative WPT,that is,microwave power transmission(MPT),shows the perspects to work at long distances,but it also faces plenty of challenges in its practical process.The contradiction between the antennas’ aperture and the transfer efficiency bears the brunt.How to improve the transfer efficiency for long-distance with a limited-size aperture is the key issue.Then,antennas based on conventional metals materials are generally opaque.Thus,achieving optical transparency of antennas will be a high-profile conformal design solution for space-tight applications such as satellites and indoors.Furthermore,for micro-power applications,how to achieve wideband/multi-frequency,wide-angle high-efficiency ambient wireless energy harvesting(WEH)and directional WPT under the premise of antennas’ miniaturization;while for high-power applications,how to address the issue of single-channel rectifying power capacity.The in-depth exploration around the above key issues will be carried out as the main contents of the dissertation as follows:A general procedure to design near-field focusing(NFF)reflective metasurface with multifeed,multi-focus,unequal power allocated characteristics is outlined for efficient wireless power tranfer.First,the electromagnetic characteristics of the NFF technology are analyzed based on the scalar diffraction field theory,and the calculation method of the multi-feed and multi-focus phase compensation through reflective metasurfaces is deduced.A‘cross-dipole’structure with dual-polarization independent regulation characteristics is designed as the metasurface element.On this basis,an X-band single-feed dual-focus reflective metasurface is designed,and its multi-focus unequal power allocated characteristics as well as the focusing performance are analyzed and verified.A dual-band reflective metasurface working at 5.8 GHz and 10 GHz is also designed to simultaneously generate a quasi-diffraction zeroorder Bessel beam and a NFF beam,for the aim of comparing the performance of the two near-field beams in WPT applications.Finally,a 5.8 GHz,10 m distance WPT and WEH system is established,including a set of 1.2 m×1.2 m reflective metasurface transmitter and a 50 mm×50 mm WEH terminal,whcih includes a metasurface radio frequency(RF)harvester,a rectifying circuit and an energy management circuit.Through the system test,a LED lamp at a distance of ten meters is successfully lit.The measured received RF power is highly consistent with the full-wave simulations,reaching 89%,which is expected to provide solutions for many special wireless charging application scenarios.A design method for antenna miniaturization with the limited-size ground structure is proposed.By introducing the ’ground resonance mode’ of the designed ground structure close to the half-wavelength size,the multi-mode resonance characteristics are realized together with the original resonance mode.The capacitive loading of the parasitic patches and the inductive loading of shorting vias are adopted to reduce the Q value of the antenna,forming the wideband omnidirectional resonance from 2 GHz to 4 GHz,and excite a 5.8 GHz high-directivity resonance meanwhile.On this basis,a ’back-to-back’ microstrip antenna is proposed to improve the omnidirectional characteristics of wideband resonance,to cover the mainstream communication bands and ISM bands for ambient WEH.At the same time,the designed antenna would achieve the two-directional radiation characteristics at 5.8 GHz to realize dedicated WPT.Through antenna measurements and WEH test,the performance and the potential of this work in the energy autonomy scheme of wireless sensor network,namely ’passive collection+active charging’,is proved.The design methodology of transparent metantenna and transparent reflective metasurface based on transparent metal material ITO is emphatically explored.First,the light transmittance and conductivity of chosen ITO material(1 Ω/sq)is analyzed and measured.A transparent metantenna simultaneously realizing visible light transmission and radio frequency energy harvesting in communication bands is designed.The chosen ITO material is adopted to etch the slotted ground structure and the metasurface topology structure,achieving a light transmittance of more than 50%,an impedance bandwidth of 43.6%,and a maximum gain of 1.3 dB.Combined with the designed wideband rectifying circuit,the peak RF-DC efficiency of 65%is obtained at the input power of 0 dBm.Furthermore,in order to address the issue of low efficiency of feeding structure of transparent antennas and the difficulty in arraying,a transparent NFF reflective metasurface is innovatively proposed.On the basis of ensuring high visible light transmittance,high-efficiency WPT with NFF characteristics at 5.8GHz is realized.The principal contradiction of adopting lossy metal materials to design reflective metasurfaces is resolved,that is,balancing the phase shift characteristics(required)and efficiency loss(should be avoided)caused by resonance effects.Through the light transmittance test,planar near-field scanning test,wireless power transfer and energy harvesting test,the practicability and effectiveness of the design is verified.The rectification technology based on novel GaN Schottky barrier diode is explored,as well as its application in the kilowatt-level high-power MPT system.The F-type circuit configuration is introduced to design a 5.8 GHz rectifying circuit with simple structure,high efficiency and wide input power range characteristics.The measured rectification efficiency can achieve nearly 70%with the 2 W input power,reaching the highest level of public reporting.The 16-in-1 microstrip patch antenna array is used as an independent RF receiving unit,and an 8×8 rectenna array is designed.The input power range for a single rectifying antenna to work normally is 0.5 W to 6 W,and the measured rectification efficiency in the range of 1 W to 4 W is higher than 50%.Through introducing the partitioned molecular array method,the direct current synthesis network is designed,with the multi-level cascade mode of "parallel-series-series" to optimize the efficiency.Finally,a 5.8 GHz MPT system with a distance of 20 meters and a kilowatt level is built.The measured received RF power totals 172 W,the rectification efficiency of each channel reaches 50%～60%,and the total output power after DC synthesis is 63.2 W,which verifies the practicability and the advancement of the designed GaN-based rectifying antenna array for high-power and long-distance MPT systems. |
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