An L-band, circularly polarized, dual-feed, cavity-backed annular slot antenna has been investigated. The investigations comprise parametric studies of design parameters and measurements with different ground planes. The antenna has an impedance bandwidth of 6% around 1.59 GHz and a maximum directivity of about 7 dBi

Breinbjerg, Olav

Larsen, Niels Vesterdal

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An L-band, circularly polarized, dual-feed, cavity-backed annular slot antenna with wide-angle coverage

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Downloaded from orbit.dtu.dk on: Dec 17, 2017An L-Band, Circularly Polarized, Dual-Feed, Cavity-Backed Annular Slot Antenna withWide-Angle CoverageLarsen, Niels Vesterdal; Breinbjerg, OlavPublished in:IEEE AP-S International SymposiumLink to article, DOI:10.1109/APS.2006.1710850Publication date:2006Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Larsen, N. V., & Breinbjerg, O. (2006). An L-Band, Circularly Polarized, Dual-Feed, Cavity-Backed Annular SlotAntenna with Wide-Angle Coverage. In IEEE AP-S International Symposium (Vol. 2, pp. 1549-1552). IEEE. DOI:10.1109/APS.2006.1710850An L-band, Circularly Polarized, Dual-Feed, Cavity-BackedAnnular Slot Antenna with Wide-Angle CoverageNiels Vesterdal Larsen*1 and Olav Breinbjerg21nvl@oersted.dtu.dk, 2ob@oersted.dtu.dk,Ørsted•DTU, Technical University of Denmark, Ørsteds Pladsbuild. 348, DK-2800 Kgs. Lyngby, Denmark.AbstractAn L-band, circularly polarized, dual-feed, cavity-backed annular slot antenna hasbeen investigated. The investigations comprise parametric studies of design parame-ters and measurements with diﬀerent ground planes. The antenna has an impedancebandwidth of 6% around 1.59GHz and a maximum directivity of about 7 dBi.IntroductionThis paper presents results from an investigation of a circularly polarized cavity-backed annular slot (CBAS) antenna. The antenna operates in the L-band arounda center frequency of 1.6GHz and yield wide-angle coverage. It is possible to obtaincircular polarization with only a single feed by employing various perturbations tothe slot [1] or by shorting the slot [2], however, such solutions are usually associatedwith low axial ratio bandwidths and instead two feeds in phase quadrature are used.Similar antennas, though of diﬀerent implementation, were previously reported in[3, 4]. Initial design investigations comprise extensive numerical computer simula-tions using Ansoft’s HFSS ver. 9.2. Subsequently, an antenna prototype has beenconstructed and measured at the DTU-ESA Spherical Near-Field Antenna Test Fa-cility. Three diﬀerent ground planes were used to investigate the inﬂuence of theseon the wide-angle performance of the antenna. A comprehensive documentation ofthe work on this antenna can be found in [5].Antenna Design and Impedance CharacteristicsThe CBAS antenna is shown in ﬁgure 1. It consists of an annular slot, with meanradius ρs and width ws, which is etched in the single copper layer of a copper-cladRogers RO-4003 substrate with the relative permittivity r and thickness t. Theannular slot is backed by a circular metallic cavity with interior radius ρc and depthhc and mounted on a small circular ground plane. The annular slot is excited bytwo 50Ω coaxial cables, denoted feed 1 and feed 2, respectively. Feed 2 is phasedwith a 90o lag with respect to feed 1 resulting in right-hand circular polarizationin the boresight direction. The harmonic time dependence ejωt, with t and ω beingthe time and angular frequency, respectively, is assumed and suppressed.The values of the above design parameters were chosen on basis of extensive numer-ical simulations and are summarized in ﬁgure 1. The simulations have led to thefollowing observations on the qualitative behavior of the resonance frequency fresand impedance bandwidth BW .1-4244-0123-2/06/$20.00 ©2006 IEEE 1549ρc2ρczFeed 2Feed 1hcwsFeed 1SuperstrateGround planetρsφyx xρc = 40.0mm, hc = 20.0mm, t = 0.79mmρs = 32.0mm, ws = 2.0mm, r = 3.38Rin = 450Ω, fres = 1.59GHz, BW = 6 %Figure 1: Geometry of the CBAS antenna and the chosen design parameters. Forsimplicity the dielectric superstrate is not shown on the top view.• fres decreases for increasing values of the cavity dimensions ρc and hc as wellas slot parameters ρs, r, and t. It increases for increasing values of ws.• BW increases for increasing values of ρc, hc, and ws but has been found to beindependent on ρs. It decreases for increasing values of r and t.The input impedances Zin,i = Rin,i + jXin,i and input reﬂection coeﬃcients Γin,i atthe two ports, i = 1, 2, can be determined from the antenna scattering matrix S,Zin,i = Z0(1 + Sii)(1− Sjj) + SijSji + 2SijV +j(1−Sjj )−V+iSjiV +i(1−Sii)−V+jSij(1− Sii)(1− Sjj)− SijSji, (1)Γin,i =Zin,i − Z0Zin,i + Z0, j = 3− i, i = 1, 2, (2)where V +i is the incident voltage wave at the i’th feed port (deﬁned at the inter-section of the slot and coaxial cable). Equation (1) can be slightly reduced sinceS12 = S21, and even further if perfect phase quadrature between the incident voltagewaves is assumed, i.e., V +2 = −jV+1 , but, with (1) a non-ideal feed network can betaken into account.In the left part of ﬁgure 2 the input impedances of the two ports are plotted as-suming V +2 = −jV+1 for both simulated and measured scattering parameters. Atthe resonance frequency (deﬁned at Xin = 0) they are approximately 450Ω for thesimulation result, whereas they are slightly larger for the measurements. By assum-ing a 50Ω to 400Ω matching network the corresponding reﬂection coeﬃcients havebeen calculated and are shown in the right part of ﬁgure 2. The resonance frequencyis approximately 1.59GHz with a bandwidth of 6% with respect to Γin < −10 dB.The results from the simulation and measurement are generally in good agreementalthough slight deviations are seen for port 2.1550100200300400500-100-2001.4 1.5 1.6 1.70RinXinFrequency [GHz]Zin[Ω]Zin,1, Meas.Zin,2, Meas.Zin,1, Sim.Zin,2, Sim.1.4 1.5 1.6 1.7-40-30-20-100Frequency [GHz]Γin[dB]Γin,1, Meas.Γin,2, Meas.Γin,1, Sim.Γin,2, Sim.Figure 2: Input impedances Zin = Rin + jXin and reﬂection coeﬃcients Γin of thetwo input ports, assumed matched to 400Ω.Radiation Characteristics for Diﬀerent Ground PlanesThe antenna, fed using a quadrature hybrid, has been measured with three diﬀer-ent ground planes attached behind the small circular one shown in ﬁgure 1. Thechangeable ground planes are denoted GP 1-3 and comprise a square one with sidelengths of 250 mm and two circular ones with diameters of 360 mm and 1500 mm,respectively. For the two small ground planes (GP 1-2) the simulations were carriedout with HFSS, however, the large problem size for GP 3 rendered this approachimpossible with the available computer resources. Instead a Method of AuxiliarySources (MAS) [6] code was developed with the annular slot represented by a mag-netic ring current. In ﬁgure 3 the radiation patterns are shown for GP 1 and GP3 for φ = 0o together with the maximum directivity and boresight axial ratio asfunctions of frequency. In general the agreement between the measurements andsimulations is good for both the co- and cross-polar components, both for HFSSand MAS simulations. As expected the amount of ripples present in the radiationpatterns increases with the ground plane size, however, the maximum directivity andhalf power beam generally attain values around 7dBi and 100o, respectively. Thefact that the axial ratio is nowhere lower than 1 dB implies that coupling betweenthe feeds deteriorates the phase quadrature of the feed currents.ConclusionAn L-band, circularly polarized, dual-feed, cavity-backed annular slot antenna hasbeen developed. The antenna was equipped with three diﬀerent ground planes andsimulated using HFSS and a MAS code. Corresponding measurements have beenconducted at the DTU-ESA Spherical Near-Field Antenna Test Facility and goodagreement resulted. The antenna has an impedance bandwidth of 6% around aresonance frequency of 1.59GHz and an input impedance of approximately 450Ω.The maximum co-polar directivity is about 7 dBi and the half power beam widtharound 100o. Both, however, vary slightly for diﬀerent ground planes.1551-90 -45 45 90-40-30-20-100010θ [degrees]Directivity[dBi]DCO, Meas.DX , Meas.DCO, Sim.DX , Sim.-90 -45 45 90-40-30-20-100010θ [degrees]Directivity[dBi]DCO, Meas.DX , Meas.DCO, Sim.DX , Sim.24681.4 1.5 1.6 1.7010Frequency [GHz]Max.Directivity[dBi]GP 1GP 2GP 312341.4 1.5 1.6 1.705Frequency [GHz]AxialRatio[dB]GP 1GP 2GP 3Figure 3: Top: directivity of the CBAS antenna for φ = 0o as function of θ for GP 1(left) and GP 3 (right) at 1.58GHz. Bottom: maximum directivity (left) and axialratio at boresight (right) as function of frequency for GP 1-3.References[1] M.K. Fries & R. Vahldieck, ”Uniplanar Circularly Polarized Slot-Ring AntennaArchitectures”, Radio Science, Vol. 38, No. 2, pp. 5.1–5.10, 2003.[2] H. Morishita et al., ”Analysis of a Cavity-Backed Annular Slot Antenna with OnePoint Shorted”, IEEE Trans. Antennas Propagat., Vol. 39, No. 10, pp. 1472-1478,1991.[3] X.M. Qing & Y.W.M. Chia, ”Circularly Polarised Circular Ring Slot AntennaFed by Stripline Hybrid Coupler”, Electron. Lett., Vol. 35, No. 25, pp. 2154-2155,1999.[4] N.C. Karmakar, ”Investigations Into a Cavity-Backed Circular-Patch Antenna”,IEEE Trans. Antennas Propagat., Vol. 50, No. 12, pp. 1706-1715, 2002.[5] N.V. Larsen & O. Breinbjerg, ”An L-band, Circularly Polarised, Dual-Feed,Cavity-Backed Annular Slot Antenna for Phased Array Applications”, in submis-sion, October, 2005.[6] H.T. Anastassiu et al., ”Electromagnetic Scattering Analysis of Coated Con-ductors With Edges Using the Method of Auxiliary Sources (MAS) in ConjunctionWith the Standard Impedance Boundary Condition (SIBC)”, IEEE Trans. Anten-nas Propagat., Vol. 50, No. 1, January 2002, pp. 59-66.1552

An L-Band, Circularly Polarized, Dual-Feed, Cavity-Backed Annular Slot Antenna with Wide-Angle Coverage

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An L-Band, Circularly Polarized, Dual-Feed, Cavity-Backed Annular Slot Antenna with Wide-Angle Coverage

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