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A Ka-Band Dual-Frequency Radiator f>or Array Applications

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A Ka-Band Dual-Frequency Radiator f>or Array Applications
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  894 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 8, 2009 A Ka-Band Dual-Frequency Radiatorfor Array Applications Emilio Arnieri  , Member, IEEE  , Luigi Boccia  , Member, IEEE  , and Giandomenico Amendola  , Member, IEEE   Abstract— In this letter, a dual-band antenna is proposed for useas an element in Tx/Rx Ka-band arrays. The antenna consists of ashortedannularpatch(SAP)antennaintegratedwithacircularra-diating waveguide. TheSAP isfed witha stripline coupledthrougha slot, while for the waveguide, an E-plane stripline probe is used.Thisletterpresentsthedesignofthedual-bandelement,alongwithsimulated and measured results. It will beshown that the proposedantenna possesses good electrical characteristics and that it can berealized using standard printed technologies.  Index Terms— Dual band, millimeter-wave antennas, waveguideantennas. I. I NTRODUCTION K A-BANDflatarrayshavepotentialrelevanceinmanyap-plications for which low-profile antennas might be re-quired, such as local multipoint distribution systems (LMDSs)and interactive high-data-rate satellite communications. Con-sumer broadband services using a satellite link at the Ka bandare already operational, and there are plans to launch satellitesthat will operate exclusively at the Ka band, providing a largethroughput [1], [2]. Although dish antennas are a cheap and ef-ficient option for fixed services, a different choice has to be un-dertaken for mobile Tx/Rx terminals to be used in civil trans-portation (e.g., cars, buses, trains, aircraft, and ships). Indeed,in this case antennas need to fit onto the vehicle structure andhave to therefore be small, flat, lightweight, and robust.Inprinciple,thehigh-frequencybandsinvolvedinsuchappli-cations already allow for a significant reduction of the overallradiator size. However, a further reduction could be achievedif the transmit and receive arrays could be integrated togetherto form a single radiating aperture. To this end, the first crit-ical concern is the design of an antenna capable of operatingat both the Tx and Rx bands. Dual-band, dual-polarised arrayshave been extensively studied by researchers, in particular assynthetic aperture radar sensors, and many ingenious solutionshavebeenproposed.Dependingonthefrequencyseparationbe-tween the two bands, interleaved or overlapped architecturescould be used. Two-radiator arrays have been presented em-ploying various configurations, such as slot-patch structures,perforated patches, and overlapping patches [3], [4]. However, Manuscript received June 05, 2009; revised July 01, 2009. First publishedJuly 21, 2009; current version published August 18, 2009.The authors are with the Dipartimento di Elettronica, Informatica e Sis-temistica, University of Calabria, 87036 Rende, Cosenza, Italy (e-mail:earnieri@deis.unical.it; luigi.boccia@deis.unical.it; amendola@deis.unical.it).Color versions of one or more of the figures in this letter are available onlineat http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/LAWP.2009.2028086 all these solutions have been developed for frequencies lowerthan the Ka band, and they can become difficult to implement if their size has to be reduced even further.In this letter, a hybrid, low-profile, dual-band antenna ele-ment is presented. The proposed configuration is based on ashort-circuited patch resonating at 20 GHz, which incorporatesa dielectrically filled, open-ended circular waveguide radiatingat 30 GHz. It will be shown that the integration of two dif-ferent classes of radiators is of great benefit when developingdual-band arrays where the two frequencies are far apart. A de-tailed description of all the antenna components will be pre-sented, followed by an account of their design. With a view toapply the proposed element to dual-band Ka arrays, two arrayconfigurations will be suggested analyzing also the mutual cou-pling between the array elements.II. A NTENNA  D ESIGN The proposed hybrid dual-band antenna configuration isbased on a combination of two different antenna technologies:circular waveguides and shorted annular patches (SAPs). SAPsare annular microstrip antennas that have their inner bordershorted to the ground plane. These radiators have been widelyinvestigated [5]–[8], and a number of interesting radiatingfeatures have been pointed out. Essentially, they behave likecircular patches, but with an additional degree of freedom thatis due to their geometry. Indeed, once the dielectric charac-teristics are fixed, the SAP operation frequency and radiationcharacteristics are controlled by the size of both the outer andinner radii. As was suggested earlier [9], dual-band behaviorcan be achieved by integrating into the shorted boundary anopen-ended circular waveguide operating at a different fre-quency. Therefore, the use of a dielectrically filled waveguideis here proposed as a means for obtaining better control at thetwo operating frequencies.The geometry of the proposed dual-band antenna is shownin Figs. 1 and 2. A SAP radiator with outer radius andinner radius is printed on the top layer of a dielectric (D1)and then coupled to an asymmetrical stripline through an an-nular slot. As shown in Fig. 2, the metallized via hole formingthe SAP-shorted boundary extends over the dielectrics D2, D3,and D4, constituting the sidewalls of the circular waveguide.The waveguide operates in the mode and is fed using theapproach proposed in [10], where an E-plane stripline probe isusedincombinationwithasmalllengthofback-shortedcircularwaveguide (D5). As a consequence, the two stripline feeds areindependent,beingsandwichedindifferentlayerswithhighiso-lation between them. In order to facilitate the integration of thewaveguidewithinthe SAP radiator,a dielectricfillingwas used. 1536-1225/$26.00 © 2009 IEEE  ARNIERI  et al. : Ka-BAND DUAL-FREQUENCY RADIATOR FOR ARRAY APPLICATIONS 895 Fig. 1. Dual-band SAP-waveguide antenna.Fig. 2. Dual-band SAP-waveguide antenna geometry: side view. Although the dielectric used to fill the waveguide and that em-ployed for the stripline feed could be of different material, theywere of the same material in the present design proposal.The proposed geometry was designed to serve as a single ele-ment of a Tx/Rx array for Ka-band applications. In general, thelarge frequency separation between the transmitting and the re-ceiving channel can be a critical issue in the design of a dual-band antenna. However, the hybrid dual-band design proposedhere can be easily configured to achieve a high upper-to-lowerfrequency ratio through a proper choice of physical dimensionsand dielectric materials. For the present study, the SAP radi-ator was designed to operate at 20 GHz, and the waveguideat 30 GHz. The waveguide radius was hence fixed atmm, which corresponds to a cutoff frequency of 24 GHzwhenthefillingcontainsadielectrichavingrelativepermittivityof . The dual-band antenna geometry was definedthrough full-wave simulations [11]. The operation frequency of the shorted ring was properly tuned by setting the outer radiusat3.1mm,thedielectricheight at0.762mm,andthedi-electricconstant at3.58.Therelativepermittivityofalltheother substrates was chosen as ,while their heights were set at mm, mm,mm, and mm. Thicker dielectric slabs wereinserted between the two feeding layers (D3) and at the baseof antenna (D5) to increase the waveguide length so that a suf-ficient attenuation of high order modes is achieved. Matching TABLE IG EOMETRICAL  P ARAMETERS OF THE  D UAL -B AND  A NTENNA S HOWN IN  F IGS . 1  AND  2Fig. 3. Dual-band SAP-waveguide antenna prototype. between the 50- feeding stripline and the SAP antenna wasobtained by opportunely dimensioning the annular slot and thestripline stub. The final geometrical parameters are reported inTableI.Thecircularwaveguidewasmatchedto100 bymeansof an E-plane probe, whose geometry is shown in Fig. 1. Asingle-stageimpedancetransformerwasusedtomatchthe50-output stripline of width mm. This stub-loadedprobe terminates in a rectangular pad of width and length, located at distance from the waveguide center. Althougha simpler linear probe with wider bandwidth might be used, thismore complex geometry was chosen with a view to future de-velopment of an antenna with two polarizations.III. R ESULTS A prototype of the proposed dual-band antenna was fab-ricated employing standard multilayer printed circuit board(PCB) technology (Fig. 3). Substrates were lined up andpressed together with the aid of alignment posts and using a40 mm 40 mm metallic base and cover. The metallic coverwas 3 mm thick with a cylindrical hole of radius 10 mm, whilethe base served also to back-short the waveguide termination.Two Anritsu K102F coax-to-stripline transitions were mountedon metallic sidewalls located orthogonally to the striplines andconnected to the striplines by means of stress relief contacts.Machined dielectric rods were inserted into the top andbottom waveguide sections.The reflection coefficient was taken by connecting to a vec-torial network analyzer (VNA) the coaxial connectors that feed  896 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 8, 2009 Fig.4. Reflectioncoefficientofthedual-bandantenna:(a)SAP;(b)waveguide.Solid line: measurements; dashed line: simulations.Fig. 5. Copolar and cross-polar radiation patterns of the dual-band antenna:(a) E-plane and (b) H-plane at 20 GHz; (c) E-plane and (d) H-plane at 30 GHz.Solid line: measurements; dashed line: simulations. the SAP antenna and the cylindrical waveguide. Measurementsat both frequencies are given in Fig. 4, along with simulatedresults. As can be seen, good results were achieved at both fre-quencies.Themeasuredbandwidthsareabout950and630MHzfor the SAP and waveguide antenna, respectively. In the simu-lations,a small shift of both operation frequencies can be seen,as well as a reduction of the 10-dB bandwidth. These can be Fig. 6. Simulated mutual coupling at 20 GHz. Solid line: E-plane; dashed line:H-plane.Fig. 7. Simulated mutual coupling at 30 GHz. Solid line: E-plane; dashed line:H-plane. ascribed to imprecision during the fabrication process and mis-alignments of the coaxial-to-stripline transitions. As expected,measuredisolationbetweenthetwoportsishigherthan37dBat20 and 30 GHz, which confirms that thetwo antennas are nearlyuncoupled and can therefore be designed independently.Radiation patterns were measured at 20 and 30 GHz in theE- and H-plane of each radiator, and the results are shown inFig. 5 together with the simulated values. The SAP radiator hasagainof7.1dBinthebroadsidedirection,withacross-pollevelbetter than 14 dB below co-pol. HPBW is roughly 96 and 56in the E- and H-plane, respectively. The radiating behavior of the open-ended circular waveguide shows lower gain and lesspolarization purity. In fact, the measured boresight gain is about1.9 dB at 30 GHz, while the polarization ratio is about 12 dB.Thepoorpolarizationpuritymaybecausedbythecomplexfeedstructure, which indeed excites cross-polar field components aswell. Although measured results are consistent with simulatedpatterns, some discrepancies can be observed at low elevationangles, where the diffractions from the top cover cause a slightdeterioration in the radiation pattern. However, this effect mightwell disappear in array applications.  ARNIERI  et al. : Ka-BAND DUAL-FREQUENCY RADIATOR FOR ARRAY APPLICATIONS 897 Fig. 8. Two possible configurations of 20–30 GHz dual-band arrays based onthe proposed SAP-waveguide element: a) equal polarization in the two bands;b) polarization planes at the two frequencies are displaced 45 deg. IV. A RRAY  C ONFIGURATIONS With a view to possible use of the proposed element for dual-band array applications, interelement mutual coupling was nu-merically evaluated. Following the approach proposed in [6],mutual coupling between two dual-band hybrid antennas wasanalyzed with full-wave simulations [11] at various distancesin the E-plane and in the H-plane. In Figs. 6 and 7 the resultsare shown at 20 and 30 GHz, respectively. As can be observed,when the interelement distance is greater than , mutualcoupling is lower than 25 dB, ensuring sufficient isolation be-tween the array elements.In the design of dual-band arrays, the large separation be-tween the two operating frequencies can be balanced by in-terleaving dual-band elements with single-band antennas op-erating at the higher frequency. The single-band radiator cansimply consist of the inner waveguideof the dual-band element.One possible configuration of a dual-band array is shown inFig. 8(a), where the interelement spacing is indicated as andfor the 20- and 30-GHz elements, respectively. Using thedual-band antenna proposed in this letter, a possible array con-figuration can be designed setting mm (i.e., )and mm (i.e., ). More uniform behavior canbeobtainedusingthearrayarchitectureproposedinFig.8(b).Inthis configuration, an interleaved arrangement is considered butwith the principal planes displaced at an angle of 45 . It is easytoshowthatwhenthehigh-frequencyradiatorsareplacedonthediagonal, the separation at the two operating frequencies turnsout roughly equal, ensuring the same beamwidth in the two fre-quency bands. For instance, one could set mm (i.e.,) and mm (i.e., ).V. C ONCLUSION This letter has presented a hybrid dual-band planar antennasuited for use as a radiating element in a Tx/Rx Ka-band array.Dual-frequency operation is achieved by using a low-profileconfiguration, where a SAP antenna is integrated with a dielec-trically filled circular waveguide. The two radiating elementsturn outto be strongly decoupled,and can therefore be designedalmost independently. Furthermore, a high-frequency separa-tion between Tx and Rx bands can easily be achieved. The an-tenna can be created using any of the more common techniquesavailable, such as standard PCB technology or LTCC.R EFERENCES[1] Eutelsat, [Online]. Available: www.eutelsat.com[2] ViaSat, [Online]. Available: www.viasat.com[3] D. Pozar, D. Schaubert, S. Targonski, and M. Zawadski, “A dual-banddual-polarized array for spaceborne SAR,” in  Proc. IEEE AntennasPropag. Soc. Int. Symp. , 1998, vol. 4, pp. 2112–2115.[4] L. Shafai, W. Chamma, M. Barakat, P. Strickland, and G. Seguin,“Dual-band dual-polarized perforated microstrip antennas for SARapplications,”  IEEE Trans. Antennas Propag. , vol. 48, no. 1, pp.58–66, Jan. 2000.[5] D. Jackson, J. Williams, A. Bhattacharyya, R. Smith, S. Buchheit, andS. Long, “Microstrip patch designs that do not excite surface waves,”  IEEE Trans. Antennas Propag. , vol. 41, no. 8, pp. 1026–1037, Aug.1993.[6] M. Khayat, J. Williams, D. Jackson, and S. Long, “Mutual couplingbetween reduced surface-wave microstrip antennas,”  IEEE Trans. An-tennas Propag. , vol. 48, no. 10, pp. 1581–1593, Oct. 2000.[7] E.Arnieri,L.Boccia,G.Amendola,andG.DiMassa,“Acompacthighgain antenna for small satellite applications,”  IEEE Trans. AntennasPropag. , vol. 55, no. 2, pp. 277–282, Feb. 2007.[8] L. Boccia, G. Amendola, and G. Di Massa, “Performance evaluationof shorted annular patch antennas for high-precision GPS systems,”  Microw., Antennas Propag. , vol. 1, pp. 465–471, Apr. 2007.[9] C. Ravipati and A. Zaghloul, “A hybrid antenna element for dual-bandapplications,” in  Proc. IEEE Antennas Propag. Soc. Int. Symp. , 2004,vol. 4, pp. 4020–4023.[10] J. Navarro, “Wide-band, low-profile millimeter-wave antenna array,”  Microw. Opt. Technol. Lett. , vol. 34, pp. 253–255, 2002.[11] Ansoft HFSS. ver. 11, Ansoft Corporation.
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