Breinbjerg, Olav

Kim, Oleksiy S.

Smith, Thomas Gunst

Vesterager Gothelf, Ulrich

Vesterdal Larsen, Niels

A concentric dual split-loop element is designed and investigated for reflectarray antenna design in the emerging 20 GHz and 30 GHz Ka-band satellite communication spectrum. The element is capable of providing adjustment of the phase of reflection coefficients for circular plane waves in two separate frequency bands, by rotation of the individual split-loops. Cross-polar reflection is simultaneously minimized by optimizing the gaps in the split-loops. Based on the element characteristics, an iterative design procedure is proposed and used to design a front-fed reflectarray antenna. The aperture efficiencyexceeds 56% with an on-axis axial ratio below 0:21 dB in both frequency bands

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Downloaded from orbit.dtu.dk on: Dec 20, 201720/30 GHz dual-band circularly polarized reflectarray antenna based on the concentricdual split-loop elementSmith, Thomas Gunst; Vesterdal Larsen, Niels; Vesterager Gothelf, Ulrich; Kim, Oleksiy S.; Breinbjerg,OlavPublished in:Proceedings of 34th ESA Antenna WorkshopPublication date:2012Link back to DTU OrbitCitation (APA):Smith, T. G., Vesterdal Larsen, N., Vesterager Gothelf, U., Kim, O. S., & Breinbjerg, O. (2012). 20/30 GHz dual-band circularly polarized reflectarray antenna based on the concentric dual split-loop element. In Proceedings of34th ESA Antenna Workshop20/30GHZ DUAL-BAND CIRCULARLY POLARIZED REFLECTARRAY ANTENNABASED ON THE CONCENTRIC DUAL SPLIT-LOOP ELEMENTThomas Gunst Smith1,2, Niels Vesterdal Larsen1, Ulrich Vesterager Gothelf1, Oleksiy S. Kim2, and OlavBreinbjerg21Thrane & Thrane A/S, Lundtoftega˚rdsvej 93D, DK-2800 Kgs. Lyngby, Denmark2Department of Electrical Engineering, Electromagnetic Systems, Technical University of Denmark, Ørsteds Plads,Building 348, DK-2800 Kgs. Lyngby, DenmarkABSTRACTA concentric dual split-loop element is designed and in-vestigated for reflectarray antenna design in the emerg-ing 20GHz and 30GHz Ka-band satellite communica-tion spectrum. The element is capable of providing ad-justment of the phase of reflection coefficients for circu-lar plane waves in two separate frequency bands, by ro-tation of the individual split-loops. Cross-polar reflectionis simultaneously minimized by optimizing the gaps inthe split-loops. Based on the element characteristics, aniterative design procedure is proposed and used to designa front-fed reflectarray antenna. The aperture efficiencyexceeds 56% with an on-axis axial ratio below 0.21 dBin both frequency bands.1. INTRODUCTIONA reflectarray antenna generally consists of an array ofscattering elements and a feeding antenna. The operat-ing principle is derived from classical reflector antennas,where a shaped surface can be designed to produce e.g.a pencil or contoured beam. However, for planar reflect-arrays, the scattering elements are designed to reflect theincident field with a desired phase, such that a given far-field can be achieved [1].For high-gain antennas, printed reflectarrays are ofparticular interest due to their low cost, ease of manufac-ture and reconfigurability [2]. Thus a printed reflectar-ray is a feasible candidate for user terminal antennas forsatellite communication in the emerging Ka-band spec-trum. When choosing the reflectarray element topology,the requirements to polarization and bandwidth are essen-tial. For circular polarization, commonly used elementsinclude stub-tuned patches [3], crossed dipoles [4, 5]and single split-loops [6, 7]. However, for large ref-lectarrays the differential spatial phase delay is signifi-cant [8] making it extremely challenging to cover the en-tire 20 − 30GHz frequency band, thus it is preferred toutilize a reflectarray element which allows separate phaseadjustment in the 20 and 30GHz frequency bands. Forthis purpose, a concentric dual split-loop element utiliz-ing the element rotation technique is proposed. The el-ement is investigated and optimized for dual-frequencycircular polarization, specifically LHCP from 19.7 to20.2GHz (RX) and RHCP from 29.5 to 30.0GHz (TX).This element was suggested in [1, p. 122], but to the bestof our knowledge it has not been subject to detailed in-vestigations.2. ELEMENT DESIGN AND INVESTIGATIONThe commercially available software Ansoft Designerhas been employed to optimize the element geometry. Byusing periodic boundary conditions and plane wave exci-tation the element is simulated as if it was located in atwo dimensional infinite square grid array, with an inter-element spacing of 5mm. The element has shown stableperformance over relevant incidence angles; hence onlynormal incidence is considered in this paper.2.1. Element Rotation TechniqueA local (u, v) coordinate system is assigned to the ele-ment under consideration, which in Figure 1 is exempli-fied by a simple dipole though the following is valid forarbitrary element geometries. The rotation of the localcoordinate system, and thus the element, can be describedin the global (x, y) coordinate system by the rotation an-gle ψ.An incident RHCP plane wave is considered travelingin the negative z-directionEi = (xˆ+ jyˆ)E0ejkz , whereE0 is the electric field amplitude and k is the free-spaceuvxyψFigure 1. Global (x, y) and local (u, v) coordinate sys-tem for element rotation.wavenumber. The time-factor ejωt is assumed and sup-pressed. By decomposing this incident field along the u-and v-directions and employing the associated linear re-flection coefficients, Ru and Rv , it can readily be shownthat the reflected far-field plane wave can be described byEs =12((Ru −Rv) [xˆ− jyˆ) ej2ψ+(Ru +Rv) (xˆ+ jyˆ)]E0e−jkz,(1)where Ru, Rv are the reflection coefficients of the linearelectric field components along uˆ, vˆ, respectively. It isevident that for Ru = Rvejpi , the scattered field is a pureRHCP plane wave traveling in the positive z direction andthe phase scales linearly in ψ. However, if either |Ru| 6=|Rv|, or if the phase difference is not equal to pi, cross-polarization (here LHCP) is introduced.It is important to note that for elements placed in a ref-lectarray element grid, the coupling to the neighboring el-ements will generally change when the element is rotated.It is therefore crucial to modify the element geometry ac-cordingly to keep Ru = Rvejpi, such that the cross-polarreflection is suppressed [7]. This is usually achieved byan anisotropic design where only one of the two linearorthogonal polarizations excites a resonant element, e.g.a dipole on a grounded dielectric, as depicted in Fig-ure 1. In the case of resonance, the polarization parallel tothe dipole will see a reflection with zero phase, whereasthe orthogonal polarization will be reflected with a phaseequal to pi due to the infinite ground-plane. Therefore, ifthe dipole length is adjusted properly, a phase-differenceof pi is ensured. However, since the losses increase sig-nificantly around the resonance, the amplitude of the par-allel component will be smaller than the orthogonal. Itis thus clear that the losses put a constraint on the cross-polarization level that can be achieved.xyψoψigogi2ro 2riwwFigure 2. Concentric dual split-loop reflectarray elementgeometry.2.2. Concentric Dual Split-Loop ElementThe concentric dual split-loop element proposed in thispaper is illustrated in Figure 2. The outer and inner split-loop is characterized by the radii ro and ri, respectively,measured from the center to the outside of the conductor,the rotation angles ψo and ψi, the gap sizes go and gi, andequal conductor width w. The element is printed on agrounded Rogers RT/Duroid 5880 substrate with dielec-tric constant r = 2.20, dissipation factor tan δ = 0.0009and thickness of 1.016mm.The concept of the concentric dual split-loop ele-ment is that the inner and outer split-loop can be rotatedsuch that desired reflection phases are obtained at 20 and30GHz simultaneously. As noted in Section 2.1, the ge-ometry of the concentric dual split-loop element must beadjusted such that the orthogonal reflection coefficientshave a phase difference of pi, so that no cross-polarizationoccurs. However, when the outer and inner split-loopsare rotated their coupling to each other and to neighbor-ing elements change. Therefore, by adjusting the gapsof the split-loops, go and gi, each element is optimizedto minimize cross-polar reflection, for all rotation angles.In this way, the cross-polar reflection coefficient at both20 and 30GHz is suppressed below −31 dB for all rota-tion angles. The optimized element geometry is given inTable 1. Due to the symmetry of the concentric dual split-loop element, all properties are 180◦ periodic in rotation,hence results are shown for ψo and ψi in the interval 0◦to 180◦.Table 1. Reflectarray element geometry data.ro 1.9mmri 1.4mmw 0.2mmψo, ψi [0◦; 180◦]go [0.46mm; 0.52mm]gi [0.24mm; 1.01mm]0 50 100 150050100150ψo [deg]ψi[deg]∠Rll [deg]100 200 300 400 500Figure 3. Phase of 20 GHz LHCP-to-LHCP reflection co-efficient.At 20GHz, the reflection coefficient is primarily de-termined by ψo and almost unaffected by ψi, which canbe seen in Figure 3, where ∠Rll is the phase of theLHCP-to-LHCP circular reflection coefficient. The con-tour curves are almost vertical and equidistant, indicatingthat the rotation of the outer split-loop provides the de-sired linear phase adjustment over rotation angle ψo forany value of ψi. For a fixed value of ψo, the phase varieswithin ±5.0◦ when ψi is varied.At 30GHz, the reflection coefficients are found to de-pend on both ψo and ψi, which is evident in Figure 4,where ∠Rrr is the phase of the RHCP-to-RHCP reflec-tion coefficient. However, an important feature is thatfor a fixed value of ψo, the phase is monotonic in ψi,which means that for a given rotation angle of the outersplit-loop, there is a unique solution for the rotation an-gle of the inner split-loop which gives a desired reflectionphase.The phase range of the circular co-polar reflection co-efficient is exactly 360◦, for ψo or ψi in the interval 0◦to 180◦, at 20 and 30GHz, respectively. This range is ofcourse necessary for the use of the element in the reflect-array design. For all rotation angles of both the inner andouter split-loops, the amplitude of the co-polar reflectioncoefficient is above 0.26 and 0.35 dB over the entire RXand TX frequency bands, respectively.The above mentioned characteristics of the circu-lar reflection coefficients enables a design of a reflect-array antenna, where the reflectarray is first optimizedfor 20GHz performance by determining ψo for all ele-0 50 100 150050100150ψo [deg]ψi[deg]∠Rrr [deg]100 200 300 400 500Figure 4. Phase of 30 GHz RHCP-to-RHCP reflectioncoefficient.ments, and afterwards adjusting ψi for 30GHz perfor-mance without affecting the radiation at 20GHz signifi-cantly.3. REFLECTARRAY DESIGNBased on the element characteristics presented in Sec-tion 2, an iterative design procedure has been developedto determine ψo and ψi for each reflectarray element. Inthis section, the proposed design procedure is describedand a concept reflectarray antenna design is presented.3.1. Design ProcedureAs stated in Section 2, the reflection coefficient at 20GHzis almost unaffected by rotation of the inner split-loop,hence the reflectarray may be optimized for 20GHz andsubsequently 30GHz. The reflectarray is designed fora single frequency in each of the two separate frequencybands, and the design procedure can generally be outlinedas follows:1. go and gi are optimized for minimum cross-polar re-flection using Ansoft Designer, for a finite set of ψoand ψi2. ψo and ψi of all reflectarray elements are set to zero3. ψo is determined for all reflectarray elements(a) A one dimensional 20GHz phase-curve is gener-ated for varying ψo, while keeping ψi fixed(b) ψo is determined from the phase-curve and the de-sired reflected phase4. ψi is determined for all reflectarray elements(a) A one dimensional 30GHz phase-curve is gener-ated for varying ψi, while keeping ψo fixed(b) ψi is determined from the phase-curve and the de-sired reflected phase5. The scattered aperture field over the reflectarray iscalculated6. The radiated field of the entire reflectarray is calcu-lated and the feed pattern is addedStep 3 and step 4 can be repeated such that the actualnon-zero rotation angle of ψi is included when choosingψo. This may be necessary for concentric dual split-loopgeometries where the coupling between the two loops islarger at 20GHz. However, for the present design a sin-gle iteration is sufficient.The procedure has been implemented in a numericalcode that calculates the radiated far-field by the equiva-lent currents technique [9, technique II].3.2. Concept DesignTo validate the proposed reflectarray element, a front-fedcircular aperture reflectarray has been designed. The ref-lectarray is designed to have a broadside pencil beam to-wards θ = 0◦ and consists of approximately 11, 300 el-ements. The developed code allows for an arbitrary feedantenna, but here a Gaussian beam feed model is used.The specifications of the reflectarray and feed antenna aresummarized in Table 2.The rotation angles of the concentric dual split-loopelements are shown in Figure 5. Due to the rotationalsymmetry of the reflectarray only a cross-section from thecenter to the edge is shown, where element 61 and 120 arethe element near the center and edge, respectively. It isfound that the rotation angle of the outer split-loop scaleslinearly with the distance from the element to the feedantenna, which is due to the linear 2ψo reflection phasebehavior and independence of ψi, as discussed in Sec-tion 2. However, the rotation angle of the inner split-loopis highly aperiodic. This is caused by the complex behav-ior of the reflection phase, as also discussed in Section 2.Table 2. Specifications of front-fed reflectarray antenna.RX 19.7GHz− 20.2GHz LHCPTX 29.5GHz− 30.0GHz RHCPDiameter 60 cmFocal length 46 cmEdge taper −12 dB60 70 80 90 100 110 120050100150ElementRotationangle[deg]ψoψiFigure 5. Rotation angles of reflectarray elements alongradial cut from centre to edge of the circular aperture.−90 −45 0 45 90−2002040θ [deg]D[dBi]RHCPLHCPFigure 6. Radiation pattern of the designed reflectarrayat 20 GHz.The radiation pattern of the front-fed reflectarray isdepicted in Figure 6 for 20GHz and in Figure 7 for30GHz, with a zoom of the 30GHz radiation patternin Figure 8. At 20.0GHz, the maximum directivity is39.8 dBi, which corresponds to an aperture efficiency of60%, and the sidelobe level is 28.4 dB below peak. At30.0GHz, the maximum directivity is 43.2 dBi, which isan aperture efficiency of 58%, and the sidelobe level is30.4 dB below peak.Over the RX and TX frequency bands, the on-axis co-polar directivity and axial ratio (AR) is plotted in Figure 9and 10, respectively. The designed reflectarray antennahas an aperture efficiency exceeding 56% and an on-axisAR below 0.21 dB over both the RX and TX frequencybands.−90 −45 0 45 90−2002040θ [deg]D[dBi]RHCPLHCPFigure 7. Radiation pattern of the designed reflectarrayat 30 GHz.−20 −10 0 10 20010203040θ [deg]D[dBi]RHCPLHCPFigure 8. Zoom of the radiation pattern of the designedreflectarray at 30 GHz.4. CONCLUSIONSA concentric dual split-loop reflectarray element for dual-band circular polarization has been presented. The ele-ment provides means for adjusting the phase of circularreflection coefficients in two separate frequency bands.By optimizing the element geometry, the cross-polar re-flection coefficient of the element has been suppressedbelow −31 dB for all element rotation angles, at both de-sign frequencies simultaneously.An iterative design-procedure has been presented, anda front-fed reflectarray antenna has been designed fordual-band 20 and 30GHz application. The reflectarrayshows satisfying performance in both frequency bands,with an aperture efficiency above 56% and on-axis ARbelow 0.21 dB.19.7 19.8 19.9 20 20.1 20.238.53939.54040.5D0f [GHz]D0[dBi]00.10.20.30.4ARAR[dB]Figure 9. On-axis directivity and AR in RX frequencyband.29.5 29.6 29.7 29.8 29.9 3041.54242.54343.5D0f [GHz]D0[dBi]00.10.20.30.4AR AR[dB]Figure 10. On-axis directivity and AR in TX frequencyband.A prototype reflectarray is planned to be manufac-tured and measured in the near future to validate the pre-sented results.REFERENCES[1] J. Huang and J. A. Encinar. Reflectarray Antennas.IEEE Press, 2008.[2] J. A. Encinar. Recent advances in reflectarray anten-nas. In Proceedings of the European Conference onAntennas and Propagation, pages 1–6, 2010.[3] J. Huang and R. J. Pogorzelski. A Ka-band mi-crostrip reflectarray with elements having variablerotation angles. IEEE Transactions on Antennas andPropagation, 46(5):650–656, 1998.[4] E. B. Felstead, J. Shaker, M. R. Chaharmir, andA. Petosa. Enhancing multiple-aperture ka-bandnavy SATCOM antennas with electronic tracking andreflectarrays. In MILCOM 2002. Proceedings, vol-ume 2002 of 1, pages 168–172. IEEE, October 2002.[5] R. Chaharmir, J. Shaker, and M. Cuhaci. Develop-ment of dual-band circularly polarised reflectarray. InIEE Proceedings - Microwaves, Antennas and Prop-agation, volume 153, pages 49–54, 2006.[6] B. Strassner, C. Han, and K. Chang. Circularly polar-ized reflectarray with microstrip ring elements hav-ing variable rotation angles. IEEE Transactions onAntennas and Propagation, 52(4):1122–1125, April2004.[7] A. Yu, F. Yang, A. Z. Elsherbeni, J. Huang, andY. Kim. An offset-fed x-band reflectarray an-tenna using a modified element rotation technique.IEEE Transactions on Antennas and Propagation,60(3):1619–1624, March 2012.[8] D. M. Pozar. Bandwidth of reflectarrays. ElectronicsLetters, 39(21):1490–1491, 2003.[9] M. Zhou, S. B. Sorensen, E. Jorgensen, P. Meincke,O. S. Kim, and O. Breinbjerg. An accurate tech-nique for calculation of radiation from printed ref-lectarrays. IEEE Antennas and Wireless PropagationLetters, 10:1081–1084, 2011.

20/30 GHz dual-band circularly polarized reflectarray antenna based on the concentric dual split-loop element

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20/30 GHz dual-band circularly polarized reflectarray antenna based on the concentric dual split-loop element

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