Microwave Wireless Power Transfer System Based on a Frequency Reconﬁgurable Microstrip Patch Antenna Array

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Article Microwave Wireless Power Transfer System Based on a Frequency Reconﬁgurable Microstrip Patch Antenna Array

Haiyue Wang , Lianwen Deng , Heng Luo * , Junsa Du, Daohan Zhou and Shengxiang Huang

School of Physcis and Electronics, Central South University, Changsha 410083, China; [email protected] (H.W.); [email protected] (L.D.); [email protected] (J.D.); [email protected] (D.Z.); [email protected] (S.H.) * Correspondence: [email protected]; Tel.: +86-139-7585-1402

Abstract: The microwave wireless power transfer (MWPT) technology has found a variety of applications in consumer electronics, medical implants and sensor networks. Here, instead of a magnetic resonant coupling wireless power transfer (MRCWPT) system, a novel MWPT system based on a frequency reconfigurable (covering the S-band and C-band) microstrip patch antenna array is proposed for the first time. By switching the bias voltage-dependent capacitance value of the varactor diode between the larger main microstrip patch and the smaller side microstrip patch, the working frequency band of the MWPT system can be switched between the S-band and the C-band. Specifi- cally, the operated frequencies of the antenna array vary continuously within a wide range from 3.41 to 3.96 GHz and 5.7 to 6.3 GHz. For the adjustable range of frequencies, the return loss of the antenna array is less than −15 dB at the resonant frequency. The gain of the frequency reconfigurable antenna array is above 6 dBi at different working frequencies. Simulation results verified by experimental results have shown that power transfer efficiency (PTE) of the MWPT system stays above 20% at different frequencies. Also, when the antenna array works at the resonant frequency of 3.64 GHz, the PTE of the MWPT system is 25%, 20.5%, and 10.3% at the distances of 20 mm, 40 mm, and 80 mm, respectively. The MWPT system can be used to power the receiver at different frequencies, which has great application prospects and market demand opportunities. Citation: Wang, H.; Deng, L.; Luo, H.; Du, J.; Zhou, D.; Huang, S. Keywords: microwave wireless power transfer; antenna array; power transfer efficiency; fre- Microwave Wireless Power Transfer quency reconfigurable System Based on a Frequency Reconfigurable Microstrip Patch Antenna Array. Energies 2021, 14, 415. https://doi.org/10.3390/en14020415 1. Introduction

Received: 26 November 2020 The power supply technology used for various electronics is important, and the con- Accepted: 8 January 2021 ventional power supply technology is provided by using cables, which can be hazardous, Published: 13 January 2021 inconvenient, or impossible [1,2]. The transformation of charging technology from using traditional cables to wireless power transfer (WPT) is suitable for the booming development Publisher’s Note: MDPI stays neu- of portable devices, consumer electronics, robots, and other fields [3–5]. The WPT technol- tral with regard to jurisdictional clai- ogy has attracted great attention due to its promising application potential in the field of ms in published maps and institutio- charging [6,7]. One of the interesting and highly requested directions for wireless energy nal affiliations. transfer is sensor applications [8,9]. There are many types of WPT technologies including magnetic resonant coupling, microwaves, inductive coupling, radio waves, lasers, and so forth [10,11]. Out of these, the microwave wireless power transfer (MWPT) technology can transfer energy over relatively long distances with miniaturized antennas, which has great Copyright: © 2021 by the authors. Li- censee MDPI, Basel, Switzerland. market demand [12]. This article is an open access article At present, the MWPT technology has been applied in various fields, such as electric distributed under the terms and con- vehicles and mobile phones powered by electromagnetic waves and the MWPT systems ditions of the Creative Commons At- inside buildings [13,14]. The microstrip patch antenna array has advantages of a simple tribution (CC BY) license (https:// process, small volume, light weight, low profile, and easy integration, which is why creativecommons.org/licenses/by/ it is widely used in the field of WPT. A series of theoretical and experimental results 4.0/). about MWPT systems with the microstrip patch antenna arrays were conducted [15]. Also,

Energies 2021, 14, 415. https://doi.org/10.3390/en14020415 https://www.mdpi.com/journal/energies Energies 2021, 14, 415 2 of 12

many various methods have been employed to increase the power transfer efficiency (PTE), performances, and robustness of the MWPT systems by designing transmitting arrays [16–19]. A MWPT system operating at the frequency of 5.8 GHz by using an 8 × 8 transmitting antenna array and a 4 × 4 receiving antenna array was presented, and the PTE of the system was 33.4% at the distance of 40 cm [20]. Additionally, in order to improve the efficiency of the system, an optimized MWPT system using an 8 × 8 transmitting patch antenna array and a 8 × 8 receiving patch antenna array at the frequency of 5.8 GHz and the distance of 100 cm. The measured results show that the PTE of this MWPT system is 46.9% [21]. However, for the above mentioned WPT systems, receivers only catch the energy at a single frequency, regardless of their demands. This may cause malfunctions and failures, and the PTE of the system may therefore be reduced. In order to resolve this issue, many methods have been adopted in the magnetic resonant coupling wireless power transfer (MRCWPT) system [22–25]. Frequency reconfigurable MRCWPT systems were developed in previous works [22,23] by adding adaptively switching resonant capacitances. In the reference [24], a new approach based on reconfigurable magnetic resonant structures is proposed to achieve high energy efficiency and low volt-amp ratings. This basic principle is to create more than one efficiency-load curve. By combining a variable coupling technique, a frequency reconfigurable MRCWPT system has been proposed by varying the distance between the transmitter and the receiver [25]. In addition, a shape-reconfigurable modularized MRCWPT system in [26] achieves shape reconfigurability by using different structures and positional combinations of resonant modules. The reconfigurability in the reconfigurable MRCWPT system in [27] is achieved by adaptively switching between different sizes of drive loops and load loops. Here, instead of a MRCWPT system, a novel MWPT system based on a frequency reconfigurable (covering the S-band and C-band) microstrip patch antenna array is proposed for the first time to power the receiver at the different frequencies. This paper is organized as follows. The frequency reconfigurable microstrip patch antenna array is described in Section2. In Section3, the simulation and the measurement of the proposed frequency reconfigurable MWPT system are presented. The experimental results are compared with the simulation results. Finally, Section4 draws conclusions.

2. Frequency Reconfigurable Microstrip Patch Antenna Array 2.1. The Configuration and Design of the Patch Antenna Array A conventional antenna array only works at one frequency. This is because when the conventional antenna is used as the transmitter for wireless power transfer (WPT), the conventional antenna used as the receiver only receives the energy from the transmitter at the same working frequency. Also, when the working frequency of the receiver changes, the power transfer efficiency of the system will reduce. In addition to the conventional antenna only being able to power the receiver at one working frequency, it cannot power the receiver for different frequencies, regardless of energy demands. This may cause malfunctions and failures, and the PTE of the system may decrease as a result. The frequency reconfigurable antenna continuously adjusts the working frequency within a certain frequency range. The WPT system based on a frequency reconfigurable antenna can power the receivers at different frequencies. It was shown that the PTE of the WPT system remains stable at different frequencies. Also, the PTE of the system does not clearly decrease as the resonant frequency of the receiver changes. Therefore, the frequency reconfigurable array more important than the conventional antenna array for wireless power transfer. The operating mechanism of the array is that the array radiates the electromagnetic wave if four patches are excited. Under the excitation of the main mode, the direction of the electric field changes along the length of the half-wavelength patch. The antenna array radiates energy mainly through the gap between the open edge of the patch and the floor, and the electromagnetic wave radiated over the narrow seam can be equivalent to the magnetic flux. According to the array antenna theory, the radiation field at the center of two arrays is the largest, as the direction of patch normal, while the radiation field Energies 2021, 14, 415 3 of 12

deviating from the corresponding direction with the normal direction becomes smaller. The electric field of the two radiated edges can be decomposed into the vertical and horizontal components relative to the ground plane. The length of the radiation patch is set as a half wavelength. According to the transmission line theory of half wavelengths, the vertical components of the two radiation edge crevice decomposition cancel each other out in the far field and generate no radiation, while the horizontal component has the same direction and produces the strongest radiation, meaning that the radiation field is the strongest in the direction perpendicular to the antenna surface. The frequency reconfigurable antenna array is fed by a coaxial feed. The feed network of the antenna array uses the power divider. The power divider is designed to provide the same excitation power. For a frequency tuning operation, varactor diodes are used. The varactor diode is placed between the larger main patch and the smaller side patch, which has the variable bias voltage provided by the adjustable DC power source. By changing the capacitance of those varactors, the resonance frequency of the patch antenna array can be tuned. The antenna dimensions can be derived from the working frequency of the microstrip patch antenna. The variable symbol W represents the width of the patch, which can be derived as follows [28]: − 1 c ε + 1 2 W = ( r ) (1) 2 f 2

where the variable symbol c is the speed of light, and the variable symbol εr is the relative dielectric constant of the dielectric material. The dielectric material is epoxy resin, while the variable symbol f is the working frequency of the patch antenna. The thickness of dielectric material is h. The metal layer is copper and the effective dielectric constant εe can be derived as follows [29]:

− 1 ε + 1 ε − 1 h 2 ε = r + r (1 + 12 ) . (2) e 2 2 W The length of the radiation gap is ∆L and the length of the patch is L [30].

(ε + 0.3)(W/h + 0.264) ∆L = 0.412h e (3) (εe − 0.258)(W/h + 0.8) c L = √ − 2∆L (4) 2 f εe The simulation model of the frequency reconfigurable microstrip patch antenna array is shown in Figure1. The patch antenna array mainly consists of 4 main patches, 16 side patches and the feed network. By placing the varactor diode (SMV1281) inside the larger main patch and the smaller side patch, the antenna can perform as a frequency reconfigurable antenna array. The SMV1281 varactor diode has an adequate range of capacitance values from 0.69 pF to 13.3 pF, and it has low series resistance. Adding 4 varactors on the side of the patch is used to changing the working frequency of the frequency reconfigurable antenna array. The feed network is used to feed the antenna array and the lumped port is used in the simulation software HFSS (High Frequency Structure Simulator) to excite the patch antenna array. Energies 2021, 14, x FOR PEER REVIEW 4 of 12

Energies 2021, 14, 415 4 of 12 Energies 2021, 14, x FOR PEER REVIEW 4 of 12

Unit: mm l0=5 Unitw: m0=m2 l =5 M=1 0 w =2 0 L=16.75 W=19 M=1 L=16.75 W=19

Feed network Variode

FR4 Main Patch Variode FR4 Side Patch Lumped Port Main Patch Variode Lumped Port Figure 1.S Theide P ageneraltch geometry of the patch antenna array (four smaller patches are all connected

to the larger main patch). Figure 1. The general geometrygeometry ofof thethe patchpatch antennaantenna arrayarray (four(four smaller smaller patches patches are are all all connected connected to theto the largerThe larger mainfrequency main patch). patch). reconfigurable antenna array has two working states. There are four smaller patches around the larger main patch, as shown in Figure 1. When four smaller The frequency reconfigurable antenna array has two working states. There are four patchesThe are frequency all connected reconfigurable to the larger antenna main arraypatch has by usingtwo wor fourking varactors, states. There the frequency are four smaller patches around the larger main patch, as shown in Figure1. When four smaller smallerreconfigurable patches patch around antenna the larger array main works patch at frequenc, as shownies fromin Figure 3.41 GHz1. When to 3.96 four GHz. smaller The patches are all connected to the larger main patch by using four varactors, the frequency patchesdetails of are simulation all connected in this to the case larger are shownmain patch in Figure by using 2a. Thefour varactorvaractors, diode the frequency model in reconfigurable patch antenna array works at frequencies from 3.41 GHz to 3.96 GHz. The reconfigurableHFSS is replaced patch by the antenna lumped array RLC works boundary. at frequenc The returnies from losses 3.41 at GHz different to 3.96 frequencies GHz. The details of simulation in this case are shown in Figure2a. The varactor diode model in detailsfor the of general simulation reconfigurable in this case antenna are shown array in a reFigure illustrated 2a. The in varactor Figure 2b. diode The modelworking in HFSS is replaced by the lumped RLC boundary. The return losses at different frequencies HFSSfrequency is replaced belongs by to the S- band.lumped In RLCthis case, boundary. the return The returnlosses arelosses less at than different −15 dB frequencies when the for the general reconfigurable antenna array are illustrated in Figure2b. The working forcapacitance the general values reconfigurable are changed. antenna Then, the array working are illustrated frequency in of Figurethe patch 2b. antenna The working array frequency belongs to S-band. In this case, the return losses are less than −15 dB when frequencyare adjusted. belongs When to just S-band. one smaller In this case,patch the is connectedreturn losses to theare largerless than main −15 patch, dB when the fre-the the capacitance values are changed. Then, the working frequency of the patch antenna capacitancequency reconfigurable values are changed.patch antenna Then ,array the working can work frequency at frequenc of theies patchfrom 5.7antenna GHz toarray 6.3 arrayare adjusted. are adjusted. When When just one just smaller one smaller patch patch is connected is connected to the to larger the larger main main patch, patch, the fre- the frequencyGHz. The reconfigurabledetails of simulation patch antennain this case array are can shown work in at Figure frequencies 3a. The from varactor 5.7 GHz diode to quencymodel in reconfigurable HFSS is also replacedpatch antenna by the array lumped can RLCwork boundary. at frequenc Theies fromreturn 5.7 losses GHz atto dif-6.3 6.3GHz. GHz. The The details details of ofsimulation simulation in inthis this case case are are shown shown in in Figure Figure 33a.a. TheThe varactorvaractor diodediode modelferent frequencies in HFSS is also in this replaced case are by theillustrated lumped in RLC Figure boundary. 3b. The The working return frequency losses at different belongs modelto C-band in HFSS, and isthe also return replaced losses b arey the also lumped less than RLC − 15boundary. dB at different The return working losses frequen- at dif- frequenciesferent frequencies in this in case this are case illustrated are illustrated in Figure in Figure3b. The 3b. workingThe working frequency frequency belongs belongs to cies. − C-band,to C-band and, and the the return return losses losses are alsoare also less less than than15 − dB15 atdB different at different working working frequencies. frequencies.

Side Patch 0 Main Patch Side Patch -5 Main Patch 0 -10 c=0.69 pf -5 c=0.89 pf

c=2 pf (dB) -15 c=0.69c=4.8 pf pf 11 -10

S c=0.89c=8.6 pf pf

c=2c=13.3 pf pf (dB) -20

-15 c=4.8 pf 11

S c=8.6 pf -20-25 c=13.3 pf 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 Varactor Diode Lumped RLC -25 Model Boundary Frequency (GHz) Varactor Diode Lumped RLC 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 (a) (b) Model Boundary Frequency (GHz) FigureFigure 2. (a).) DetailsDetails of(a the) simulation model model (four (four smaller smaller patches patches are are all all( bconnected) connected to to the the larger larger

mainmain patch).patch). (b)). The The returnreturn losses losses at at different different frequencies frequencies for for the the reconﬁgurable reconfigurable antenna antenna array. array. Figure 2. (a). Details of the simulation model (four smaller patches are all connected to the larger main Thepatch) patch. (b). The antenna return array losses is at designeddifferent frequencies on a double-sided for the reconfigurable substrate using antenna the array planar. technology, while the feed network is designed on a single-sided substrate. In assembling the microstrip patch antenna array, the non-copper side of the feed network is placed in

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Energies 2021, 14, 415 5 of 12 Side Patch 0

Main Patch -5

direct contact with the ground-10 of the patch antenna array, as shown in Figure4a. This leads

to a low proﬁle and compact-15 structure. The dimensions of the frequency C=1 pf reconﬁgurable patch antenna array in Figures1 and4a are listed in Table1. The fabricated C=1.5 pf patch antenna C=2 pf -20 array is shown in Figure(dB) 4b,c for measurement. The SMA (SubMiniature C=3 pf Version A)

11 C=4 pf connector is fed to the feed-lineS through the substrate material and the ground plane. The Energies 2021, 14, x FOR PEER REVIEW -25 5 of 12 reconﬁgurable antenna array and the SMA connector are designed on the FR4 substrates, which have a dielectric constant-30 of 4.4 and a loss tangent of 0.02. -35 Varactor Diode Lumped RLC 5.50 5.75 6.00 6.25 6.50 Side Patch 0 Model Boundary Frequency (GHz) Main Patch (a) -5 (b)

-10 Figure 3. (a). Details of the simulation model (one smaller patch is connected to the larger main patch). (b). The return losses at different frequencies. -15 C=1 pf C=1.5 pf

C=2 pf -20 (dB) C=3 pf

The patch antenna array is 11 designed on a double-sided substrate C=4 using pf the planar technology, while the feed networkS -25 is designed on a single-sided substrate. In assembling the microstrip patch antenna array, the non-copper side of the feed network is placed in -30 direct contact with the ground of the patch antenna array, as shown in Figure 4a. This leads to a low profile and compact-35 structure. The dimensions of the frequency reconfig- Varactor Diode Lumped RLC 5.50 5.75 6.00 6.25 6.50 urable patch antenna array in Figures 1 and 4a are listed in Table 1. The fabricated patch Model Boundary Frequency (GHz) antenna array is shown in Figure 4b,c for measurement. The SMA (SubMiniature Version (a) (b) A) connector is fed to the feed-line through the substrate material and the ground plane. FigureFigure 3. 3.( a(a).). DetailsDetails ofof theThethe simulation simulationreconfigurable modelmodel antenna (one(one smaller smaller array patchpatch and is is the connected connected SMA connector to to the the larger larger are main main designed patch). patch) .on ( b(b). ).theThe The FR4 returnreturn sub- losseslosses at at different different frequencies. frequencies.strates, which have a dielectric constant of 4.4 and a loss tangent of 0.02.

Substrate of feeThed n epatchtwork antenna array is designed on a double-sided substrate using the planar Substrate of Patch technology, while the feed network is designed on a single-sided substrate. In assembling h h1 the microstrip patch antenna array, the non-copper side of the feed network is placed in direct contact with the ground of the patch antenna array, as shown in Figure 4a. This leads to a low profile and compact structure. The dimensions of the frequency reconfigurable patch antenna array in Figures 1 and 4a are listed in Table 1. The fabricated patch antenna array is shown in Figure 4b,c for measurement. The SMA (SubMiniature Version A) connector is fed to the feed-line through the substrate material and the ground plane. The reconfigurable antenna array and the SMA connector are designed on the FR4 substrates, which have a dielectric constant of 4.4 and a loss tangent of 0.02. Patch SMA Substrate Foef efde eNde ntwetowrokrk SubGstrraanted ooff PPaattcchh Antenna (a) h (b) (c) h1 FigureFigure 4. ( 4.a).( Assemblya). Assembly schematic schematic of antenna of antenna.. (b) (Topb) Top and and (c) (bottomc) bottom views views of the of the fabricated fabricated antenna. antenna.

TableTable 1. Dimensions 1. Dimensions of the of the antenna antenna array array.. Parameters Values Parameters Values W 19.0 mm L W 16.819.0 mm mm L 16.8 mm w0 2.0 mm w0 2.0 mm Patch l0 5.0 mm SMA Feed Network l0 5.0 mm Grand of Patch Antenna M M 1.0 1.0mm mm (a) h h (b) 0.8( 0.8mmc) mm h1 h1 1.6 1.6mm mm Figure 4. (a). Assembly schematic of antenna. (b) Top and (c) bottom views of the fabricated antenna.

Table 1. Dimensions of the antenna array. Parameters Values W 19.0 mm L 16.8 mm w0 2.0 mm l0 5.0 mm M 1.0 mm h 0.8 mm h1 1.6 mm

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Energies 2021, 14, 415 6 of 12 2.2. Simulation and Measurement Results Simulation results are obtained in the simulation software HFSS, and the return losses2.2.Simulation are measured and Measurement by the vector Results network analyzer (Agilent Technologies N5230A, whichSimulation is manufactured results are by obtained Agilent in in the Palo simulation Alto, USA. software) to verify HFSS, the and accuracy the return of losses the frequencyare measured reconfigurable by the vector antenna network array. analyzer When (Agilent four smaller Technologies side patches N5230A, are all which con- is nectedmanufactured to the larger by Agilentmain patch, in Palo the Alto,simulated USA.) and to verifymeasured the accuracyreturn losses of the at different frequency frequenciesreconfigurable for the antenna reconfigurable array. When antenna four array smaller are illus sidetrated patches in Figure are all 5a. connected The resonant to the frequencylarger main of the patch, antenna the simulated array changes and measured significantly return from losses 3.41 atGHz different to 3.96 frequencies GHz by var- for yingthe the reconfigurable DC bias voltage antenna from array 0 to are20 V. illustrated The working in Figure frequency5a. The from resonant 3.41 GHz frequency to 3.96 of GHzthe belongs antenna to array the S changes-band (2 significantly–4 GHz). The from simulated 3.41 GHz and tomeasured 3.96 GHz return by varying losses at the dif- DC ferentbias voltagefrequencies from that 0 to are 20 V.obtained The working when frequencyjust one smaller from 3.41 side GHz patch to is 3.96 connected GHz belongs to the to largerthe S-band main patch (2–4 GHz). are illustrated The simulated in Figure and measured5b. The working return losses frequency at different changes frequencies signifi- cantlythat are from obtained 5.7 GHz when to 6.3 just GHz one smaller by varying side the patch DC is bias connected voltage to from the larger 3 to 11main V. patch The workingare illustrated frequency in Figure in this5 b.case The belongs working to frequencythe C-band changes (4–8 GHz). significantly The frequency from 5.7 which GHz belongsto 6.3 GHzto the by S- varyingband and the the DC C- biasband voltage have been from widely 3 to 11 used V. The in workingthe fields frequency of radar, satel- in this litecase communication belongs to the and C-band so on. (4–8 Now GHz). other The widely frequency used which forms belongs of technology to the S-band such and as Bluetooth,the C-band ZIGBEE, have been wireless widely routing, used in wireless the fields mouse of radar, device satellite and communicationa wide range of andelec- so tronicon. Nowproducts other also widely use this used frequency forms of, technologywhich belongs such to as the Bluetooth, S-band and ZIGBEE, C-band. wireless The resonantrouting, frequency wireless mouseof the antenna device andarray a shifts wide toward range of highe electronicr values products with an alsoincrease use thisin DCfrequency, bias voltages which and belongs a decrease to the S-band in capacitance and C-band. values The resonantof the varactor frequency diodes. of the For antenna the adjustablearray shifts range toward of frequencies, higher values the withreturn an loss increase of the in antenna DC bias array voltages is less and than a decrease−15 dB at in thecapacitance resonant valuesfrequency. of the There varactor is a diodes.slight differ For theence adjustable between range simulation of frequencies, results and the measurementreturn loss of results the antenna because array of the is fabrication less than − tolerance15 dB at theand resonantsmall measurement frequency. Thereerrors. is Also,a slight the ohmic difference loss of between the varactor simulation diodes results cannot and be precisely measurement modeled results in the because simulation of the softwarefabrication HFSS. tolerance Also, there and smallare a number measurement of modulations errors. Also, that the are ohmic needed loss because of the varactorlots of varactordiodes diodes cannot are be welded precisely between modeled the in larger the simulation main patches software and the HFSS. smaller Also, side there patch- are a number of modulations that are needed because lots of varactor diodes are welded es, and lots of wires are used. The frequency reconfigurable antenna array is affected by between the larger main patches and the smaller side patches, and lots of wires are used. interference caused by using lots of wires. The frequency reconfigurable antenna array is affected by interference caused by using lots of wires.

Simulation Results Simulation Results 0 Measurement Results 0 Measurement Results

-5 -5

-10 -10

(dB) (dB)

11 -15

-15 11 S S 11 V& 3.2 V& 0 V& 1 pF -20 4 pF -20 13.3 pF 7.5 V& 1 V& 4.7 V& 1.5 pF 8.6 pF 3 V& 20 V& 3 pF 6 V& 13 V& -25 -25 4.8 pF 0.69 pF 6 V& 2 pF 0.89 pF 2 pF -30 3.00 3.25 3.50 3.75 4.00 4.25 4.50 5.50 5.75 6.00 6.25 6.50 Frequency (GHz) Frequency (GHz) (a) (b)

FigureFigure 5. 5. (a().a). Simulated Simulated and and measured measured return losseslosses forfor the the antenna antenna (four (four smaller smaller side side patches patches are are all connectedall connected to the to larger the largermain main patch). patch (b).) Simulated. (b). Simulated and measured and measured return return losses (onelosses smaller (one smaller side patch side is patch connected is connected to the larger to the main larger patch). main patch). The antenna pattern refers to the pattern in which the relative field intensity of radiationThe antenna field changes pattern withrefers the to the direction pattern at in a which certain the distance relative from field the intensity antenna. of ra- The diationantenna field pattern changes is an with important the direction pattern toat measurea certain thedistance antenna from performance. the antenna. The The antenna an- tennapattern pattern is a diagramis an important used to pattern show the to directivitymeasure the of ante thenna antenna. performance. The parameters The antenna of the patternantenna is cana diagram be observed used fromto show the antennathe directivity pattern. of Thethe frequencyantenna. The reconfigurable parameters antennaof the antennais continuously can be observed adjusted from at thethe workingantenna pattern. frequency The within frequency a certain reconfigurable frequency antenna range. A good frequency reconfigurable antenna should have a relatively stable antenna pattern. We show the antenna pattern of the frequency reconfigurable antenna array at different

Energies 2021, 14, x FOR PEER REVIEW 7 of 12

Energies 2021, 14, 415 is continuously adjusted at the working frequency within a certain frequency range.7 ofA 12 good frequency reconfigurable antenna should have a relatively stable antenna pattern. We show the antenna pattern of the frequency reconfigurable antenna array at different frequencies. This pattern indicates that the frequency reconfigurable antenna array works frequencies. This pattern indicates that the frequency reconﬁgurable antenna array works normally at different working frequencies. As a result, the antenna is very important for normally at different working frequencies. As a result, the antenna is very important for the wireless power transfer system. The radiation gain patterns of the antenna array in the wireless power transfer system. The radiation gain patterns of the antenna array in the the xz plane and yz plane at different frequencies are shown in Figure 6. The xz plane xz plane and yz plane at different frequencies are shown in Figure6. The xz plane means meansthat Phithat is Phi 0 deg, is 0 and deg, the and yz the plane yz meansplane means phi is 90 phi deg. is 90 Radiation deg. Radiation gain patterns gain patterns maintain maintainsimilar shapessimilar atshapes different at different states. Instates. summary, In summary, with 16 with varactors, 16 varactors, this array this canarray operate can operatewith the with maximum the maximum gain varying gain varying from 7.87 from to 7.87 7 dBi to when 7 dBi thewhen operating the operating frequency frequency is tuned is fromtuned 3.96 from to 3.96 6.21 to GHz. 6.21 GHz.

0 7 dBi Phi=0 deg 0 7.2 dBi 0 7.5 dBi 10 10 Phi=0 deg 10 Phi=0 deg

Phi=90 deg Phi=90 deg Phi=90 deg

0 0 0 300 60 300 60 300 60

-10 -10 -10

-20 -20 -20

-10 -10 -10 240 120 240 120 240 120 0 0 0

10 10 10 180 180 180 (a) 3.41 GHz (b) 3.53 GHz (c) 3.64 GHz 0 7.87 dBi 0 7.64 dBi Phi=0 deg Phi=0 deg 0 7.76 dBi 10 10 10 Phi=0 deg Phi=90 deg Phi=90 deg Phi=90 deg

0 0 300 60 300 60 300 60 -10 -10 -10

-20

-20 -20

-10 -10 -10 240 120 240 120 240 120 0 0 0

10 10 10 180 180 180 (d) 3.75 GHz (e) 3.85 GHz (f) 3.96 GHz

0 7.32 dBi 0 7.61 dBi 0 7.82 dBi 10 Phi=0 deg 10 Phi=0 Deg 10 Phi=0 Deg

Phi=90 deg Phi=90 Deg Phi=90 Deg

0 0 0 300 60 300 60 300 60

-10 -10 -10

-20 -20 -20 -10 -10 -10 240 120 0 240 120 240 120 0 0

10 10 180 180 10 180 (g) 5.79 GHz (h) 6.01 GHz (i) 6.21 GHz FigureFigure 6. 6.RadiationRadiation gain gain patterns patterns of ofthe the antenna antenna array array in inthe the xz xz plane plane and and yz yz plane plane at atdifferent different frequencies. frequencies.

TheThe gain gain of ofthe the single single patch patch antenna antenna and and the the gain gain of of the the patch patch antenna antenna array array without without anyany smaller smaller side side patches patches are are simulated. simulated. The The gains gains of ofthe the single single patch patch antenna antenna and and the the patchpatch antenna antenna array array are are shown shown in in Figure Figure 7.7 The. The gain gain of of the the single single patch patch antenna antenna is is 4.4 4.4 dBi. dBi. Also,Also, the the gain gain of of the the single single patch patch antenna antenna array array is is7.8 7.8 dBi. dBi. Therefore, Therefore, the the gain gain is isimproved improved byby using using th thee array. array. As As a aresult, result, we we updated updated the the gains gains of of the the frequency frequency reconfigurable reconﬁgurable antennaantenna array. array. The gains of the antenna array at different frequencies between the S-band are shown in Figure 8a. Also, the simulated and measured gains are illustrated in Figure 8b when one smaller side patch is connected to the larger main patch. Simulated results have shown that the gain is above 7 dBi. Measurement results have shown that the gain of the antenna array is above 6 dBi at different frequencies. There was an error rate of

Energies 2021, 14, x FOR PEER REVIEW 8 of 12

about 6% between the simulation results and the measurement results. Measured gains were slightly lower than the simulated gains due to measurement errors and the fabrication tolerance. In short, the gains of the antenna array are relatively stable within the range of the frequency. The gains of the frequency reconfigurable antenna array are slightly lower than the gains of the patch antenna array without any smaller side patches. Also, these gains are reduced as the capacitance of the varactor increases due to increases Energies 2021, 14, 415 in internal resistance. In short, the radiation patterns and the gains of the antenna array8 of 12 are relatively stable within the range of the frequency.

Energies 2021, 14, x FOR PEER REVIEW 8 of 12 10 7.8 dBi patch, Phi=0 deg patch, Phi=90 deg 5 4.4 dBi array, Phi=0 deg array, Phi=90 deg about 6% between the simulation results and the measurement results. Measured gains 0 were slightly lower than the simulated gains due to measurement errors and the fabrica- -5 tion tolerance. In short, the gains of the antenna array are relatively stable within the range of the frequency. The gains of the frequency reconfigurable antenna array are

Gain (dBi) Gain -10 slightly lower than the gains of the patch antenna array without any smaller side patches. Also,-15 these gains are reduced as the capacitance of the varactor increases due to increases

in internal-20 resistance. In short, the radiation patterns and the gains of the antenna array are relatively-160 -120 stable-80 -40 within0 40 the80 range120 of160 the frequency. Theta (Deg) FigureFigure 7. The 7. The gains gains of the of thesingle single patch patch antenna antenna and and the thepatch patch antenna antenna array. array. 10 7.8 dBi patch, Phi=0 deg patch, Phi=90 deg The5 gains of the antenna4.4 dBi array array, atdifferent Phi=0 deg frequencies between the S-band are shown in Figure8a. Also, the100 simulated and array, measured Phi=90 deg gains are illustrated in100 Figure8b when one Simulation Results 10 0 90 10 Simulation Results 90 smaller Measurement side patch Results is connected to the larger main patch. Measurement Simulated Results results have shown Error 80 Error 80 that the-5 gain is above 7 dBi. Measurement results have shown that the gain of the antenna 8 8 array is above 6 dBi70 at different frequencies. There was an error rate of70 about 6% between

Error (%) Error (%) 60 Gain (dBi) Gain -10 60 6 the simulation results and the6 measurement results. Measured gains were slightly lower than the simulated gains50 due to measurement errors and the fabrication50 tolerance. In short, -15 40 4 the gains of the antenna40 array are4 relatively stable within the range of the frequency. The

Gain (dBi) Gain 30 Gain (dBi) Gain 30 gains of-20 the frequency reconﬁgurable antenna array are slightly lower than the gains of the -160 -120 -80 20-40 0 40 80 120 160 20 2 patch antenna array without any2 smaller side patches. Also, these gains are reduced as the 10Theta (Deg) 10 capacitance of the varactor increases due to increases in internal resistance. In short, the 0 0 0 0 5.8 radiationFigure6.0 7. The patterns gains6.2 of and the the single gains patch3.4 of theantenna3.5 antenna and3.6 the array3.7 patch are 3.8antenna relatively3.9 array. stable4.0 within the range Frequencyof the (GHz) frequency. Frequency (GHz) (a) (b) 100 100 Simulation Results 10 90 10 Simulation Results 90 Figure 8. (a). Simulated and measured gains Measurement of the Results antenna (four smaller side patches are all connected Measurement to Results the larger main Error 80 Error 80 patch). (b). Simulated8 and measured gains (one smaller side patch is 8connected to the larger main patch). 70 70

Error (%) 60 Error (%) 60 6 3. The Microwave Wireless Power Transfer6 System 50 50 The simulation model of the MWPT system can be built by using the40 simulation 40 4 4 software HFSS. The model of the proposed MWPT system is shown in Figure 9a. The Gain (dBi) Gain 30 Gain (dBi) Gain 30 2 frequency reconfigurable antenna20 array2 is used as the transmitter of the MWPT20 system to transmit energy. The antenna10 is also used as the receiver of the MWPT system10 to receive 0 energy. The capacitance value0 of the0 varactor diode changes with different0 voltage, 5.8 6.0 6.2 3.4 3.5 3.6 3.7 3.8 3.9 4.0 whichFrequency has (GHz) been shown in Figure 9a. This suggesFrequencyts that the (GHz) frequency reconfigurable antenna array can work at the continuous switchable resonant frequency within a certain (a) (b) range. The energy is transmitted from the transmitter to the receiver through the electromagnetic field at the different working state, as shown in Figure 9a. The variable FigureFigure 8.8. ((aa).). SimulatedSimulated andand measuredmeasured gainsgains ofof thethe antennaantenna (four(four smallersmaller sideside patchespatches areare allall connectedconnected toto thethe largerlarger mainmain symbol d is the distance between the transmitter and the receiver. patch).patch). ((bb).). SimulatedSimulated andand measuredmeasured gainsgains (one(one smallersmaller sideside patchpatch isis connectedconnected toto thethe largerlarger mainmain patch).patch). The experimental platform for the MWPT system is illustrated in Figure 9b. The transmitter3.3. The Microwave and the receiver Wireless are Power Powerconnected TransferTransfer to port SystemSystem 1 and port 2 of the vector network an- TheThe simulation simulation model of the the MWPT MWPT system system can can be be built built by by using using the the simulation simulation softwaresoftware HFSS.HFSS. TheThe modelmodel ofof thethe proposedproposed MWPTMWPT systemsystem isis shownshown inin FigureFigure9 a.9a. The The frequencyfrequency reconﬁgurablereconfigurable antennaantenna arrayarray isis usedused asas thethe transmittertransmitter ofof thethe MWPTMWPT systemsystem toto transmittransmit energy.energy. TheThe antennaantenna isis alsoalso usedused asas thethe receiverreceiver ofof thethe MWPTMWPT systemsystem toto receivereceive energy.energy. TheThe capacitance capacitance value value of the of the varactor varactor diode diode changes changes with different with different voltage, voltage, which haswhich been has shown been shown in Figure in 9Figurea. This 9a suggests. This sugges thatts the that frequency the frequency reconﬁgurable reconfigurable antenna an- arraytenna canarray work can atwork the continuousat the continuous switchable switchable resonant resonant frequency frequency within within a certain a certain range. Therange. energy The isenergy transmitted is transmitted from the from transmitter the transmitter to the receiver to the through receiver the through electromagnetic the electromagnetic field at the different working state, as shown in Figure 9a. The variable symbol d is the distance between the transmitter and the receiver. The experimental platform for the MWPT system is illustrated in Figure 9b. The transmitter and the receiver are connected to port 1 and port 2 of the vector network an-

Energies 2021, 14, x FOR PEER REVIEW 9 of 12

alyzer (Agilent Technologies N5230A, which is manufactured by Agilent in Palo Alto, USA.), respectively. The adjustable DC voltage source (KEYSIGHT E3633A, which is manufactured by KEYSIGHT in Shenzhen, China.) is used to provide the variable DC Energies 2021, 14, 415bias voltage for the varactor diode. The transmitter is fixed in a certain position, and the 9 of 12 receiver is placed in a coaxial position of the transmitter. PTE of the frequency reconfigurable MWPT system is calculated by the equation in [21], which is:

S /10 ﬁeld at the differentPTE working10 state,21  as100% shown. in Figure9a. The variable(5 symbol) d is the distance between the transmitter and the receiver.

④ 14 12 Capacitance ⑷ 10 ⑶ 8 6 4

Capacitance (pf) Capacitance 2 0 0 2 4 6 8 10 12 14 16 18 20 patch Voltage (V) ⑥ PTE ③ state 1

⑤ PTE f1 f ② state 2 ① f2 f d PTE ⑴ ⑵ ⑦ ⑧ state n

fn f (a) (b)

Figure 9. (a).Figure The model 9. (a). ofThe the model microwave of the wirelessmicrowave power wireless transfer power (MWPT) transfer system (MWPT) ( 1 : transmitter, system (① :2 transmitter,: receiver, 3 : varactor ② ③ ④ ⑤ ⑥ ⑦ diode, 4 : DC: voltagereceiver, source, : varactor 5 : lumped diode, port : 1,DC 6 voltage: lumped source, port 2, :7 lumped: feed network, port 1, 8: :lumped patch). port (b). The2, experimental: ⑧ platform forfeed the MWPTnetwork, system : patch). ((1): transmitter, (b). The experimental (2): receiver, platform (3): DC for voltage the MWPT source, system (4): vector ((1): transmitter, network analyzer). (2): receiver, (3): DC voltage source, (4): vector network analyzer). The experimental platform for the MWPT system is illustrated in Figure9b. The The S21 oftransmitter the MWPT andsystem the is receiver measured are by connected the vector to portnetwork 1 and analyzer. port 2 ofSimula- the vector network tion and measurementanalyzer (Agilentresults of Technologies PTE of the MWPT N5230A, system which at is different manufactured frequencies by Agilent be- in Palo Alto, tween S-band USA.), and C- respectively.band are illustrated The adjustable in Figure DC 10a voltage,b, respectively. source (KEYSIGHT The measured E3633A, which is results have shownmanufactured that the byPTE KEYSIGHT of the MWPT in Shenzhen, system stays China.) at around is used to20% provide at different the variable DC bias frequencies. Thevoltage measured for the results varactor agree diode. with The the transmitter simulatedis results, fixed in but a certain there is position, an error and the receiver rate of about 10%is placed between in athe coaxial simulation position results of the and transmitter. the measurement PTE of results. the frequency Also, the reconfigurable measured resultsMWPT are lower system than is calculated the simulated by the results equation due to in the [21], fabricated which is: tolerance and measurement errors. The MWPT system based on the frequency reconfigurable antenna array can be used to power the receiver at differentPTE = frequencies,10|S21|/10 × and100%. the PTE of the sys- (5) tem does not decrease as the resonant frequency of the receiver changes. The S21 of the MWPT system is measured by the vector network analyzer. Simulation and measurement results of PTE of the MWPT system at different frequencies between S-band and C-band are illustrated in Figure 10a,b, respectively. The measured results have shown that the PTE of the MWPT system stays at around 20% at different frequencies. The measured results agree with the simulated results, but there is an error rate of about 10% between the simulation results and the measurement results. Also, the measured results are lower than the simulated results due to the fabricated tolerance and measurement errors. The MWPT system based on the frequency reconfigurable antenna array can be used to power the receiver at different frequencies, and the PTE of the system does not decrease as the resonant frequency of the receiver changes. Simulation and measurement results of power transfer efficiency of the system at the different distance are shown in Figure 11. Obviously, the PTE of the MWPT system decreases as the distance value increases. After repeated testing, the relative error between the simulation and experimental results is distributed at around 10%. The measurement results are relatively lower than the simulation results because of the practical fabrication tolerance. When the antenna array works at the resonant frequency of 3.64 GHz, the PTE of the MWPT system is 25%, 20.5% and 10.3% at distances of 20 mm, 40 mm and 80 mm, respectively. Energies 2021, 14, x FOR PEER REVIEW 10 of 12

100 100 Simulation Results 90 Simulation Results Measurement Results Measurement Results 90 30 Error 30 80 Error 80 70 70

60 Error(%) 20 20 60 Energies 2021, 14, x FOR PEER REVIEW 50 50 10 of 12 Energies 2021, 14, 415 10 of 12 40 40 30 10 10 30 (%) Error 20 20

10100 10100 Simulation Results Simulation Results

Power Transfer Efficiency (%) Efficiency Transfer Power 090 0 Measurement Results 0 Measurement Results 090 30 3.4 3.6 Error3.8 4.0 (%) Efficiency Transfer Power 30 5.8 5.9 6.0 6.1 6.2 80 Error 80 Frequency (GHz) Frequency (GHz) 70 70 (a) 60 Error(%) (b) 60 20 20 50 50 Figure 10. (a)The power transfer efficiency (PTE) of the MWPT system at different frequencies (four smaller side patches 40 40 are all connected to the larger main patch). (b) The power transfer efficiency (PTE) of the MWPT system at different fre- 30 quencies (one10 smaller side patch is connected to the larger main patch).10 30 (%) Error 20 20 Simulation and measurement10 results of power transfer efficiency of the10 system at the Power Transfer Efficiency (%) Efficiency Transfer Power 0 different distance are shown0 in Figure0 11. Obviously, the PTE of the MWPT0 system de- 3.4 3.6 3.8 4.0 (%) Efficiency Transfer Power 5.8 5.9 6.0 6.1 6.2 Frequencycreases (GHz)as the distance value increases. After repeatFrequencyed testing, (GHz) the relative error between the simulation and experimental results is distributed at around 10%. The measurement (a) (b) results are relatively lower than the simulation results because of the practical fabrication tolerance. When the antenna array works at the resonant frequency of 3.64 GHz, the PTE Figure 10. (a)The The power power transfer efﬁciencyefficiency (PTE)(PTE) ofof thethe MWPTMWPT system system at at different different frequencies frequencies (four (four smaller smaller side side patches patches are of the MWPT system is 25%, 20.5% and 10.3% at distances of 20 mm, 40 mm and 80 mm, areall connectedall connected to the to largerthe larger main main patch). patch) (b) The. (b) powerThe power transfer transfer efﬁciency efficiency (PTE) (PTE) of the MWPTof the MWPT system system at different at different frequencies fre- respectively. quencies(one smaller (one side smaller patch side is connected patch is connected to the larger to the main larger patch). main patch).

S30imulation and measurement results of power100 transfer efficiency of the system at the Simulation Results different distance are shown in Figure Measurement 11. ResultsObviously,90 the PTE of the MWPT system decreases as the distance value increases. Error After repeat80ed testing, the relative error between the simulation and experimental results is distributed at around 10%. The measurement 20 70

results are relatively lower than the simulation results Error (%) because of the practical fabrication 60 tolerance. When the antenna array works at the resonant frequency of 3.64 GHz, the PTE 50 of the MWPT system is 25%, 20.5% and 10.3% at distances of 20 mm, 40 mm and 80 mm, 40 respectively.10 30 20 30 100 0 Simulation Results 10 Measurement Results 90 Power Transfer Efficiency (%) Efficiency Transfer Power 0 20 30 40 50 Error60 70 80 80 20 Distance (mm) 70

Error (%) 60 FigureFigure 11. 11. TheThe measurement measurement results results of of the the four four-coil-coil MWPT MWPT system. system. 50 AsAs10 we we know, know, the the energy energy is istransmitted transmitted from from the40 the transmitter transmitter to the to receiver, the receiver, while while the antennathe antenna is operated is operated as the as transmitter the transmitter to transmit to transmit energy,30 energy, as well as as well being as operated being operated as the receiveras the receiver to receive to receiveenergy. energy.The research The research based on based the20 transmitter on the transmitter and the andreceiver the receiveris very is very important. In this paper, we proposed the idea that the frequency reconfigurable important.0 In this paper, we proposed the idea that10 the frequency reconfigurable mi- microstrip patch antenna array can be used for a microwave wireless power transfer system. crostrip(%) Efficiency Transfer Power patch antenna array can be used for a microwave0 wireless power transfer sys- 20 30 40 50 60 70 80 tem.This This would would allow allow the the energy energy tobe to transmittedbe transmitted to theto the receiver receiver at differentat different frequencies. frequen- Also, when the energyDistance is transmitted (mm) to the load from the receiver in the wireless power cies. Also, when the energy is transmitted to the load from the receiver in the wireless transfer system, there should be a rectifier circuit which converts alternating current to power transfer system, there should be a rectifier circuit which converts alternating cur- Figuredirect 11. current. The measurement In this paper, results we onlyof the verify four-coil the MWPT efficiency system. of the power transmitted to the rent to direct current. In this paper, we only verify the efficiency of the power transmitted receiver from the transmitter. Notably, the rectifier circuit is not involved in power transfer fromAs the we transmitter know, the toenergy the receiver. is transmitted As a result, from the the transmitter rectifier circuit to the is receiver, not in our while present the antennawork. The is operated systematic as efficiencythe transmitter involving to transmit the rectifier energy, circuit as well will as be being focused operated on in futureas the receiverdesign research. to receive energy. The research based on the transmitter and the receiver is very important. In this paper, we proposed the idea that the frequency reconfigurable mi- crostrip4. Conclusions patch antenna array can be used for a microwave wireless power transfer system. ThisIn this would paper, allow a MWPT the energy system to based be transmitted on the frequency to the receiver reconfigurable at different antenna frequen- array cies.was proposed.Also, when The the frequency energy is reconfigurable transmitted to antenna the load array from works the receiver at different in the frequencies wireless powercovering transfer a wide system, range from there 3.41 should to 3.96 be GHz a rectifier and 5.5 circuit to 6.3 GHz,which and converts the working alternating frequency cur- rentband to could direct be current. switched In betweenthis paper, the we S-band only andverify the the C-band. efficiency The of frequency the power reconfigurable transmitted

Energies 2021, 14, 415 11 of 12

antenna array can work with good performance at different resonant frequencies. The return loss of the frequency reconfigurable antenna array is less than −15 dB at the different frequencies. Radiation patterns of the antenna array at different frequencies are relative stable. Also, the gains of the antenna array are above 6 dBi at different working frequencies. In addition, PTE of the MWPT system based on the frequency reconfigurable antenna remains above 20% at different frequencies. Also, PTE of the MWPT system at the frequency of 3.64 GHz is 25%, 20.5% and 10.3% at the distances of 20 mm, 40 mm and 80 mm, respectively. The MWPT system based on a frequency reconfigurable antenna array can be used to power the receiver at different frequencies, and PTE of the system does not decrease in a clear way as the resonant frequency of the receiver changes, which enables this system to greatly meet the demands of users.

Author Contributions: H.W. proposed the main idea. H.L. checked and discussed the results and the whole manuscript. L.D., J.D., D.Z. and S.H. contributed to the discussion of this study. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Key Research and Development Program of China (Grant No. 2017YFA0204600), and the National Natural Science Foundation of China (Grant No. 51802352). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data sharing not applicable. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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