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WE2B-6

A HIGH CONV ERSION EFFICIENCY 5.8 GHZ RECTENNAJames 0 McSpadden, Lu Fan, and Kai Chang

Department of Electrical EngineeringTexas A M University

College Station, Texas 77843-3 128ABSTRACT

A high efficiency rectenna element has beendesigned and tested at 5.8 GHz for applicationsinvolving microwave power transmission. Thedipole antenna and filtering circuitry are printedon a thin duroid substrate. A silicon Schottky-barrier mixer diode with a low breakdownvoltage is used as the rectifLing device. Therectenna element is tested inside a waveguidesimulator and achieves an RF to DC conversionefficiency of 82% at an input power level of 50mW. The antenna and circuit design is based ona full-wave electromagnetic simulator.

I. INTRODUCTIONThe rectenna is coined from the wordsrectifLing and antenna. This receivingantenna efficiently converts microwave powerinto direct current (DC) power. A rectenna is afundamental component in microwave powertransmission systems. Microwave powertransmission has applications in solar powersatellites, remotely powered vehicles, andpowering high altitude communication andsurveillance platforms [13, [2].Components for microwave power transmissionhave traditionally been focused at 2.45 GHz dueto its low attenuation through the atmosphere,low cost technology base, and location at thecenter of an ISM band. The next higher ISMband is centered at 5.8 GHz. This frequency isappealing for beamed power transmission due to

5470-7803-3814-6/97/$5.00 EEE

smaller component sizes and a greatertransmission range over 2.45 GHz. Theefficiency at which rectennas convert themicrowave energy into D is the rectennascritical figure of merit. Aluminum bar-typerectennas developed in the 1970s achievedconversion efficiencies greater than 90% at 2.45GHz [3]. Later, a thin-film class of rectennadesign was developed at 2.45 GHz whereconversion efficiencies of 85% were achieved~41.The first C-band rectenna achieved a 70% overallefficiency and a 80% conversion efficiency at5.87 GHz [5]. These efficiencies were measuredin a waveguidle simulator with a input powerlevel approximately 700 mW per element. ThisC-band rectenna used a printed dipole that fedinto a Si Schottky diode quad bridge. However,little information is provided on the design of thematching circuit between the dipole antenna andthe diode.In testing rectennas in a waveguide simulator, anoverall efficiency(q,) and a conversion efficiency,qc)are defined as

DC Output PowerIncident RF PowerDC Output Power

7 =

Incident RF Power Reflected RF PowerMeasurements performed in a waveguidesimulator can accurately monitor reflectedpower.

1997 IEEE MTT-S Digest

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The rectenna element developed in this paperoperates efficiently (>8O ) at much lowerincident power levels of 30 to 60 mW. Thischaracteristic has two important applications inmicrowave power beaming systems: 1 powercan be converted efficiently at the edge of therectenna where power densities are lower thanthe center elements, and 2 power can beconverted efficiently when the transmissiondistance is large and power density is low.

II RECTENNA E LEMENT DESIGN

DipoleAntenna

Low PassFilter RF Short Capacitor

Fig. 1. 5.8 GHz rectenna element,The structure of the printed rectenna element isshown in Figure 1. The entire element iscomposed of a two plane format where theprinted rectenna circuit is placed horizontally to ametal reflecting plane. The horizontal dipoleantenna and coplanar stripline (CPS)transmission lines are printed on one side of 10mil Rogers 5880 duroid ( E ~ 2.2). A low passfilter composed of three printed strips on theopposite side of the coplanar stripline (CPS)transmission lines passes the operating frequencyof 5 8 GHz and rejects higher order harmonicsproduced by the rectifLing diode. The filter alsotransforms the input impedance of the dipoleantenna to the input impedance of the diode. A47 pF chip capacitor is used to effectively shortthe RF energy and pass the DC power to aresistive load. The chip is also used to maximizethe diodes conversion efficiency. The distancebetween the diode and the chip constitutes aninductance which tunes the capacitance of thediode. The resistive load typically 1.2 to 1.3times the diode input resistance is then placed atthe output CPS terminals [6]. Both the low passfilter and the chip output capacitor are used tostore RF energy during the off period of thediode. A reflecting metal plane is then placedbehind the substrate typically 0.2 A.

A commercially available full waveelectromagnetic simulator, IE3D was used todesign the dipole, CPS, and low pass filter [7].The steps taken to design the rectenna begin atthe dipole. The length and input resistance of theresonant dipole were first simulated to beapproximately 26.5 mm and 92 SZ respectively.The gap between the dipole terminals isdetermined by the characteristic impedance of theCPS. This gap is designed to accommodate thesize of the diode package. Using IE3D thecalculated characteristic impedance for a 2.1 mmgap and 1.6mm wide strips is 254 a. The diodeinput impedance is taken to be 254 R o reducethe loss associated with the built-in voltage of thediode. The low pass filter is designed to pass 5.8GHz and transform the 92 t dipole impedanceto the 254 R diode impedance. E 3 D was usedin an iterative process to correctly size the threelow pass filter strips and the spacing between thestrips to achieve this characteristic. Theefficiency loss caused by the built-in voltage Vb)drop across the Schottky barrier is approximatelygiven by [3]

vvdc 4 v

Loss =

where Vdc is the voltage measured across theresistive load. Forcing the diode inputimpedance to be large will decrease the lossassociated with Vb.

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A MA-COM Si Schottky diode (MA40150- 119)used for X-band mixer applications is therectifying device. The measured built-in andbreakdown voltages at 10 pA are 0.224 V and4.44 V respectively. This low breakdownvoltage limits the diode's power handlingcapabilities.

III. MEASUREMENTSA waveguide simulator was designed to test andmonitor the rectenna element. The shortdimension of a standard WR 137 waveguide wasexpanded to equal the wide dimension. Thus, therectenna is placed in a square opening (3.485 cmx 3.485 cm). Based on the cutoff frequency ofthe WR 137 waveguide and the operatingfrequency of 5.8 GHz the angle of incidence iscalculated to be 47.9 from boresight [6]. Thus,the rectenna efficiency proves to be very non-directional. In applications where the rectenna isplaced on a moving target, this aspect allows therectenna to receive power efficiently at variousangles of incidence.Figure 2shows the measurement setup. The P83622A synthesized sweeper provides the inputRF power and allows the power and frequency tobe varied. In addition to built-in isolators of thesweeper, a coaxial isolator provides additionalprotection from the reflected energy. A coax towaveguide adapter is used to connect to awaveguide slotted line to monitor the reflectedRF power. VSWR measurements by a voltmeterare taken using the mounted detector on thecarriage. The waveguide expander transitionsfrom WR 137 waveguide to the square output ofthe waveguide simulator. The rectenna elementis placed between the waveguide expander and asquare waveguide. This square waveguidehouses an adjustable reflecting plane to tune therectenna element.

0 8 4 e q - pHP 83622A Waveguide ISynthesized Slotted Line Waveguide TunerSweeper Carriage Expander

Fig. 2. Measurement setup.Figure 3 shows overall and conversionefficiencies of the rectenna using a 326 R resistorload. A placement of 11.1 mm (0.21 h,measured between the back of the substrate andthe reflected plane resulted in the highestrectenna efficiency. A maximum conversionefficiency of 82% is achieved at an input powerof 50 mW. The differences between the overallefficiency and conversion efficiency indicate verylittle power is reflected from the measurementsystem.

I Conversion Efficiency0 10 20 30 40 50 60 70Input Power (mw)

326 oad.Fig. 3. Rectlenna performance at 5.8 CiHz with a

Figure 4 shows the effects of varying the resistiveload on the rectenna at 5.8 GHz. The range ofloads used indicate very small changes inconversion eEciency. Overall efficiency wasalso monitored but again there was littledifference from conversion efficiency. Thus, therectenna element converts power efficiently whenthe load varies by 10%.

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V ACKNOWLEDGMENTS80

s-60820s o

g 403CA

0 10 20 30 40 50 60 70Input Power (mw)Fig. 4. Conversion efficiency comparison.

Figure 5 shows the rectenna conversionefficiency when the frequency is varied at aconstant input power of 5 5 mW. Changes infrequency influences rectenna performance byelectrically changing the dipole impedance andreflecting plane spacing. A maximum conversionefficiency of 81% occurs at 5.775 GHz. Theefficiency remains above 75% over the entireISM band located between 5.725 GHz to 5.875GHz.

90 ?iw 80706050

5.7 5.75 5.8 5.85 5.9Frequency (GHz)Fig. 5 Frequency variation.

IV. CONCLUSIONS

This research was supported in part by theNASA Center for Space Power and the ArmyResearch Office. The evaluation copy of IE3Dby Zeland Software is acknowledged.

REFERENCES[ l ] W. C. Brown, The history of powertransmission by radio waves, IEEE Trans.Microwave n e o r y Techn. vol. MTT-32, no.9, pp. 1230-1242, Sept. 1984.

[2] J. J. Schlesak, A. Alden, and T. Ohno, Amicrowave powered high altitude platform,in I988 IEEE MTT-S InternationalMicrowave Symposium Digest 1988, pp.

[3] W. C. Brown, Electronic and mechanicalimprovement of the receiving terminal of afree-space microwave power transmissionsystem, Raytheon Company, Wayland, MA,Tech. Report PT-4964, NASA Report No.

[4] W. C. Brown and J. F. Triner, Experimentalthin-film, etched-circuit rectenna, in 1982IEEE MTT-S International MicrowaveSymposium D igest 1982,pp. 185-187.

[5] S . S. Bharj, R. Camisa, S. Grober, F.Wozniak, and E. Pendleton, High efficiencyC-band 1000 element rectenna array formicrowave powered applications, in 1992IEEE MTT-S International MicrowaveSymposium Digest 1992, pp. 301-303.

[6] W. C. Brown, Rectenna technologyprogram: ultra light 2.45 GHz rectenna and20 GHz rectenna, Raytheon Company,Waltham, MA, Tech. Report No. PT-6902,

283-286.

CR-135194, Aug. 1977.

A rectenna element has been developed which NASA Report No. CR-179558, March 1987.has the highest recorded conversion efficiency of [7] E3D, version 3.0, Zeland Software, Inc.,82% at 5.8 GHz with an input power of 50 mW. Fremont, CA.The circuit design was based on electromagnetic [SI P . W. Hannan and M. A. Balfour,Simulation of a phased-array antenna inand circuit analysis and accurately tested in a waveguide, IEEE Trans. Antennas andclosed environment. Propagat. vol. AP-13, pp. 342-353, May

1965.

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