A Single-Feeding Circularly Polarized Microstrip Antenna With the Effect of Hybrid FeedingHyungrak Kim,Byoung Moo Lee,and Young Joong Yoon ,Member,IEEEAbstract—In this paper,a single series feeding cross-aperture coupled microstrip antenna with the effect of hybrid feeding is pro-posed and demonstrated.To better understand this antenna,the characteristics according to the variation of parameters are shown.This proposed antenna has the following advantages of the effect of hybrid feeding,improved axial ratio bandwidth (4.6%),high gain (8dBi),and flat 3-dB gain bandwidth (above 16.7%).In measured radiation patterns,we have 3-dB beamwidthof 30and good F/B of 20dB.Index Terms—Effect of hybrid feeding,microstrip antenna.I.I NTRODUCTIONWITH rapid development of wireless communication system,many kinds of circularly polarized (CP)antennas have been studied since CP antennas are often preferred in satellite communication,Global Positioning System (GPS),and radar system.In general,feeding structure of CP antenna may be divided into single and hybrid feeding.A single-feeding CP antenna provides simple structure,easy manufacture,and advantage in array with small size.However,it has narrow axial ratio bandwidth.Hybrid feeding gives complex structure,difficult manufacture,and increased antenna size,but it provides wide axial ratio bandwidth.Thus,in the design of CP antenna,a tradeoff of characteristics between two feeding methods is required.In CP antenna,axial ratio bandwidth is the most important factor in design since it is the most limiting factor for oper-ating factor.Therefore,many kinds of CP antennas have been studied to obtain wide axial ratio bandwidth [1]–[4].Recently,CP antennas to obtain wide axial ratio bandwidth using single feeding have been studied to improve disadvantages of hybrid feeding,e.g.,large antenna size and complex structure.Cross-aperture coupled microstrip antennas [5],[6]were proposed and analyzed,but it still has narrow axial ratio bandwidth (2.5%),narrow gain bandwidth (3.27%for 3-dB),and low antenna gain (5dBi).Another improvement was suggested by Aloni et al.[7],where traveling wave type CP antenna was introduced.How-ever,it has very low gain and low radiation efficiency,and nar-rower gain bandwidth than reasonably wide axial ratio band-width and impedance bandwidth.Therefore,not only wide axial ratio and impedance bandwidth,but also other enhanced charac-teristics,e.g.,high gain,flat-gain bandwidth,and similar radia-tion patterns in operating frequencies are needed in CP antenna for practical wireless communication system.Manuscript received February 20,2003;revised April 9,2003.The authors are with the Department of Electrical and Electronic Engi-neering,Yonsei University,Seoul,Korea (e-mail:okebari@mwnat.yonsei.ac.kr;binny@mwnat.yonsei.ac.kr;yjyoon@mwnat.yonsei.ac.kr).Digital Object Identifier10.1109/LAWP.2003.813382(a)(b)Fig.1.(a)Side view and (b)bottom view of the proposed antenna.In this paper,we propose a resonant type single series feeding CP microstrip antenna.Series feeding is suggested to obtain wide axial ratio bandwidth and flat gain bandwidth.Also,cross-aperture with short length is used to provide high gain.II.A NTENNA D ESIGNThe configuration of the proposed antenna is shown in Fig.1.It is composed of the two layers and air-gap.The rectangular patch,whose physical dimensions are 45mm 45mm at center frequency of 2.4GHz,is on the upper layer,and series feeding line under the lower layer is positioned close behind cross-aper-ture.For the upper and lower layer,Duroid 5880substrate with 0.5-oz copper,62-mil substrate height,and dielectric constant of 2.2and FR-4substrate with 1-oz copper,0.8-mm substrate height,and dielectric constant of 4.6are used,respectively.As shown in Fig.1(b),series feeding line is placed behind cross-aperture,and a quarter-wavelength section of feeding line is positioned between each arm of aperture to create the 90phase difference for circular polarization.Series feeding brings into sequential rotation of current on the surface of radiating1536-1225/03$17.00©2003IEEEpatch.Therefore,the proposed feeding structure has the effect of hybrid feeding without external hybrid components.Also,we made efforts to achieve high gain.In this study,by shorter aperture length than that of reported researches [5],[6],front to back ratio (F/B)of the proposed antenna is enhanced.As a result,we achieve F/B over than about 20dB at center frequency.Basically,the reciprocity method,as developed by Pozar [8],has been used for analyzing a conventional rectangular aperture antenna,where the aperture loadimpedance ,as seen from the feeding line,was expressedas(1)where,,,,,and are the microstrip charac-teristic impedance,the normalized modal voltage discontinuity,the aperture admittances looking into the antenna,the aperture admittances looking into the feeding line,and self admittance of the patch,respectively.In this study,cross-aperture and series feeding line are used.To analyze this structure,mutual interactions between each dif-ferent coupling points of the cross-slot should be considered.Aperturevoltages,in each four arm of the cross-aper-ture are expressed as[7](2)(3)(4)(5)whereisthenormalizedelectricfieldamplitudeintheaper-ture with respect to the feeding line current amplitude.Therefore,reflectioncoefficientsand slotimpedances for nth arm of cross-aperture are evaluated,respectively,by(6)(7)To optimize performance of the proposed antenna,the simu-lation software Ensemble is used in this study.We present simu-lated axial ratios and return losses as the variation of the length of cross-aperture,width of cross-aperture,and length of open stub in Fig.2.The variations of axial ratios and return losses for length ofcross-apertureare shown in Fig.2(a).Design of Fig.2(a)is carried out withthe of 0.7mmand of 13.65mm.Wevaryfrom 28.05to 29.55mm.As increases,impedance bandwidth becomes wide with poor matching.We also found axial ratio is worse forlong .We achieve the best simulated resultwhen is 28.55mm.It is shown in Fig.2(b)that the variations of axial ratios andreturn losses for various widths ofcross-aperturebetween 1.5and 0.3mm.In Fig.2(b),design is achieved withtheof(a)(b)(c)Fig.2.Simulated axial ratios and return losses for (a)length of cross_aperture,(b)width of cross_aperture,and (c)length of open stub.28.55mmand of 13.65mm.The variations of character-isticsforare nearly samewith ,and we found the best simulated resultwhen is 0.7mm.Fig.2(c)shows the variations of axial ratios and return lossesfor length of openstub.In design of Fig.2(c),wechose of 28.55mmand of 0.7mm.Wevary from 18.65TABLE ID ESIGNED P ARAMETERS AND V ALUES OF THE P ROPOSED ANTENNAFig.3.Return losses of the proposed antenna.to 8.65mm.In this variation,we found axial ratio is moved tohigh frequencyasincreases,impedance matching is around 15dB at center frequency,and impedance bandwidth is nearly maintainedfor .Therefore,this parameter can be used to choose axial ratio frequency.The best simulated performanceis shown in the casewithof 13.65mm.The optimized design parameters and values of the proposed antenna are totally summarized in Table I.III.E XPERIMENTAL R ESULTS AND D ISCUSSIONThe calculated and measured return losses of the proposed antenna are shown in Fig.3.The maximum resonant frequen-cies of the both return losses are a little lower than the center frequency due to the optimized axial ratio characteristics.As shown in Fig.3,impedance matching and bandwidth of the mea-sured return loss are better and wider than those of the simu-lated return loss in operating frequencies.In higher frequencies of bandwidth,simulated and measured return losses are nearly same,but they were slightly different in lower frequencies of bandwidth,which is due to fabrication error (from slight varia-tion of air-gap between the upper and lower layers).The mea-sured impedance bandwidth(15dB)is 240MHz (10%).The calculated and measured axial ratios of the proposed an-tenna are illustrated in Fig.4.The both axial ratios are nearly same around center frequency,but difference between them is found in low and high frequencies of bandwidth.ThemeasuredFig.4.Axial ratios of the proposedantenna.Fig.5.Gains of the proposedantenna.Fig.6.Radiation patterns of the proposed antenna at 2.4GHz.axial ratio bandwidth for 3dB is 110MHz (4.6%),which is about two times wider compared with the reported literature [5],[6].Therefore,from this measured result the proposed CP an-tenna has the effect of hybrid feeding.Fig.5shows the calculated and measured gains of the pro-posed antenna.We found the both gains of the proposed antennaTABLE IIM EASURED R ESULTS OF THE P ROPOSED ANTENNAare nearly same.The measured maximum gain and gain band-width for 3dB in operating frequencies are 8dBi and above 400MHz(16.7%),respectively.The measured radiation patterns of the proposed antenna at 2.4GHz is shown in Fig.6.We found F/B of 20dB and 3dBbeamwidthof.The pattern symmetry is maintained over operating axial ratio bandwidth (2.35–2.45GHz).The measured results of the proposed antenna are summarized in Table II.IV .C ONCLUSIONIn this paper,we proposed a single-feeding circularly polar-ized microstrip antenna with the effect of hybrid feeding.Series feeding is suggested to obtain wide axial ratio bandwidth andflat gain bandwidth,and cross-aperture with short length is used to have high gain.Validity and stability of the proposed antenna is described by measured data of the proposed antenna.This proposed antenna has improved axial ratio bandwidth (4.6%),high gain (8dBi),and flat 3-dB gain bandwidth (above 16.7%).In measured radiation patterns,3-dB beamwidthofand good F/B of 20dB are achieved.R EFERENCES[1]K.R.Keith R.Carver and J.W.James W.Mink,“Microstrip antenna tech-nology,”IEEE Trans.Antennas Propagat.,vol.AP-29,pp.2–24,Jan.1981.[2] D.M.Pozar and D.H.Schaubert,The Analysis and Design of MicrostripAntennas and Arrays 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