Antenna matching Tools
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Advances in Impedance Matching Tools for Antenna Applications Jussi Rahola Optenni Ltd P.O. Box 42, FI-02211 Espoo, Finland [email protected] Abstract—Optenni Lab is a novel matching circuit optimization and impedance analysis tool designed especially for antenna ap- plications. Optenni Lab’s matching circuit optimization includes advanced circuit synthesis methods, a component library and tolerance analysis. In addition, Optenni Lab provides tools for estimating the obtainable impedance and efficiency bandwidths through matching circuits, and for estimating the worst-case iso- lation in multiport systems using the concept of electromagnetic isolation. I. I NTRODUCTION In modern antenna design for mobile devices, matching circuits constructed using inductors and capacitors and perhaps also transmission lines are increasingly being used to improve the bandwidth and efficiency of antennas. With matching circuits, broadband antennas can be designed much faster than by modifying the antenna geometry only. In addition, when the mechanics of the wireless device is changing, it is much easier to re-tune the antenna by changing the matching components than by modifying the antenna geometry. Thus, the use of matching circuits remarkably speeds up the antenna design process. In addition, matching circuits and thus the whole antenna system can easily be made tunable. When designing matching circuits it is important to consider the effect of component losses and tolerances. The losses in realistic components may actually give a better impedance match than ideal components, but of course at the expense of efficiency. It is also critical to evaluate the effect of component tolerances to verify that the matching circuits can be mass manufactured within reasonable performance variation. There is a clear need for new tools to speed up the matching circuit design for antenna applications. As compared to filter design, antenna matching is a more difficult task because the load impedance is complex and highly frequency dependent and therefore the analytic matching solutions are of little use. In addition, also the generator impedance may be complex, making the problem even more difficult. Optenni Lab [1] is a novel matching circuit optimization and impedance analysis software designed especially for an- tenna applications. Optenni Lab incorporates latest advances in antenna analysis and circuit simulation, but the complexity is hidden from the user so that also non-specialists can easily use the software. Optenni Lab’s user interface is designed to be intuitive for which we have received a lot of positive feedback from our customers. II. MATCHING CIRCUIT DESIGN Matching circuit design in Optenni Lab is made fast and intuitive for the end user, who only specifies the antenna impedance (in Touchstone format), the desired frequency ranges and the number of matching components. In a matter of seconds, Optenni Lab proposes several optimized matching circuit topologies to the user. The circuits can be interactively tuned and the simulation results can be stored in various formats. Optenni Lab uses the power wave definition of S parameters which correctly describes the propagation of power in mi- crowave networks with complex termination impedances [2], [3]. Thus, the matching circuit efficiency is measured by the scattering parameter |S 21 | 2 , also representing the transducer power gain. Optenni Lab is not trying to get the best possible impedance match between the antenna and the generator, but instead it maximizes the power transferred from the generator to the load through the matching circuit [4], [5]. The generator impedance can be complex and frequency-dependent, for example determined from load-pull measurements. Optenni Lab supports multiband matching with variable efficiency targets for the different bands. In addition to au- tomatically generated geometries, the user can also specify the topology manually, using inductors, capacitors, resistors, transmission lines and two-port S parameter blocks. Also the S2PMDIF format is supported. In the optimization, stop band criteria can also be entered, which gives rise to a multi- objective optimization task [6]. Optenni Lab contains an easy-to-use component library of inductors and capacitors from leading component manu- facturers, such as Murata, Taiyo Yuden and Coilcraft. The library is fully compatible with the optimization algorithms. Thus, it is easy to design and optimize matching circuit using accurate component models originally provided by the component manufacturers. These models take into account the frequency-dependent losses and parasitic effects in the components. In addition, the sensitivity of the results with respect to the component tolerances can be easily analyzed. Optenni Lab also contains a two-way link to CST STUDIO SUITE TM [7] so that the results simulated in CST can be easily opened in Optenni Lab and the generated matching circuits can be transferred back to CST DESIGN STUDIO R . Additional interfaces to other software packages are also planned. 978-1-4673-0462-7/12/$31.00 ©2012 IEEE
Transcript of Antenna matching Tools
Advances in Impedance Matching Tools forAntenna Applications
Jussi RaholaOptenni Ltd
P.O. Box 42, FI-02211 Espoo, [email protected]
Abstract—Optenni Lab is a novel matching circuit optimizationand impedance analysis tool designed especially for antenna ap-plications. Optenni Lab’s matching circuit optimization includesadvanced circuit synthesis methods, a component library andtolerance analysis. In addition, Optenni Lab provides tools forestimating the obtainable impedance and efficiency bandwidthsthrough matching circuits, and for estimating the worst-case iso-lation in multiport systems using the concept of electromagneticisolation.
I. INTRODUCTION
In modern antenna design for mobile devices, matchingcircuits constructed using inductors and capacitors and perhapsalso transmission lines are increasingly being used to improvethe bandwidth and efficiency of antennas. With matchingcircuits, broadband antennas can be designed much faster thanby modifying the antenna geometry only. In addition, when themechanics of the wireless device is changing, it is much easierto re-tune the antenna by changing the matching componentsthan by modifying the antenna geometry. Thus, the use ofmatching circuits remarkably speeds up the antenna designprocess. In addition, matching circuits and thus the wholeantenna system can easily be made tunable.
When designing matching circuits it is important to considerthe effect of component losses and tolerances. The losses inrealistic components may actually give a better impedancematch than ideal components, but of course at the expense ofefficiency. It is also critical to evaluate the effect of componenttolerances to verify that the matching circuits can be massmanufactured within reasonable performance variation.
There is a clear need for new tools to speed up the matchingcircuit design for antenna applications. As compared to filterdesign, antenna matching is a more difficult task because theload impedance is complex and highly frequency dependentand therefore the analytic matching solutions are of little use.In addition, also the generator impedance may be complex,making the problem even more difficult.
Optenni Lab [1] is a novel matching circuit optimizationand impedance analysis software designed especially for an-tenna applications. Optenni Lab incorporates latest advancesin antenna analysis and circuit simulation, but the complexityis hidden from the user so that also non-specialists can easilyuse the software. Optenni Lab’s user interface is designed to beintuitive for which we have received a lot of positive feedbackfrom our customers.
II. MATCHING CIRCUIT DESIGN
Matching circuit design in Optenni Lab is made fast andintuitive for the end user, who only specifies the antennaimpedance (in Touchstone format), the desired frequencyranges and the number of matching components. In a matterof seconds, Optenni Lab proposes several optimized matchingcircuit topologies to the user. The circuits can be interactivelytuned and the simulation results can be stored in variousformats.
Optenni Lab uses the power wave definition of S parameterswhich correctly describes the propagation of power in mi-crowave networks with complex termination impedances [2],[3]. Thus, the matching circuit efficiency is measured by thescattering parameter |S21|2, also representing the transducerpower gain. Optenni Lab is not trying to get the best possibleimpedance match between the antenna and the generator, butinstead it maximizes the power transferred from the generatorto the load through the matching circuit [4], [5]. The generatorimpedance can be complex and frequency-dependent, forexample determined from load-pull measurements.
Optenni Lab supports multiband matching with variableefficiency targets for the different bands. In addition to au-tomatically generated geometries, the user can also specifythe topology manually, using inductors, capacitors, resistors,transmission lines and two-port S parameter blocks. Also theS2PMDIF format is supported. In the optimization, stop bandcriteria can also be entered, which gives rise to a multi-objective optimization task [6].
Optenni Lab contains an easy-to-use component libraryof inductors and capacitors from leading component manu-facturers, such as Murata, Taiyo Yuden and Coilcraft. Thelibrary is fully compatible with the optimization algorithms.Thus, it is easy to design and optimize matching circuitusing accurate component models originally provided by thecomponent manufacturers. These models take into accountthe frequency-dependent losses and parasitic effects in thecomponents. In addition, the sensitivity of the results withrespect to the component tolerances can be easily analyzed.
Optenni Lab also contains a two-way link to CST STUDIOSUITETM [7] so that the results simulated in CST can be easilyopened in Optenni Lab and the generated matching circuits canbe transferred back to CST DESIGN STUDIO R©. Additionalinterfaces to other software packages are also planned.
978-1-4673-0462-7/12/$31.00 ©2012 IEEE
III. ESTIMATION OF IMPEDANCE AND EFFICIENCYBANDWIDTH
In addition to matching circuit optimization, Optenni Labalso provides innovative tools for estimating the obtainablebandwidth of antennas through matching circuits. As a startingpoint, the quality factor or Q of antennas can be estimatedfrom the impedance data directly using the formulas presentedin [8]. However, the concept of bandwidth potential [9],[10] offers a more realistic performance measure, as the Qfactor analysis is only limited to single-resonant narrowbandantennas [11] and is also very sensitive to measurement orsimulation errors.
The bandwidth potential calculation makes it possible tocompare differently matched and even non-resonant antennasand it can be used in an early design stage to rank dif-ferent antenna prototypes and concepts. It is also useful ininterpreting the results of parameter sweeps as changing theantenna parameters typically changes the antenna impedancein a complex way so that direct visual estimation of thebandwidth is often misleading.
The standard bandwidth potential calculation uses two idealcomponents for the estimation of the obtainable impedancebandwidth. To make the situation more realistic, the compo-nent losses can be taken into account in the calculation ofthe obtainable efficiency bandwidth [12]. This technology ismost useful in the selection of the best topology for tunablematching circuits. One or more components in a given circuittopology can be made tunable and thereafter an optimizationmaximizes the obtainable efficiency bandwidth through thematching circuit. In the results it can be seen what kinds ofefficiencies can be obtained at various frequencies (and variousvalues of the tuning components).
IV. ELECTROMAGNETIC ISOLATION
When studying the isolation in multiantenna systems, insome cases a seemingly good isolation is obtained as aresult of poor antenna impedance match. Thus, the effect ofthe imperfect matching should be eliminated from isolationcalculations. In Optenni Lab this can be done using the conceptof electromagnetic isolation [13], which calculates the worst-case isolation of two-antenna system. In real systems theisolation will always be better than this, because there isalways some impedance mismatch at the antenna ports.
The concept of electromagnetic isolation can be used to seethe effect of isolation-enhancing materials (e.g. metamaterials)to the antenna isolation. Metamaterials usually change theantenna input impedance and thus their effect on the isolationis difficult to evaluate from the raw impedance data. To makea fair comparison, the antennas should be re-tuned after theinsertion of the metamaterial. In Optenni Lab this re-tuningis implicitly done in the electromagnetic isolation calculation.For filter and switches the electromagnetic isolation shows thelowest possible insertion loss (which might not happen at 50Ohm termination).
V. CONCLUSIONS
Optenni Lab provides a fast and intuitive tool for optimizingmatching circuits especially for antenna applications. Theuser does not have to choose the matching circuit topologiesin advance as Optenni Lab provides automatically severaloptimized topologies to the user. In the optimization the usercan use either generic components or a component libraryof commonly used inductors and capacitors from leadingcomponent manufacturers. Both approaches support toleranceanalysis of the generated matching circuits.
Optenni Lab also includes several innovative tools foranalyzing the obtainable bandwidth through matching circuits.With these tools the user can rank antenna prototypes in anearly design stage and select the best topology for tunablematching circuits. The electromagnetic isolation removes theeffect of antenna matching in isolation calculations so thatdifferently matched antenna systems can be compared in termsof isolation.
Optenni Lab also contains innovative tools for teachingpurposes, such as the impedance trace that shows how thecomponents in a matching circuit transform the impedance ata given frequency. This tool combined with interactive tuningdemonstrates to students and antenna engineers how matchingcircuits are operating in practice.
Optenni Lab will be further developed based on recent sci-entific advances and customer requirements. In the near futurewe will focus on the development of multiantenna analysistools, while continuing to put effort in the intuitiveness of ouruser interface.
REFERENCES
[1] Optenni Lab antenna analysis software, www.optenni.com.[2] K. Kurokawa, “Power waves and the scattering matrix,” IEEE Trans.
Microw. Theory Tech., vol. MTT-13, no. 3, pp. 194–202, Mar. 1965.[3] J. Rahola, “Power waves and conjugate matching,” IEEE Trans. Circuits
Syst. II, Express Briefs, vol. 55, pp. 92–96, 2008.[4] ——, “Estimating the performance of matching circuits for antennas,”
in Proceedings of the EuCAP 2010 conference, Barcelona, April 12-16,2010.
[5] ——, “Optimization of matching circuit for antennas,” in Proceedingsof the EuCAP 2011 conference, Rome, April 11-15, 2011.
[6] ——, “Antenna matching circuit optimization including stop banddefinitions,” in Proceedings of the 2011 IEEE International Symposiumon Antennas and Propagation and USNC/URSI National Radio ScienceMeeting, Spokane, WA, July 3-8, 2011.
[7] CST STUDIO SUITE electromagnetic simulator, www.cst.com.[8] A. Yaghjian and S. Best, “Impedance, bandwidth, and Q of antennas,”
IEEE Trans. Antennas Propagat., vol. 53, pp. 1298–1324, 2005.[9] J. Villanen, J. Ollikainen, O. Kivekas, and P. Vainikainen, “Coupling el-
ement based mobile terminal antenna structures,” IEEE Trans. AntennasPropagat., vol. 54, pp. 2142–2153, 2006.
[10] J. Rahola, “Bandwidth potential and electromagnetic isolation: Tools foranalysing the impedance behaviour of antenna systems,” in Proceedingsof the EuCAP 2009 conference, Berlin, March 23-27, 2009.
[11] ——, “Estimating antenna bandwidth using the bandwidth potential andQ value techniques,” in Proceedings of the 15th International Symposiumon Antennas and Propagation, Macao, November 23-26, 2010.
[12] J. Rahola and R. Valkonen, “Using the concept of obtainable efficiencybandwidth to study tunable matching circuits,” in Proceedings of theEuCAP 2012 conference, Prague, March 26-30, 2012, to appear.
[13] J. Rahola and J. Ollikainen, “Analysis of isolation of two-port antennasystems using simultaneous matching,” in Proceedings of the EuCAP2007 conference, Edinburgh, November 11-16, 2007.
