Broadband circularly polarized Annular -Ring Microstrip Antenna

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Moreover,the effective bandwidth of38%for the proposed annular-ring antenna is also much wider than those for the other annular-ring patch an-tennas in the literature.Index Terms—Circular-polarization,microstrip antennas,wideband an-tennas.I.I NTRODUCTIONCircularly polarized(CP)patch antennas have attracted much atten-tion for mobile wireless and satellite communications because they al-lows for greaterflexibility in orientation angle between transmitter and receiver antennas,better mobility and weather penetration,and reduc-tion in multipath reflections.The general limitations of the CP patch an-tennas are the achievable impedance and axial-ratio(AR)bandwidths. Traditionally,the allowable3-dB AR bandwidth is typically less than 10%for the single-feed cases[1]–[4].Much wider impedance and AR bandwidths can be achieved for dual-feed and quadruple-feed CP patch antennas[5]–[13].Patch antennas[12],[13]with quadruple broad-band feeds[14],[15]were found to experimentally achieve10dB impedance bandwidths of around70 80%and3dB AR bandwidths of 80%.Although the quadruple-feed CP patch antenna offers much wider impedance and AR bandwidths,it may result in a larger footprint beneath the patch and could be a problem for forming antenna arrays. Thus,the dual-feed CP patch antennas are still much of interest. Annular-ring patch antennas have received considerate attention [16]–[22].When operated in the fundamental TM11mode,the size of the annular ring patch antenna is smaller than that of the circular or rectangular patch for a given frequency.In application to array designs,this allows the antenna elements to be more densely packed, thereby reducing the grating-lobe problem.When operated at the TM12mode,it has a relatively wider impedance bandwidth.Most CP annular ring patch antennas in the reported articles have narrow bandwidths less than10%[17]–[22].Manuscript received November02,2008;revised January19,2009.First pub-lished June05,2009;current version published August05,2009.This work was supported in part by the Singapore Ministry of Education Academic Research Fund Tier1project R-263-000-534-133.Y.-X.Guo is with the Department of Electrical and Computer Engineering, National University of Singapore,Singapore117576,Singapore(e-mail: yxguo@).L.Bian and X.Q.Shi are with the Nanjing University of Science and Tech-nology,Nanjing,Jiangsu,China,210094.Digital Object Identifier10.1109/TAP.2009.20245840018-926X/$26.00©2009IEEEFig.1.Geometry of the CP circular patch antenna and the annular-ring patch antenna G=180mm,L=11mm,R=1mm,H=20mm,t=0:8mm, "=3:38,"=1;Patch antenna:D=76:5mm,d=0mm,L=35mm,S=9mm;Annular-ring patch antenna:D=70mm,d=20mm, L=30mm,S=19mm.In this paper,a wideband CP annular-ring patch antenna with two L-probe feeds is presented.The current distributions on the conven-tional circular patch are found not in order for CP operation at higher frequencies of the passband.A smaller concentric circular slot cut on the original larger circular patch is proposed to form an annular-ring patch antenna.As such,the current distributions at the upper band can be improved to be more symmetrical.Thus the proposed antenna delivers a wider AR bandwidth than that of the circular patch using the identical feeding technique.Considering the common overlapped bandwidth limited by the impedance,AR and gain,the proposed annular-ring patch antenna exhibits an effective bandwidth of38% from1.5–2.2GHz,which is wider than the corresponding bandwidth of29.7%from1.35–1.82GHz for the conventional circular patch. Moreover,the obtained effective bandwidth of38%for the proposed antenna is much wider than those for the other annular-ring patch antennas in the literature[17]–[22].II.A NTENNA C ONFIGURATION AND D ESIGNThe antenna shown in Fig.1has a copper circular patch(d=0mm) of diameter D1=76:5mm positioned at a height of20mm above a square PCB ground plane of180mm by180mm.The two copper L-probe feeds are of diameter2.0mm,horizontal length,L h,11mm, and vertical length,L v1,35mm,were soldered onto the respective output ports of a wideband feed network.The circular patch is excited by the two copper L-probe feeds orientated to have phases of0 and 90 .The employed broadband microstrip feed network provides equal amplitude and90 phase shift across a wide bandwidth[12].In this design,a planar feed network operating at the central frequency of1.8 GHz was designed on the microwave substrate RO4003having thick-ness0.8mm and dielectric constant3.38.By investigating the current distributions for the circular patch as shown in Fig.2,it is found that the current distributions on the circular patch are not in order for CP operation at the center part at higher fre-quencies.From Fig.2,a current corner was found for the case at2.2 GHz,which could exhibit more linear polarization.As such,to improve the AR bandwidth of the circular patch antenna,a smaller concentric circular slot is proposed to be cut on the original larger circular patch to form an annular-ring patch,as shown in Fig.1.With such an ar-rangement,the current distribution at2.2GHz has been improved to be more symmetrical and a wider AR bandwidth is achieved.The an-nular ring patch antenna has a copper annular patch of outer diameter D2=70mm and inner diameter d=20mm at a height of20mm above the square PCB ground plane of180mm by180mm.The two copper L-probe feeds are of diameter2.0mm,horizontal length,L h, 11mm,and vertical length,L v2,30mm.It is seen from the simula-tion in Fig.4that the operating frequencies for the annular-ring patch are from1.15–2.23GHz with a relative bandwidth of64%for AR less than3dB.These results reveal significant enhancements in the AR bandwidth for the annular-ring patch over the circular patch antenna. The circular slot cut on the patch only improves the AR bandwidth by adjusting the current distribution in the patch especially at higher fre-quencies.The software IE3D is used in the optimization.Note that the current distributions for both circular patch and annular-ring patch an-tennas were taken at a certain time.In the simulation by IE3D,it is observed that the currents for CP antennas rotate all the time and the current directions can be at any angle.Moreover,we also changed the feed substrate dielectric constant to shift the operation frequency and found that the effect of using the ring slot cut on the patch to improve the current distribution was still valid.III.R ESULTS AND D ISCUSSIONSFigs.3–5depict simulated and measured results for the SWR,AR and gain of the proposed CP annular-ring patch antenna.It is observed from Fig.3that the CP annular-ring patch antenna exhibits measured and simulated impedance bandwidths of67%from1.1–2.2GHz and 65%from1.12–2.2GHz for SWR<2,respectively.Note that the regular single L-probe single element rectangular patch antenna has a typical impedance bandwidth(SWR<2)of around30%[23]. It is observed that the circular patch antenna fed by two proximity-coupled L-probes orientated to have phases of0 and90 ,using a narrow-band90 hybrid couplers,delivers a10-dB return loss band-width of42%[11].As compared with the results in[11],[23],it is seen that much wider impedance bandwidth was achieved for the CP an-nular-ring patch antenna with the proposed wideband90 hybrid feed networks.Referring to Fig.4,the CP annular-ring patch antenna ex-hibits a measured3-dB AR bandwidth of60%from1.23–2.28GHz, which is derived from the measured E-and H-plane linear radiation pat-terns.The standard linear-polarization horn antennas were employed for the antenna radiation measurements.The simulated3-dB AR band-width is64%from1.15–2.23GHz.From Fig.5,it is seen that the CP annular patch antenna exhibits a measured and simulated gain band-widths of38%from1.5–2.2GHz and41%from1.5–2.28GHz for the gain>3dBi,respectively.Meanwhile,it is seen from Figs.3–5that the patch antenna shows a simulated impedance bandwidth of60.4% from1.18–2.20GHz for SWR<2,a simulated AR bandwidth of43% from1.18–1.82GHz for AR<3dB,and a simulated gain bandwidthFig.2.Current distributions on the circular patch and the annular-ring patch (a)1.4GHz (b)1.8GHz (c)2.2GHz.Fig.3.SWR of the CP circular and annular-ring patch antenna.of 50%from 1.35–2.25GHz for gain >3dBi.Considering the common overlapped bandwidth limited by the impedance,AR and gain,the pro-posed annular-ring patch antenna exhibits an effective bandwidth of 38%from 1.5–2.2GHz,which is much wider than thecorrespondingFig.4.Axial ratio of the CP circular and annular-ring patchantenna.Fig.5.Gain of the CP annular-ring patch antenna.bandwidth of 29.7%from 1.35–1.82GHz for the conventional circular patch.Moreover,the obtained effective bandwidth of 38%for the pro-posed antenna is much wider than those for the other annular-ring patch antennas in the literature [17]–[22].Fig.6and Fig.7show the measured radiation patterns in the X-Z and Y-Z planes for the CP annular-ring patch antenna at three different frequencies 1.5,1.8,and 2.1GHz.The radiation patterns are stable in the passband.The results reveal a significant enhancement in the AR bandwidth for the proposed annular-ring over the circular patch antenna and the other annular-ring patch antennas in the literature [17]–[22].The CP bandwidth of the annular-ring patch antenna is enhanced because its current distributions can be adjusted by the circular slot cut on it.The impedance bandwidths of the antennas are enhanced due to using the L-probe feeding technique and a thick substrate.IV .C ONCLUSIONIn this paper,a wideband CP annular-ring patch antenna with two proximity-coupled L-probe feeds connected to the broadband 90 hy-brid has been presented.Considering the common overlapped band-width limited by the impedance,AR and gain,the proposed annular-ring patch antenna exhibits an effective bandwidth of 38%from 1.5–2.2GHz,which is wider than the corresponding bandwidth of 29.7%from 1.35to 1.82GHz for the conventional circular patch.Moreover,the ef-fective bandwidth of 38%for the proposed annular-ring antenna is also much wider than those for the other annular-ring patch antennas in theFig.6.Measured radiation patterns in the X-Z plane for the CP annular-ring patch antenna.(a)1.5GHz;(b)1.8GHz;(c)2.1GHz.Fig.7.Measured radiation patterns in the Y-Z plane for the CP annular-ring patch antenna.(a)1.5GHz;(b)1.8GHz;(c)2.1GHz.literature.The gain bandwidth could be further improved using stacked patches and other gain enhancement techniques.R EFERENCES[1]H.Iwasaki,“A circularly polarized small-size microstrip antennawith a cross slot,”IEEE Trans.Antennas Propag.,vol.44,no.10,pp.1399–1401,Oct.1996.[2]W.K.Lo,J.L.Hu,C.H.Chan,and K.M.Luk,“L-shaped probe-feed circularly polarized microstrip patch antenna with a cross slot,”Microw.Opt.Technol.Lett.,vol.25,no.4,pp.251–253,May 2000.[3]F.S.Chang,K.L.Wong,and T.W.Chiou,“Low-cost broadband cir-cularly polarized patch 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