Low-Profile Circularly Polarized Rectifying Antenna for Wireless Power Transmission at 5.8 GHz
Abstract
Portable devices operating without batteries aresometimes desired in wireless applications such as RFID (RadioFrequency Identification) and telemetry. A novel circularlypolarized shorted annular ring-slot rectenna (rectifying antenna)on a 0.5 mm thick flexible microwave laminate is proposed for thepowering of these devices. Output dc voltage of 1.3 V and axialratio of 1.5 dB were measured when 32 dBm microwave powerwas transmitted at 5.8 GHz over a distance of 2 m. Comparing theresults with a rectenna designed on a thick rigid laminate, similarperformance was observed.Index Terms—Circular polarization, rectifier, slot antenna.
I. INTRODUCTION
ARECTENNA is a RF power receiver that converts the receivedpower into dc power that can then be consumed,e.g., by an active RFID tag [1] integrated into the rectenna structure.The advantage of a circularly polarized (CP) rectenna overa linearly polarized one is that nearly constant dc output canbe achieved even if the rectenna’s rotation angle, relative to thetransmitter, changes.Several antennas for CP radiation have been designed overthe last few years using a circular ring, e.g., with stripline hybridcoupler feed at 2.45 GHz [2], with microstrip feed at 1.5GHz [3], and with coplanar waveguide and coaxial line feed at2.45 GHz [4]. To the best of our knowledge, few rectenna forCP radiation have been proposed so far [5], [6]. Furthermore,these rectennas are intended for the operation at relatively highpower densities.An annular ring-slot rectenna designed for low power densitiesand operating at 5.2 GHz was introduced in [7]. In thisletter, an enhanced design operating at 5.8 GHz is proposed andthe performance of the two designs is compared.II. TEST STRUCTURERectenna was designed on a thin high-frequency laminateRO3003 ( , , , copperthickness 35 ). The layout of the rectenna is shown in Fig. 1.Antenna and rectifier (interface indicated with a dashed line inFig. 1) were initially designed, manufactured and measured separatelyin order to validate their performance prior to the realizationof the complete rectenna.A shorted annular ring-slot structure, first introduced in [8],was utilized as an antenna element. Antenna feed was composedof a transmission line TL1 and two quarter-wave transformersManuscript received June 5, 2003; revised October 28, 2003.The authors are with the Tampere University of Technology, Institute of Electronics,FIN-33101 Tampere, Finland (e-mail: jouko.heikkinen[at]tut.fi).Digital Object Identifier 10.1109/LMWC.2004.827114TL2 and TL3. Antenna was designed for 50 input impedancefor measurement purposes. The mean slot radiuswas preliminarily designed so that the slot wavelength [9] atthe desired center frequency 5.8 GHz was equal to the mean slotcircumference(1)Such a structure was simulated using MOM (Method of Moments)so that the input impedance was calculated at the outeredge of the slot (at the end of TL1). It was found out that (1)predicted the vicinity of the antennas first resonant frequencyat reasonable accuracy, but the frequency for the optimum CPradiation was about 1.4 times higher. That was mainly because(1) does not include the effect of the ring-slot feed. Mean slotradius, slot width and short angle were thereforevaried in simulations to find the minimum axial ratio at 5.8 GHz.All simulations were performed with ADS (Advanced DesignSystem) Momentum using strip metallization layers. Final dimensionsof the antenna are listed in Table I.Rectifier was composed of a microwave Schottky detectordiode pair D, a bypass capacitor C, a load resistor R and achoke inductor L. The diode pair (HSMS-2862) was connectedin a shunt-series manner (so-called voltage doubler circuit) inorder to lower the input impedance and maximize the output dcvoltage. The input impedance of the rectifier circuit was transformedinto 50 with transmission lines TL4 and TL5 andtwo symmetrical sections of an open Stub. Rectifier was designedwith ADS EM/Circuit co-simulation feature using stripmetallization layers. Component values and dimensions of thematching elements are given in Table I.III. RESULTSSeparate antenna and rectifier as well as the completerectenna were manufactured by wet etching. Measured andsimulated input return loss of the antenna and the rectifier(input power 5 dBm) are shown in Fig. 2. Theoreticaland experimental results agreed fairly well and a quite goodimpedance match was achieved in the vicinity of 5.8 GHz.The difference between simulated and measured results wasmainly due to manufacturing tolerances and an error due to theconnector used in measurements.Antenna performance for CP radiation was tested in an anechoicchamber. A linear gain of 4 dB and an axial ratio of 2 dBwere measured. A CP gain corresponding to these results wastherefore 6.1 dB [10]. Performance was quite similar with a5.2 GHz antenna designed on a nearly three times thicker rigidlaminate


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