The paper presents the results of an investigation into patch antenna elements that would be capable of pro- viding good circular polarization not only in the broadside direction, but also over a wide range of elevation angles

Guzda, Dominik

Hornik, Monika

Hornik, Paweł

Kabacik, Paweł

Biblioteka Cyfrowa Instytutu Łączności / National Institute of Telecomunications: Digital Library

PaperOptimizing circular polarizationwithin a beam of patch antenna elementsMonika Hornik, Paweł Hornik, Dominik Guzda, and Paweł KabacikAbstract— The paper presents the results of an investigationinto patch antenna elements that would be capable of pro-viding good circular polarization not only in the broadsidedirection, but also over a wide range of elevation angles.Keywords— lightweight antennas, circularly polarized antennas,microstrip antennas.1. IntroductionWhile many researchers have concentrated their studies onbandwidth widening techniques for microstrip antennas,there has been less concern paid to optimizing their po-larization quality oﬀ the broadside direction. The increas-ing popularity of wireless services has brought about thedesigning of antennas capable of generating a high qual-ity circular polarization not only in the broadside direction,but also over a broad angular range of beam, and is be-coming a crucial issue. It is strongly advisable to providesuch good circular polarization properties without sacriﬁc-ing the antenna low proﬁle – a property that is a primaryadvantage and is of paramount importance when undertak-ing application decisions.In order to achieve the satisfaction of these requirements,we considered a number of antenna concepts, with a focuson shaping the antenna feed slots. Primary attention wasalso given to the patch shape, with parasitic elements inits proximity. Our research is being driven by the needsof two minisatellite missions. The ﬁrst of them was com-pleted successfully in October 2005 when SSETI-Expresswas launched and put into orbit, the second, ESEO, isscheduled for launch in 2008.2. Methods of improving circularpolarization in patch antennasFigure 1 shows the general structure of the investigated an-tennas. It consists of a 31-mil thick microwave laminatebearing the ground and metal patch printed on the thin di-electric foil, which are separated by 10 mm thick dielectricfoam. These values of substrate heights were selected in thecourse of lengthy simulations aiming at bandwidth broad-ening. The foam material features highly stable electricalparameters over a wide temperature range. In order to en-sure broad impedance bandwidths, we designed our anten-nas using through-slot coupled transmission lines. Circularpolarization for these kinds of antennas can be achievedusing either a single slot or a dual slot feed (due to spacelimitations, we abandoned the interest in feeds made withtwo pairs of coupling slots). We focused on a dual slot feedFig. 1. General structure of the investigated antennas.because we believe it oﬀers much better axial ratio (AR) val-ues and a larger axial ratio bandwidth than the single feedmethod as far as we are capable of providing a high qualityexternal polarizer. Both output signals of the polarizer musthave an equal magnitude and diﬀer by a 90◦ phase shiftin the whole operating bandwidth. A three-strip couplerproved to be of the highest utility for such requirements.Typically, for a 2.4 GHz center frequency, it features phasevariations within 3◦ in roughly a band of 1 GHz. Manufac-Fig. 2. One of the patches in the investigated antennas.18Optimizing circular polarization within a beam of patch antenna elementstured on a thin dielectric membrane, it does not contributemuch to the weight and volume of the antenna and can beperfectly integrated with its structure. The methods of de-velopment for such couplers are described by Sachse andSawicki [3].Our investigation focused on determining the most advanta-geous shape of the slots and the geometry of their arrange-ment to be used. We compared a wide range of possibledimensions and shapes for the coupled slots [1]; the shapeof the slots in a pair may be diﬀerent, or the slots may beof identical shape and dimensions.In order to improve the axial ratio over the broad angle ofa main beam, we decided to use parasitic elements in theproximity of the patch, usually produced as a copper stripdivided into two or four sectors (Fig. 2).3. Discussion of resultsIn our analysis of printed, dielectric antennas, we used En-semble of Ansoft. Several antenna models were manu-factured and measured. Figure 3 presents the calculatedFig. 3. Simulated return loss characteristics for antennas fed witha various pairs of slots (2C, 2H, C+R).Fig. 4. Measured return loss characteristics for CP antennas fedwith 2H and 2C slot pairs.return loss characteristics for three shapes of slots. Whenthe slots are in a T arrangement, there are some diﬃcultiesin reaching equal impedance matches on two feed ports. Inmost cases, the impedance bandwidth of one port is widerthan the other. These diﬀerences can be minimized if de-signed carefully. When the slots are in an L arrangement,these problems do not occur. The return loss characteris-tics, shown in Fig. 4, were measured for assembled CP an-tenna models (Fig. 1). The measured 20 dB bandwidth wasabout 600 MHz for the dual C-slot feed (2C), and 300 MHzfor both 2H slot and C+R feeds.One of the conditions for generating an excellent circularpolarization is the high isolation between the orthogonalports. A remarkable increase in the degree of isolationcan be obtained when the slots are in a T arrangement [2],especially when one of the slots is pushed a little to theside of the symmetry axis of the patch. Owing to the T ar-rangement, the isolation between the ports was increasedby 12 dB in all cases. Thus, we had no problems achievingisolation better than 30 dB. However, it is diﬃcult to obtaina compact feed line arrangement. The calculated isolationbetween the ports in the 2C case slightly exceeds 30 dB inthe band. The calculated isolation values for 2H and C+Rfeed ranged between 22 and 30 dB. Measured characteris-tics were consistent with the calculated ones. The isolationwas only frequency dependent to a small degree.Fig. 5. The in-house developed measurement system in the mi-crowave laboratory at the Institute of Telecommunications, Tele-informatics, and Acoustics, Wrocław University of Technology.19Monika Hornik, Paweł Hornik, Dominik Guzda, and Paweł KabacikMost of electromagnetic computer tools and measurementsystems are capable of determining axial ratio character-istics only in some cut-planes (usually in two orthogonalplanes ϕ = 0◦ and ϕ = 90◦). In our research, we had to ex-amine the axial ratio within the whole forward hemisphere.This has become possible with the measurement systemshown in Fig. 5, developed in-house in our microwave lab-oratory.Figure 6 presents the measured values of the axial ra-tio (in dB) within the forward hemisphere at 2400 MHzfor a dual C-slot CP antenna. In the broadside direction(Θ = 0◦), the measured axial ratio was 0.6 dB. The param-eters of these antennas were additionally measured in SaabEricsson Space antenna laboratories, where their excellentpolarization quality was conﬁrmed. Identical antennas wereFig. 6. The measured values of the axial ratio (in dB) of the2C antenna at 2400 MHz in the forward hemisphere.used onboard the SSETI-Express minisatellite. For theother two introduced antenna models, we obtained the ax-ial ratio values 1.34 dB and 2.36 dB at the broadside for2H-slot and C+R slots (Fig. 7), respectively.However, the greatest interest was in providing a good de-gree of circular polarization in a possible wide angularrange in the forward hemisphere of the radiation pattern.A great impact on this parameter was the shape and ar-rangement of the feed slots. The T arrangement, whichsigniﬁcantly improves the isolation between the feed lineports, raises concerns in regard to symmetry. One of theslots must be pushed a little to the side of the symme-try axis of the patch. This leads to some asymmetry inthe radiation pattern and in the CP quality of the anten-nas, which considerably diﬀerentiates the values of theaxial ratio between the cut-planes. While there are excel-lent values at one cut-plane across the hemisphere, weshould expect poorer axial ratio values at high elevationFig. 7. The measured values of the axial ratio (in dB) of theC+R antenna at 2400 MHz in the forward hemisphere.angles in the other principal cut-plane. Only with 2C and2H antenna types, AR has taken the values below 2 dBin the broad angular range of the main beam. We ob-served that the 2C antenna features favorable AR valueswithin at least 60◦ of the elevation angle (θ ). In somecut-planes at elevation angles close to endﬁre, the circu-lar polarization quality can deteriorate abruptly and actu-ally become linear polarization within the narrow angularstep (AR drop by more than 10 dB). Even with the best2C type antennas, this drawback occurs at two diagonalcut-planes.Our investigations have shown that in some cases, the po-larization quality of the antenna can be signiﬁcantly im-proved when a conductive parasitic strip partitioned intotwo or four sectors surrounds its patch. We have foundthat the axial ratio values may considerably depend onthe angular orientation of the gaps between the sectorsof the strip in respect to the feed slots (ξ angle shownin Fig. 2).This claim is substantiated by plots (Figs. 8 and 9) show-ing the results of the simulations carried out for the an-tenna feed with two rectangular slots. In all considereddirections (θ = 0◦), (θ = 40◦, ϕ = 0◦), (θ = 40◦, ϕ = 90◦),etc., the AR value was the lowest for ξ ∈ 〈30◦,40◦〉.20Optimizing circular polarization within a beam of patch antenna elementsFig. 8. Simulated values of the axial ratio in speciﬁed directionsof the antenna as a function of angular orientation of the gapsbetween the strip sectors.Fig. 9. Simulated values of the axial ratio for the diﬀerent angularorientation of gaps between the strip sectors and for the patchwithout the strip in the operating band of the antenna.In Fig. 9, the axial ratio of the antennas with patchessurrounded by a strip (for varying ξ ) are compared withpatches without a strip in the 2350 MHz to 2450 MHzfrequency band.4. ConclusionsHigh quality of circular polarization over a broad angleof an antenna beam has been achieved with low proﬁlemicrostrip patches. One of the antennas we had devel-oped is characterized with favorable values (< 6 dB) ofthe axial ratio in some cut-planes even from θ = −90◦to θ = +90◦. Good polarization properties have beenshown in the large range of the forward hemisphere witha few other microstrip patch designs. The prerequisites ofgood polarization properties are compact forms of cou-pling slots used in the element feed (i.e., C), their care-ful arrangement under the patch (i.e., T geometry) and theuse of parasitic, small conductive shapes in the proxim-ity of an antenna patch. The axial ratio values are thenlikely to match those known from bulkier shapes andvolumes, and until now, widely used antenna elements(e.g., helix).The patches optimized and presented in this paper are tobe used mostly in space communication, polarimetric radarand in radio interfaces of wireless services.References[1] P. Kabacik, K. Wincza, M. Kamaszuk, and P. Hornik, “Optimiz-ing circular polarization in broadband lightweight patch antennas”, inIEEE AP Soc. Int. Symp., Washington, USA, 2005.[2] S. K. Padhi, N. C. Karmakar, C. L. Law, and S. Aditya, Sr., “A dualpolarized aperture coupled circular patch antenna using a C-shapedcoupling slot”, IEEE Trans. AP, vol. 51, no. 12, pp. 3295–3298,2003.[3] K. Sachse and A. Sawicki, “Quasi-ideal multilayer two- and three-stripdirectional couplers for monolithic and hybrid MIC”, IEEE Trans.MTT, vol. 47, no. 9, pp. 1873–1882, 1999.Monika Hornik was born inBielsko-Biała, Poland, in 1982.From 2001 to 2006 she stud-ied electrical and computer en-gineering at Wrocław Univer-sity of Technology, Poland.In July 2006 she graduatedwith M.Sc. degree, with spe-cialization in telecommunica-tion, following presentation ofher thesis on “Gain enhance-ment methods for lightweight microwave antennas”. Shewas admitted on Ph.D. course in electrical engineering atWrocław University of Technology. For three years shehas taken part in the project Student Space Exploration andTechnology Initiative (SSETI). She has carried on researchon lightweight, low-proﬁle, microstrip antennas for the useonboard International Space Station.e-mail: Monika.Hornik@pwr.wroc.plInstitute of Telecommunications, Teleinformatics,and AcousticsWrocław University of TechnologyWybrzeże Wyspiańskiego st 2750-370 Wrocław, Poland21Monika Hornik, Paweł Hornik, Dominik Guzda, and Paweł KabacikPaweł Hornik was born inOpole, Poland, in 1982. From2001 to 2006 he studied electri-cal and computer engineering atWrocław University of Technol-ogy, Poland. In July 2006 hegraduated with M.Sc. degree,with specialization in telecom-munication, following presenta-tion of his thesis on “Cylindri-cal lightweight microstrip an-tenna array for space borne application”. For three years hehas taken part in the project Student Space Exploration andTechnology Initiative (SSETI). He has carried on researchon lightweight, low-proﬁle, microstrip antennas for the useonboard International Space Station.e-mail: hpol@poczta.fmInstitute of Telecommunications, Teleinformatics,and AcousticsWrocław University of TechnologyWybrzeże Wyspiańskiego st 2750-370 Wrocław, PolandDominik Guzda was born inKrapkowice, Poland, in 1981.He studied telecommunica-tions on Faculty of Electricaland Computer Engineering atWrocław University of Technol-ogy, Poland. He has taken partin the project Student SpaceExploration and TechnologyInitiative (SSETI) for threeyears. Currently he is workingin Siemens Networks company and is engaged in develop-ment of intelligent networks.e-mail: dominik.guzda@gmail.comInstitute of Telecommunications, Teleinformatics,and AcousticsWrocław University of TechnologyWybrzeże Wyspiańskiego st 2750-370 Wrocław, PolandPaweł Kabacik was born inWrocław, Poland, in 1963.He received the M.Sc. degreein telecommunications (high-est honors) from Wrocław Uni-versity of Technology, Poland,in 1986. In 1996 he receivedthe Ph.D. in electrical engi-neering (award). In January1987, he joined the Insti-tute of Telecommunications andAcoustics, Wrocław University of Technology. He wasa visiting scholar to the Technical University of Denmark(1991/1992) and to the University of Queensland (1997,2001, and 2006). He chaired the Mulitband Antennas andConformal Array Mini-Team acting within the COST 260Project and he chairs one of four research tasks in thecurrent COST 284 Project. He is an expert of EuropeanCommission. His research interests include highly inte-grated antenna arrays, conformal antennas, terminal anten-nas, phased arrays, advanced antenna measurements andcommunication subsystems of small spacecraft. He wasa Principal Investigator to several contracts funded by theNational Research Council, Poland, and by industry. Heis a member of Technical Committees at several interna-tional conferences. He received several recognitions forhis research work, of which the most valuable is the “2000Harold A. Wheeler Applications Prize Paper Award Honor-able Mention” of the IEEE. In 1993 he received the Awardfor Young Scientists at the 7th National URSI Symposium,Poland. He was listed in six editions of “Marquis’s Who’sWho in Science and Engineering”.e-mail: Pawel.Kabacik@pwr.wroc.plInstitute of Telecommunications, Teleinformatics,and AcousticsWrocław University of TechnologyWybrzeże Wyspiańskiego st 2750-370 Wrocław, Poland22

Optimizing circular polarization within a beam of patch antenna elements, Journal of Telecommunications and Information Technology, 2007, nr 1

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Optimizing circular polarization within a beam of patch antenna elements, Journal of Telecommunications and Information Technology, 2007, nr 1

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