Compact Antenna in 3D Configuration for Rectenna Wireless Power

sensors

Communication Compact Antenna in 3D Conﬁguration for Rectenna Wireless Power Transmission Applications

Alassane Sidibe 1,2,* , Alexandru Takacs 1, Gaël Loubet 1 and Daniela Dragomirescu 1

1 Laboratoire d’Analyse et d’Architecture des Systèmes du Centre National de la Recherche Scientiﬁque (LAAS-CNRS), Université de Toulouse, Centre National de la Recherche Scientiﬁque (CNRS), Institut National des Sciences Appliqués de Toulouse (INSA), Université Paul Sabatier, Toulouse III (UPS), 31400 Toulouse, France; [email protected] (A.T.); [email protected] (G.L.); [email protected] (D.D.) 2 Uwinloc, 9 Rue Humbert Tomatis, 31200 Toulouse, France * Correspondence: [email protected]

Abstract: This work presents methods for miniaturizing and characterizing a modified dipole antenna dedicated to the implementation of wireless power transmission systems. The antenna size should respect the planar dimensions of 60 mm × 30 mm to be integrated with small IoT devices such as a Bluetooth Lower Energy Sensing Node. The provided design is based on a folded short-circuited dipole antenna, also named a T-match antenna. Faced with the difficulty of reducing the physical dimensions of the antenna, we propose a 3D configuration by adding vertical metallic arms on the edges of the antenna. The adopted 3D design has an overall size of 56 mm × 32 mm × 10 mm at 868 MHz. Three antenna-feeding techniques were evaluated to characterize this antenna. They consist of soldering a U.FL connector on the input port; vertically connecting a tapered balun to the antenna; and integrating a microstrip transition to the layer of the antenna. The experimental results of the selected feeding techniques show good agreements and the antenna has a maximum Citation: Sidibe, A.; Takacs, A.; Loubet, G.; Dragomirescu, D. gain of +1.54 dBi in the elevation plane (E-plane). In addition, a final modification was operated to Compact Antenna in 3D the designed antenna to have a more compact structure with a size of 40 mm × 30 mm × 10 mm at Configuration for Rectenna Wireless 868 MHz. Such modification reduces the radiation surface of the antenna and so the antenna gain Power Transmission Applications. and bandwidth. This antenna can achieve a maximum gain of +1.1 dBi in the E-plane. The two Sensors 2021, 21, 3193. https:// antennas proposed in this paper were then associated with a rectifier to perform energy harvesting doi.org/10.3390/s21093193 for powering Bluetooth Low Energy wireless sensors. The measured RF-DC (radiofrequency to direct current) conversion efficiency is 73.88% (first design) and 60.21% (second design) with an Academic Editor: Razvan D. Tamas illuminating power density of 3.1 µW/cm2 at 868 MHz with a 10 kΩ load resistor.

Received: 12 February 2021 Keywords: compact antenna; wireless power transmission (WPT); energy harvesting; rectenna; Accepted: 28 April 2021 wireless sensors Published: 4 May 2021

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in 1. Introduction published maps and institutional affil- iations. Over the last decades, we have been faced with the miniaturization of electronic devices, especially in the field of wireless systems. The aim is to have multiple functionalities on an ever-smaller surface area. Recent IoT applications (Internet of Things) tend to employ tiny and low power electronic components [1]. However, batteries are still widely used for powering the devices despite their significant size and the frequent need for replacement. Copyright: © 2021 by the authors. An alternative is using a battery-free system powered by energy harvesting (EH) or wireless Licensee MDPI, Basel, Switzerland. power transmission (WPT), for instance, based on a rectenna circuit. This article is an open access article A rectenna is a combination of a rectifier and an antenna used to scavenge ambient distributed under the terms and conditions of the Creative Commons or specially generated far-field radiofrequency (RF) waves [2]. The implementation of a Attribution (CC BY) license (https:// rectenna in a battery-free system can allow increasing its lifetime, reducing its manufactur- creativecommons.org/licenses/by/ ing costs, while ensuring reliable performances for decades. Several kinds of application 4.0/).

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exist, such as, for instance, rectennas, which were developed for a biomedical device pasted on the human body [3] or used in the IoT domain [4–6]. The antenna is a ubiquitous element in IoT and other wireless applications. However, the required size stays important due to the important dependence of the geometrical elements with the targeted wavelength. Several miniaturization techniques are described in the literature. Structural modification on a Printed Circuit Board (PCB) antenna consists of acting on the geometry of the antenna or by adding another element on the antenna shape. In this sense, a coupling element, such as a rectangular ring, can allow a reduction in size and an increase in bandwidth as presented in [7]. Traditionally, small size antennas are designed by using meander lines which reduce the resonant frequency [8]. Fractal geometry is also used to miniaturize antennas [9]. Another possibility is to add reactive loading elements. This technique is employed in [10] through an LC load (a combination of a lumped inductor and a distributed capacitor). Miniaturization can also be achieved by reducing the guided wavelength through the use of a higher permittivity material, such as a ceramic—polymer composite [11]. Recent research activities were focused on developing small antennas on metamaterial as presented in [12]. In this paper, we present a miniaturization technique which consists of shaping the antenna to form a three-dimensional (3D) structure tuned for the Industrial Scientific and Medical (ISM) 868 MHz frequency band. This configuration allows us to have an electrically small antenna as described in Section2. A more compact antenna based on the previous one is designed to compare the trade-off between size and rectenna performances. Then, Section3 presents the experimental results of the fabricated antennas with different feeding methods to validate the use of the U.FL connector for the next steps. Simulations were carried out on the Ansys HFSS software and verified with far-field measurements performed in an anechoic chamber. The final section of this paper describes an original concept of powering a Bluetooth Low Energy Sensing Node embedded in concrete element with a compact and efficient rectenna design.

2. Design of a Compact 3D Dipole Antenna 2.1. Antenna Miniaturization Methods and the First Design of the Compact 3D Dipole In this study, we proposed a miniaturized antenna design for WPT applications. An antenna is a resonant structure with a proper frequency depending on its length. Therefore, there are size and performance limitations for small antennas [13,14]. The size reduction imposes a smaller radiation resistance, so a lower radiation efficiency and a bandwidth limitation. Wheeler defines a electrically small antenna as one defined by the formula given in Equation (1). It means that the antenna sphere is smaller than the radian sphere, also defined as Wheeler Cap. λ is the wavelength at the operating frequency, k represents the free-space wavenumber and a is the minimum antenna sphere radius. The choice of proposing an electrically small antenna should allow us to have an antenna design in the maximum planar size of 60 mm × 30 mm. Nevertheless, the antenna bandwidth reduction does not matter with the applications in the ISM 868 MHz frequency band. The targeted antenna gain is about +1 dBi less compared to a conventional dipole antenna gain (+2.15 dBi). 2π k·a < 1 with k = (1) λ The proposed design is based on a dipole antenna according to its multiple advantages, such as the symmetry of its shape and radiation pattern, its easy and low-cost fabrication, and the possibility of receiving balanced signals. The required dimensions of 60 mm × 30 mm allow us to have a conventional half-wavelength dipole antenna at 2 GHz, as presented in Figure1. Starting from this antenna at 2 GHz, we had investigated a technique for reducing the physical dimensions of the antenna without significantly degrading its performances (+1 dBi less of the gain). The planar structure of the final antenna design on an FR4 substrate is presented in Figure2. Overall, a folded dipole antenna designed after size optimization Sensors 2021, 21, x FOR PEER REVIEW 3 of 12

Sensors 2021, 21, x FOR PEER REVIEW 3 of 12 Starting from this antenna at 2 GHz, we had investigated a technique for reducing the physical dimensions of the antenna without significantly degrading its performances (+1 dBi less of the gain). The planar structure of the final antenna design on an FR4 Starting from this antenna at 2 GHz, we had investigated a technique for reducing Sensors 2021, 21, 3193 substrate is presented in Figure 2. Overall, a folded dipole antenna designed3 of 12 after size the physical dimensions of the antenna without significantly degrading its performances optimization (Antenna in Figure 3 stamped "None") presents a resonant frequency close (+1 dBi less of the gain). The planar structure of the final antenna design on an FR4 substrateto 1.12 is GHzpresented regarding in Figure the return2. Overall, loss a(S11) folded equal dipole to − 5.2antenna dB. designed after size optimization(Antenna (Antenna in Figure3 instamped Figure “None”)3 stamped presents "None") a resonant presents frequency a resonant close frequency to 1.12 GHzclose − to 1.12regarding GHz regarding the return the loss return (S11) equalloss (S11) to 5.2equal dB. to −5.2 dB.

Figure 1. Conventional dipole antenna simulated on the required size dimensions. (a) The input impedance representation on Smith chart; (b) Half-wavelength antenna dimensions; (c) Simulated FigureFigure 1. Conventional 1. Conventional dipole dipole antenna antenna simulated simulated on the on therequired required size size dimensions. dimensions. (a) (Thea) The input input 3D radiation pattern at 2 GHz. impedanceimpedance representation representation on Smith on Smith chart; chart; (b) ( bHalf-wavelength) Half-wavelength antenna antenna dimensions; dimensions; (c (c) )Simulated Simulated 3D radiation3D radiation pattern pattern at 2 atGHz. 2 GHz. This antenna can be considered as a half-wavelength dipole antenna at 1.12 GHz. The ThistotalThis antenna length antenna of can each can be considered befolded considered arm as (L asa1 half-wavelength+ a W half-wavelength1 + L2 = 43.4 mm) dipole dipole is closedantenna antenna to at the 1.12 quarter GHz. The wavelength total length of each folded arm (L + W + L = 43.4 mm) is closed to the quarter wavelength total (<0.25·length λofg =each 31.9 folded mm). arm Figure (L1 +31 W presents1 +1 L2 =2 43.4 the mm)width is closedtuning to of the the quarter inductive wavelength shorting loop to (<0.25·λg = 31.9 mm). Figure3 presents the width tuning of the inductive shorting loop (<0.25·makeλg = 31.9 a T-match mm). Figure structure 3 presents [15]. The the resonance width tuning frequency of the inductive is greatly shortinginfluenced loop by to the width to make a T-match structure [15]. The resonance frequency is greatly inﬂuenced by the makeand a T-match the length structure of the [15]. T-match The resonance structure. frequency As seen is in greatly Figure influenced 3, the absence by the ofwidth the T-match width and the length of the T-match structure. As seen in Figure3, the absence of the and thestructure length (None)of the T-match shows astructure. significant As resonantseen in Figure frequency 3, the ofabsence the antenna of the T-matchhigher than 1.12 T-match structure (None) shows a signiﬁcant resonant frequency of the antenna higher than structure1.12GHz GHz (None) where where showsthe the impedance impedance a significant is is (17.35 (17.35resonant + j·27.3) j·27.3) frequency Ω.. TheThe of shortingshorting the antenna line line (W higher(W = 0)= 0) connected than connected 1.12 to the GHzto twowhere the dipole two the dipole impedance arms arms downshifts downshifts is (17.35 +the j·27.3) the resonant resonant Ω. The frequencyfrequency shorting line around around (W 1.07= 0)1.07 connected GHz. GHz. Thereafter, Thereafter, to the the two thedipolegeometric geometric arms parameters parametersdownshifts (Wthe and andresonant L) L) were were frequency adjusted adjusted to around match to match the1.07 input GHz.the impedanceinput Thereafter, impedance of thethe of the geometricantennaantenna parameters (50 (50Ω )Ω at) at lower (Wlower frequency.and frequency. L) were adjusted to match the input impedance of the antenna (50 Ω) at lower frequency.

FigureFigure 2. 2. DetailedDetailed geometry of the compact, compact, T-matc T-matchh dipole antenna on an FR4 FR4 substrate substrate (Lsub = 56 Figure 2. Detailed geometry of the compact, T-match dipole antenna on an FR4 substrate (Lsub = 56 sub 1 1 2 2 3 3 mm,( LWmm,subsub= = W 5632mm mm, = ,32W L submm,1 == 20.65 32 L mm, = mm, 20.65L1 =W 20.65mm,1 = 5.95 mm,W mm, =W 5.951 L=25.95 mm,= 16.8 mm, L mm, L=2 16.8= W 16.82 mm,= mm,10.5 W Wmm,2 == 10.5 10.5L3 = mm,14 mm,L L3 = =W14 143 mm= mm, , W = 9.625W 9.625mm,3 = 9.625 E mm,1 = 0.35 mm, E1 =mm,E 0.351 = 0.35E mm,2 = mm,1.05 E2 mm,E=2 1.05= 1.05g =mm, 2.03 mm, g mm). g= =2.03 2.03 mm). mm).

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Figure 3. Evolution of the input impedance (Real andand Imaginary parts) as function ofof thethe frequency Figurefrequencywith 3. Evolution various with shorting of various the input line shor widthimpedanceting line (W) width values. (Real (W)and values.Imaginary parts) as function of the frequency with various shorting line width (W) values. The simulated simulated results results of ofthe the optimization optimization on HFSS on HFSS software software are displayed are displayed in Figure in The3Figures and simulated Figure3 and 4.4 results The. The return return loss loss (reflection (reflection coefficient coefficient S11 S)11 indicates) indicates how how much much the the power power is reflectedis reflected on on the the input input port port when when the the antenna antenna is is excited. excited. A A practical practical criterion criterion for the antenna (impedance)(impedance) inputinput matchingmatching isis generally generally specified specified at at− 10−10 dBm dBm at leastat least for for a ref- a referenceerence impedance impedance of of 50 50Ω .Ω As. As represented represented in in Figure Figure4 ,4, the the greater greater the the length length (L) (L) of of the the short-circuited line, the more the antenna resonates atat higherhigher frequency.frequency.

Figure 4. Simulated S11 (return loss) of the antenna for different configurations; L = 10 mm (black), FigureLFigure 4.= 30Simulated mm 4. Simulated (blue) S11 and (return S11 L = (return 51.4 loss) mm of loss) the (red). ofantenna the antenna for different for different configurations; configurations; L = 10 Lmm = 10 (black), mm (black), L = 30L mm = 30 (blue) mm (blue)and L and= 51.4 L =mm 51.4 (red). mm (red). For the next miniaturization step, the T-match structure parameters were fixed to Forthose theFor which next the give nextminiaturization the miniaturization lower resonant step, step,the frequency T-match the T-match (W structure = 4 structure mm parameters and parametersL = 10 mm).were wereTofixed downshift fixedto to thosethethose which operating which give the give frequency lower the lower resonant obtained resonant frequency from frequency the (W planar = (W4 mm =antenna 4 and mm L andfrom = 10 L mm).1.12 = 10 GHz mm).To downshift to To 868 downshift MHz, a the operating3Dthe configuration operating frequency frequency was obtained investigated. obtained from fromthe The planar theoperating/resonant planar antenna antenna from from1.12frequency GHz 1.12 GHzto of 868 the to MHz,antenna 868 MHz, a can a 3D configurationbe3D downshifted configuration was by investigated. was connecting investigated. twoThe metaloperating/resonant The operating/resonant strands [16]. In frequency our frequencycase, of two the capacitive of antenna the antenna can metallic can be downshiftedarmsbe downshifted with aby height connecting by connectingof 10 mmtwo aremetal two vertically metal strands strands connected[16]. In [16 our]. Into case, ourthe case,twoplanar capacitive two antenna capacitive metallic with metallic a short arms transmissionarmswith witha height aheight line of 10 (Figure ofmm 10 are mm5a). vertically areThis vertically line’s connected posi connectedtion to at the the to planar edge the planar ofantenna the antenna planar with antenna witha short a short was transmissiontunedtransmission to lineobtain (Figure line an (Figureoperating 5a). This5a). frequency Thisline’s line’s posi astion position close at asthe atpossible edge the edge of to the 868 of planar the MHz planar withantenna antenna a bandwidth was was tunedoftuned to 30.7 obtain toMHz. obtain an operating an operating frequency frequency as close as closeas possible as possible to 868 to MHz 868 MHzwith witha bandwidth a bandwidth of 30.7of MHz. 30.7 MHz.

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Figure 5. (a) Geometry of the 3D configuration antenna with the connected metallic arms; (b) SimulatedFigure 5. (3Da) Geometrygain polar ofplot the at 3D 868 conﬁguration MHz. antenna with the connected metallic arms; (b) Simu- Figurelated 5. 3D (a gain) Geometry polar plot of the at 8683D MHz.configuration antenna with the connected metallic arms; (b) Simulated 3D gain polar plot at 868 MHz. In our application at the European ISM 868 MHz frequency band, a narrow band In our application at the European ISM 868 MHz frequency band, a narrow band antenna is not critical. The radiation pattern looks like a doughnut shape and the antennaIn our is application not critical. Theat the radiation European pattern ISMlooks 868 MHz like a frequency doughnut shapeband, anda narrow the maximum band maximum simulated gain is +1.5 dBi (Figure 5b). antennasimulated is not gain critical. is +1.5 dBiThe (Figure radiation5b). pattern looks like a doughnut shape and the maximum simulated gain is +1.5 dBi (Figure 5b). 2.2.2.2. Second Second Miniaturized Miniaturized Antenna Antenna Design Design 2.2. SecondInIn terms terms Miniaturized ofof compactness,compactness, Antenna the the Design previous previous antenna antenna was was modiﬁed modified and twoand differenttwo different anten- antennasnasIn (A1 terms and(A1 of A2) and compactness, were A2) designed, were thedesigned, respectively, previous respectively, antenna the unconnected was the modifiedunconnected and connectedand andtwo connected antennadifferent to antennasantennathe metallic (A1to the arms.and metallic A2) The were horizontalarms. designed, The arms horizontal ofrespectively, the planararms of dipolethe the unconnectedplanar are folded dipole in and aare spline connectedfolded shape in toa spline shape to occupy the blue part display in Figure 6a and reduce the length (Lsub) of antennaoccupy to the the blue metallic part display arms. inThe Figure horizontal6a and reducearms of the the length planar ( L subdipole) of the are planar folded antenna. in a splinetheBy planar the shape way, antenna. to theoccupy width By thethe of blueway, the partarmsthe width display was of adjusted thein Figure arms to was 6a the and adjusted substrate reduce to width. the substratelength This (L curving subwidth.) of theThisshape planar curving was antenna. obtained shape By wasafter the obtained way, several theoptimized afterwidth several of the simulations, armsoptimized was adjusted and simulations, the size to the of and A2substrate (representedthe size width. of A2 in This(representedFigure curving6b) is shape onlyin Figure 40was mm 6b)obtained ×is 30only mm after 40× mm several10 mm.× 30 optimizedmm × 10 mm. simulations, and the size of A2 (represented in Figure 6b) is only 40 mm × 30 mm × 10 mm.

FigureFigure 6. 6. DesignedDesigned antennas antennas on HFSS: (a)) TheThe ﬁrstfirst miniaturized miniaturized antenna antenna and and the the second second miniaturized miniaturized antenna named A2: 3D FDA; (b) 3D polar plot of the radiation pattern of A2 antenna Figureantenna 6. Designed named A2: antennas 3D FDA; on (bHFSS:) 3D polar(a) The plot first of miniaturized the radiation patternantenna of and A2 the antenna second at the resonant at the resonant frequency (HFSS results). miniaturizedfrequency (HFSSantenna results). named A2: 3D FDA; (b) 3D polar plot of the radiation pattern of A2 antenna at the resonant frequency (HFSS results). TheThe simulations simulations performed performed show show an an operating operating frequency frequency at 1.15 at 1.15 GHz GHz for A1 for and A1 868 and MHz868The MHzfor simulations A2. for However, A2. However,performed there is there ashow small is an again operating small reduction gain frequency reduction of 0.4 dBi at of 1.15comp 0.4 GHz dBiared comparedfor to A1the andprevious to868 the MHzdesignedprevious for A2. antenna, designed However, so antenna, thatthere we is so stilla thatsmall respect we gain still the re respectduction first condition the of ﬁrst 0.4 dBi condition of compa maximumared of a maximumto the+1 dBi previous loss +1 dBion designedtheloss gain on theantenna,from gain a conventional from so that a conventional we still dipole respect dipoleantenna the first antenna.. The condition radiation The radiation of patternsa maximum patterns in the +1 in dBiE-plane the loss E-plane onand thetheand gain H-plane the from H-plane are a conventional presented are presented in dipoleFigure in Figure antenna7. 7. . The radiation patterns in the E-plane and the H-plane are presented in Figure 7.

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FigureFigure 7. 7.Radiation Radiation pattern pattern on on the the E-plane E-plane and and H-plane H-plane at at 868 868 MHz. MHz. (a )(a The) The antenna antenna ﬁrst first 3D 3D dipole dipole antenna antenna in in Section Section 2.1 2.1;; (b)(b The) The modiﬁed modified 3D 3D dipole dipole antenna antenna (A2). (A2).

3.3. Characterization Characterization of of the the Designed Designed Antennas Antennas TheThe designed designed antenna antenna was was manufacturedmanufactured inin an FR4 substrate substrate with with a a thickness thickness of of 0.8 0.8mm. mm.Dip Dipoleole antennas antennas require require a a balanced-unbalanced (balun)(balun) circuitcircuit oror a a microstrip microstrip transitiontransition for for a coaxiala coaxial measurement. measurement. The The balun balun allows allows canceling canceling the the flowing flowing current current on on thethe outside outside surface surface of of the the outer outer coaxial coaxial conductor conductor and and then then affects affects the the measurement measurement [17 [17].]. InIn the the literature, literature, several several dipole dipole antenna-feeding antenna-feeding techniques techniques were were proposed. proposed. A A microstrip microstrip taperedtapered balun balun was was used used as as a feedinga feeding line line in in [18 [18]] and and a microstrip-to-coplanara microstrip-to-coplanar (CPW) (CPW) feed feed networknetwork balun balun for for a flexiblea flexible bowtie bowtie antenna antenna in in [19 [19].]. On On the the other other hand, hand, in in many many electronic electronic devicesdevices especially especially in in IoT, IoT, a surfacea surface mount mount coaxial coaxial U.FL U.FL connector connector is is strongly strongly used used for for characterizationcharacterization and and RF RF connection connection for embeddedfor embedded antennas. antennas. Its advantages Its advantages are its are low its cost, low smallcost, size small and size light and weight. light weight. ToTo well well characterize characterize our our designed designed antenna, antenna, three three feeding feeding methods methods were were selected. selected. The The firstfirst feeding feeding technique technique carried carried in thisin this paper paper is to is use to ause U.FL a U.FL connector connector [20]. The[20]. pads The ofpads the of connectorthe connector were were soldered soldered on the on antenna the antenna feeding feed pads,ing pads, as seen as seen in Figurein Figure8a. 8a. The The second second methodmethod consists consists of of vertically vertically connecting connecting a a designed designed tapered tapered microstrip microstrip balun balun (Figure (Figure8b). 8b). TheThe last last one one is is a microstripa microstrip transition: transition: while while one one pad pad of of the the antenna antenna is is connected connected to to a 50a 50Ω Ω transmissiontransmission line, line, the the other other balanced balanced pad pad is is connected connected to to the the ground ground plane plane(Figure (Figure8c) 8c).. For For thethe return return loss loss (S (S11)11 and) and the the gain gain measurements, measurements, a a compatible compatible coaxial coaxial cable cable was was connected connected toto the the vector vector network network analyzer analyzer (VNA).(VNA). TheThe meas measuredured return return loss loss is is plotted plotted in in Figure Figure 99 (by (byusing using three three different different connection connection methods). methods). They They all allfit fitwell well to toeach each other other but but present present a 15 a 15MHz MHz frequency frequency shift shift compared compared to to the the simu simulation.lation. The samesame frequencyfrequency shiftshift can can be be observedobserved in in Figure Figure 10 10between between simulation simulation and and measurement measurement for the for curved the curved 3D antenna 3D antenna A2. WithoutA2. Without the two the connected two connected arms, arms, the simulated the simulate andd measured and measured return return loss fit loss perfectly, fit perfectly, but oncebut theyonce are they connected, are connected, a 50 MHz a 50 frequencyMHz frequency shift appears. shift appears. The difference The difference is mainly is mainly due todue the solderingto the soldering effect and effect the verticaland the capacitivevertical capacitive arms whose arms dimensions whose dimensions are not perfectly are not respectedperfectly as respected compared as withcompared the simulated with the simulated ones. The ones. A2 antenna The A2 wasantenna measured was measured with a U.FLwith connector a U.FL connector and resonates and resonates at 878 MHz, at 878 as MHz, shown as inshown Figure in 10Figure. The 10. measured The measured and simulatedand simulated gain in gain the E-planein the E-plane are shown are shown in Figure in Figure 11. 11. Sensors 2021, 21, 3193 7 of 12 Sensors 2021, 21, x FOR PEER REVIEW 7 of 12 Sensors 2021, 21, x FOR PEER REVIEW 7 of 12 Sensors 2021, 21, x FOR PEER REVIEW 7 of 12

Figure 8. (a) Antenna with a U.FL connector named AC; (b) Antenna with connected tapered FigureFigure 8.8.( (aa)) AntennaAntenna withwith a a U.FL U.FL connector connector named named AC; AC; ( b(b)) Antenna Antenna with with connected connected tapered tapered balun balun named ATB; (c) Antenna with integrated microstrip transition named AIT. Figurenamedbalun named8. ATB; (a) Antenna (c ATB;) Antenna (c with) Antenna with a U.FL integrated with connector integrated microstrip named micr transitionAC;ostrip (b) transition Antenna named withnamed AIT. connected AIT. tapered balun named ATB; (c) Antenna with integrated microstrip transition named AIT.

Figure 9. Comparison of the measured return loss (S11) of the D1 antenna with different feeding Figure 9. Comparison of the measured return loss (S11) of the D1 antenna with different feeding methods.Figure 9. Comparison of the measured return loss (S11) of the D1 antenna with different feeding Figuremethods. 9. Comparison of the measured return loss (S11) of the D1 antenna with different feeding methods.

Figure 10. Measured and simulated reflection coefficient of the A1 and A2 antennas. Figure 10. Measured and simulated reflection coefficient of the A1 and A2 antennas. Figure 10. Measured and simulated reflection reﬂection coefﬁcientcoefficient of the A1 and A2 antennas.

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FigureFigure 11.11. Simulated (dashed line) line) and and measured measured (continuous (continuous line) line) radiation radiation pattern pattern (gain (gain plot plot in in thethe E-plane):E-plane): (a) AC antenna; ( b)) A2 A2 antenna antenna at at the the resonant resonant frequency frequency (868 (868 MHz). MHz).

TableTable1 1 compares compares inin termsterms ofof performancesperformances andand sizesize somesome state-of-the-artstate-of-the-art antennas andand thethe presentedpresented solution operating in in ISM ISM 868 868 MHz MHz or or ISM ISM 915 915 MHz MHz frequency frequency bands. bands. TheThe proposedproposed compactcompact 3D3D antennaantenna waswas implemented implemented on on an an FR4 FR4 substrate substrate that that is is lossier lossier as comparedas compared with with the the substrates substrates used used by by other other state-of-the-art state-of-the-art designs. designs.

TableTable 1. 1.Comparison Comparison withwith thethe differentdifferent compactcompact antennasantennas for rectennarectenna of IoT devices in in the the state state of of the the art. art.

Ref.Ref. Freq. (MHz)(MHz) TypeType MaxMax Gain Gain (dBi) (dBi) BWBW (MHz) (MHz) Substrate Substrate Size Size (mm (mm × ×mmmm × ×mm)mm) [21[21]] 878878 Dual Dual Band Band PIFA PIFA +1.8–+1.9+1.8–+1.9 8080 (855–937) (855–937) Duroid Duroid 5880 5880 8080 × ×4545 (0.03· (0.03λ·2λ)2 ) [22[22]] 868868 UCA UCA PIFA PIFA +0.71+0.71 2323 (857–880) (857–880) FR4 FR4 3434 × ×8080 (0.02· (0.02λ·2λ)2 ) 3 [16[16]] 915 915 Slot Slot loadedloaded DB DB folded folded dipole dipole +1.87 +1.87 Not Not available available Arlon Arlon 25N 25N 6060 × ×6060 × 60× 60(0.006· (0.006λ·3λ) ) [23] 868 3D single arm bowtie +0.19 90 Ultralam 50 × 50 × 10 (0.0006·λ3) [23] 868 3D single arm bowtie +0.19 90 Ultralam 50 × 50 × 10 (0.0006·λ3) 3 +1.54+1.54 3232 (865–897) (865–897) 5656 × ×3232 × 10× 10(0.0004· (0.0004λ·3λ) ) ThisThis work work 868 3D modiﬁedmodified T-match T-match dipole dipole FR4FR4 +1.1+1.1 2626 (862–888) (862–888) 4040 × ×3030 × 10× 10(0.0003· (0.0003λ·3λ)3 ) BW:BW: Bandwidth, Bandwidth, PIFA: PIFA: Planar Planar Inverted F-Antenna, F-Antenna, DB: DB: Dual Dual Band. Band.

4.4. RectennaRectenna and Wireless Power Transmission Experimentations TheThe antennasantennas presented presented in in this this paper paper are are more more compact compact than than the 3D the antennas 3D antennas in [16 ,23in ], and[16,23], exhibit and the exhibit best tradeoffthe best betweentradeoff size,between gain size, and bandwidthgain and bandwidth and can be and good can candidates be good forcandidates WPT applications. for WPT applications. They were They integrated were integrated in rectennas in rectennas used for used battery-free for battery-free wireless sensorswireless embedded sensors embedded in concrete in concrete structures structures [24,25]. [24,25]. TheThe rectifierrectifier partpart isis designeddesigned usingusing ADS/MomentumADS/Momentum software software from from Keysight. Keysight. The The circuitcircuit isis designeddesigned on a microstrip coupled coupled transmission transmission line line allowing allowing differential differential feeding feeding byby thethe dipoledipole antenna (Figure 12).12). It is composed of of a a SMS7630-005LF SMS7630-005LF Schottky Schottky diode, diode, an an LCLC matchingmatching network (L1-C1) ensuring 50 50 ΩΩ inputinput impedance impedance at at the the input input of of the the rectifier rectifier andand aa lowlow pass filterfilter formed by a a shunt shunt capaci capacitortor (C3) (C3) and and aa resistive resistive load load (R1). (R1). However, However, consideringconsidering the nonlinear behavior of of the the Schottky Schottky diode, diode, the the rectifier rectifier was was designed designed and and optimizedoptimized for a a low low input input power power of of−15− dBm15 dBm and and a 10 akΩ 10 resistor kΩ resistor to emulate to emulate the load the (that load (thatis the is input the input impedance impedance of the of power the power management management unit (PMU)). unit (PMU)). The simulation The simulation result resultwas wasperformed performed and anddescribed described in [24]. in [24 Two]. Two rectennas, rectennas, R1 R1and and R2, R2, in inFigure Figure 12 12 represent represent the the rectifierrectifier withwith the the antenna antenna AC AC without without any any connector connector and and A2, A2, respectively. respectively. They They were were made bymade integrating by integrating the rectifier the withrectifier the antennaswith the on antennas the same substrate.on the same Their substrate. characterization Their wascharacterization performed in was an anechoicperformed chamber in an anechoic at the distance chamber of 1.5at the m fromdistance the patch-transmittingof 1.5 m from the antenna,patch-transmitting and the harvested antenna, and DC the power harvested was measured DC power across was measured a 10 kΩ acrossload through a 10 kΩ a multimeter.load through The a multimeter. rectennas R1 The and rectennas R2 allow R1 a harvestedand R2 allow DC a voltage harvested of 788 DC mV voltage and 726of 788 mV formV 1 andµW/cm 726 mV2. for 1 µW/cm2.

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SensorsSensors 20212021, 21, 21, ,x 3193 FOR PEER REVIEW 9 of 12 9 of 12

Figure 12. Manufactured rectenna with the rectifier schematic.

The conversion efficiency as a function of the power density is presented in Figure FigureFigure 12. 12.Manufactured Manufactured rectenna rectenna with the with rectifier the rectifier schematic. schematic. 13. It is obtained by computing the formula given in Equation (2), where: The conversion efficiency as a function of the power density is presented in Figure 13. - 𝑃 Theis the conversion harvested DCefficiency power obtainedas a function across of the the load; power density is presented in Figure It is obtained by computing the formula given in Equation (2), where: - 13.𝐺𝑡 Itand is obtained 𝐺𝑟 are the by transmitting computing theantenna formula gain given and the in receiverEquation antenna (2), where: gain; - - 𝜆 isPDC theis wavelength the harvested at DC the power operating obtained frequency; across the load; - - Gt𝑃and isGr theare harvested the transmitting DC power antenna obtained gain and across the receiver the load; antenna gain; - 𝑆 is the electromagnetic (EM) power density and 𝑃 is the RF power from the source; - λ 𝐺𝑡is the wavelength𝐺𝑟 at the operating frequency; - - 𝑑 is the and distance are between the transmitting the transmitting antenna and gain receiving and the antennas. receiver antenna gain; - - S is𝜆 the is the electromagnetic wavelength (EM) at the power operating density frequency; and Pt is the RF power from the source; The variation by decade shows a difference between R1 and R2 of 13.67%, 6.01% and - - d is𝑆 the is the distance electromagnetic between the transmitting (EM) power and density receiving and antennas. 𝑃 is the RF power from the source; 1.48%. The difference can be explained by the non-linearity of the used Schottky diode. - The𝑑 variation is the distance by decade between showsa the difference transmitting between and R1and receiving R2 of 13.67%, antennas. 6.01% and By1.48%. reducing The differencethe planar can antenna be explained size of by 30%, the non-linearity we reduce ofthe the antenna used Schottky gain of diode. 0.44 BydBi and thenreducing the TheRF-DC the variation planar conversion antenna by decade efficiency size of shows 30%, of we 13% a reducedifference with the a antennapower between density gain R1 of and 0.44of 3.1 dBiR2 µW/cm of and 13.67%, then2 at 8686.01% and MHz.the1.48%. RF-DCThe secondThe conversion difference rectenna efficiency canR2 willbe of 13%explainedbe preferre with a powerbyd in the cases density non-linearity where of 3.1 theµW/cm powerof the2 at densityused 868 MHz. Schottky is very diode. lowTheBy (e.g., second reducing 31 nW/cm rectenna the2). R2planar For will our be antenna application, preferred size in cases inof the 30%, where aim we of the reducesupplying power densitythe the antenna ispower very lowgainmanagement (e.g., of 0.44 dBi and 2 unit31then , nW/cmwhich the requiresRF-DC). For our conversiona application,DC voltage efficiency inof the600 aim mV of of at supplying13% least, with R1 thewasa power power chosen. density management of 3.1 unit, µW/cm2 at 868 which requires a DC voltage of 600 mV at least, R1 was chosen. MHz. The second rectenna R2 will be preferred in cases where the power density is very 𝑃 4∙𝜋 ∙ 𝑃 𝐸 30 ∙ 𝑃 ∙ 𝐺 low (e.g., 31𝜂 nW/cm= 100 ∙2P). For= our 100 application, ∙4·π·P 𝑤𝑖𝑡ℎin the 𝑆 Eaim=2 of supplying30·=P ·G the power management(2) = · 𝑃DC = · 𝑆 ∙ 𝐺dc ∙𝜆 = 120.= 𝜋 t 𝑑t ∙ 120 ∙ 𝜋 η 100 100 2 with S 2 (2) unit , which requiresP aRF DC voltageS·Gr· λof 600 mV at120 least,·π R1d ·was120·π chosen. 𝑃 4∙𝜋 ∙ 𝑃 𝐸 30 ∙ 𝑃 ∙ 𝐺 𝜂 = 100 ∙ = 100 ∙ 𝑤𝑖𝑡ℎ 𝑆 = = (2) 𝑃 𝑆 ∙ 𝐺 ∙𝜆 120. 𝜋 𝑑 ∙ 120 ∙ 𝜋

FigureFigure 13. 13. RF-DCRF-DC conversion conversion efficiency efﬁciency for for the the rect rectennasennas atat 868868 MHzMHz for for various various decade decade power power densities.densities.

InIn the the last last setset of of tests, tests, underrun underrun in our in laboratory, our laboratory, an innovative an innovative Bluetooth Low Bluetooth Energy Low (BLE) battery-free wireless sensor network (composed of battery-free sensing nodes and a EnergyFigure (BLE) 13. RF-DC battery-free conversion wireless efficiency sensor for networkthe rectennas (composed at 868 MHz of battery-freefor various decade sensing power communicating node [26]) was operated in a harsh environment (Figure 14). Once embed- densities. nodesded, and the maina communicating objectives were node to periodically [26]) was monitoroperated the in physical a harsh data environment (e.g., temperature (Figure 14). Onceand embedded, humidity) of the the main concrete objectives and broadcast were to them periodically (by BLE) to monitor the communicating the physical node. data (e.g., temperatureIn the and last humidity) set of tests, of underrunthe concrete in ourand laboratory,broadcast theman innovative (by BLE) Bluetooth to the Low communicatingEnergy (BLE) node. battery-free wireless sensor network (composed of battery-free sensing nodes and a communicating node [26]) was operated in a harsh environment (Figure 14). Once embedded, the main objectives were to periodically monitor the physical data (e.g., temperature and humidity) of the concrete and broadcast them (by BLE) to the communicating node.

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FigureFigureFigure 14.14. 14. PhotographPhotograph Photograph ofof of thethe the experimental experimental setupsetup for for the the sensing sensing node node embedded embedded and andand powered poweredpowered by by usingusingusing WPTWPT WPT systemsystem system inin in thethe the concreteconcrete concrete structure.structure. structure.

TheTheThe sensingsensing sensing nodenode node inin in FigureFigure Figure 1515 15 is is is composedcomposed composed ofof of aa a rectennarectenna rectenna (including(including (including thethe the compactcompact compact 3D3D 3D antenna),antenna),antenna), an anan ultra-low-power ultra-low-powerultra-low-power BLE BLEBLE System SystemSystem on on Chipon ChipChip (SoC) (SoC)(SoC) (NXP (NXP(NXP QN9080) QN9080)QN9080) [27], a[27],[27], power aa powerpower man- agementmanagementmanagement unit microcontrollerunit unit microcontroller microcontroller (Texas (Texas (Texas Instruments Inst Instrumentsruments bq25570) bq25570) bq25570) [28], a capacitor[28], [28], a a capacitor capacitor of 100 µ of ofF 100 acting100 µF µF asactingacting an energy asas anan storage energyenergy element storagestorage (Panasonic elementelement (Pan EEEFK0J101P)(Panasonicasonic EEEFK0J101P)EEEFK0J101P) [29] and low-power [29][29] andand temperature low-powerlow-power andtemperaturetemperature humidity digitalandand humidityhumidity sensors (Texas digitaldigital Instruments sensorssensors (Texas(Texas HDC2080) InstrumentsInstruments [30]. The capacitorHDC2080)HDC2080) was [30].[30]. chosen TheThe aftercapacitorcapacitor evaluating waswas chosenchosen the power afterafter needed evaluatingevaluating by the thethe PMU popowerwer (870 neededneededµJ) to by startupby thethe PMUPMU from (870 deep-sleep(870 µJ)µJ) toto startupstartup mode. Throughfromfrom deep-sleepdeep-sleep a far ﬁeld mode.mode. WPT ThroughThrough system, theaa farfar illuminating fieldfield WPTWPT system,system, power over thethe illuminatingilluminating a distance of powerpower two meters overover aa candistancedistance be harvested of of two two meters meters by the can can rectenna be be harvested harvested and allow by by the poweringthe rectenna rectenna the and and sensing allow allow node.powering powering Initially the the sensing empty,sensing thenode.node. capacitor Initially Initially is empty, empty, charging the the through capacitor capacitor the is is PMUchargi chargi onngng deep-sleep through through the the mode PMU PMU up on on to deep-sleep deep-sleep 1.5 V and switchesmode mode up up ontoto 1.5fast1.5 VV charging andand switchesswitches until the onon faststoredfast chargingcharging voltage untiluntil reaches thethe stored 5.3stored V. Once voltagevoltage it has reachesreaches enough 5.35.3 energy V.V. OnceOnce stored itit hashas byenoughenough the capacitor, energyenergy stored itstored discharges byby thethe and capacitor,capacitor, allows it poweringit dischargesdischarges the and activeand allowsallows component poweringpowering on thethethe board activeactive (sensorscomponentcomponent and on on BLE the the chip) board board for (sensors (sensors a measurement and and BLE BLE chip andchip) data) for for a transmission.a measurement measurement and and data data transmission. transmission.

FigureFigureFigure 15.15. 15. DevelopedDeveloped Developed sensingsensing sensing nodenode node forfor for BLEBLE BLE communication.communication. communication.

PreliminaryPreliminaryPreliminary tests, tests,tests, with withwith an anan EIRP EIRPEIRP power powerpower of ofof +33 +33+33 dBm, dBm,dBm, allow allowallow a aa periodicity periodicityperiodicity of ofof charging, charging,charging, measuringmeasuringmeasuring and andand wireless wirelesswireless communication communicationcommunication equal equalequal to at toto most atat mostmost 190 s, 190 representing190 s,s, representingrepresenting the ﬁrst thethe charge firstfirst durationchargecharge duration duration (cold start (cold (cold + fast start start charging + + fast fast charging charging + data measurement + + data data measurement measurement and transmission). and and transmission). transmission).

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5. Conclusions An electrically small antenna operating in the ISM 868 MHz frequency band and dedicated to wireless power transmission applications is proposed in this paper. Its design is based on a folded dipole antenna with a shorting line to form a T-match antenna. A 3D configuration allows the exploitation of the Z-plane in the final implementation. Thus, the connection of two metallic arms was operated by the planar antenna to downshift the operating frequency from 1.09 GHz to 868 MHz. The design was modified to apply a spline shape to the planar antenna arms in order to reduce the overall dimensions (in the horizontal plane) from 56 mm × 32 mm to 40 mm × 30 mm. For the characterization, three feeding methods were used to provide accurate experimental results. In this application, the U.FL connector can be used for measurement but may have limitations at higher frequencies. The experimental results confirm that the proposed 3D antenna exhibits a gain of +1.54 dBi and +1.1 dBi at the operating frequency, respectively, for AC and A2. As shown in Table1, a tradeoff between the compactness, the maximal gain and the bandwidth should be performed before the design. Our solution is also the most compact and has a maximum gain higher than +1 dBi at the price of a reduced bandwidth. The combination of these antennas and an optimized rectifier performs a high efficiency of 73.88% and 60.31%, respectively, for R1 and R2 with an illuminating power density of 3.1 µW/cm2. Future works with a WPT system associated with a battery-free wireless sensor network using BLE are in progress.

Author Contributions: Conceptualization, A.S. and A.T.; methodology, A.S.; software, A.S.; valida- tion, A.S. and G.L.; investigation, A.T.; writing—original draft preparation, A.S, A.T., G.L. and D.D.; writing—review and editing, A.S., A.T., G.L. and D.D.; supervision, D.D.; project administration, A.T. and D.D.; funding acquisition, A.T. and D.D. All authors have read and agreed to the published version of the manuscript. Funding: Region OCCITANIE, France in the frame of the OPTENLOC project, supported this research. The system tests were performed in the frame of the ANR McBIM project. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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