Skrivervik, A. K.

Staub, O.

Zürcher, J. F.

Infoscience - École polytechnique fédérale de Lausanne

SMILA: A COMPACT AND EFFICIENTANTENNA FOR MOBILECOMMUNICATIONSJ.-F. Zurcher,1 O. Staub,1 and A. K. Skrivervik1¨1 Laboratoire d’Electromagnetisme et d’Acoustique´Ecole Polytechnique Federale de Lausanne´ ´CH-1015 Lausanne, SwitzerlandReceied 15 May 2000ABSTRACT: A new antenna, integrated with the shielding structure ofthe deice, is described. Despite its compact dimensions, it has excellentradiation properties.  2000 John Wiley & Sons, Inc. Microwave OptTechnol Lett 27: 155157, 2000.Key words: electrically small antennas; mobile communications; PCS;DECT; GSM; PIFA; SMILA1. INTRODUCTIONWith the development of miniaturized devices for mobilecommunications, the need for compact and efficient antennasincreases. Ideally, these antennas should be simple and ex-tremely inexpensive to manufacture, while having good elec-trical performances, and especially a high efficiency. This isimportant because, due to limited battery space, the dc poweris always limited, a fact which limits the RF power even more.On the receiver side, any additional decibels of gain providedby the antenna improve the SN ratio at no battery con-sumption cost.Typical applications for such antennas are DECT or GSM.2. ANTENNA DEVELOPMENTThe application for which this antenna has been developed isŽ .in the 1.81.9 GHz band DECTDCS , and the mechanicalconstraints were stringent: a metallic cylindrical case measur-ing 35 mm in diameter with a height of 10 mm.To optimize the performances of an electrically smallantenna, it must fill most of the available volume, while lossesshould be as low as possible. One way to attain this goal is tolocate the radiating part around the outer wall of the cylindri-cal box to maximize the antenna dimensions.The development of the following design started from theŽ .  PIFA planar inverted-F antenna 1 , which has followingadvantages: Ž .low losses no dielectric, thick metal simple concept Ž .easy to match position of the feed Ž .easy to tune adjusting the length of the arm .A classical PIFA is depicted in Figure 1. The properties ofthis antenna can be adjusted when varying the various rela-tive dimensions: an increase in H widens the bandwidth Ž .reducing W W L1 reduces the overall dimensions,but also the bandwitch L2 allows a frequency tuning every modification changes the position of the feedpointfor a given impedance.Figure 1 Standard PIFA with its main dimensionsIn its most usual shape, the PIFA has approximately theproportions shown in Figure 1. For our application, the shapeŽ .will be closer to a strip W L2 , and the width of the shortŽ .circuit will be the same as the antenna width W L1 .Furthermore, the antenna height H will be small due to thelimited space. For such an antenna, the feedpoint has to beplaced close to the short-circuited end to obtain a 50 impedance, and the bandwidth will be reduced due to thereduction in dimensions and height. On the other hand, it hasbeen shown that the bandwidth of a PIFA increases when the ground plane is reduced 1 .The study started with the design and test of a narrowŽ .PIFA W L1 10 mm, similar to the case height having aheight H between 3 and 4 mm. Simulations have been madewith HP ADSMomentum, which gave a fairly good matchwith measured results. On the realized prototypes, the groundplane, which was initially 80 80 mm large, was progres-sively reduced. The effect on the resonant frequency wasnegligible, but the 10 dB bandwidth increased. This is consistent with data published in the literature 1 .Then, a bent PIFA mounted on the side of a round boxwas simulated. The simulation could no longer be made withHP ADSMomentum since the geometry is not planar. Ten-tative attempts have been made to simulate this structurewith HP HFSS; however, the simulation times were excessive,even with a reduced number of tetrahedrons. Other testshave been made using NEC, which gave acceptable results.A prototype was constructed and tested. The frequencycould very easily be tuned by cutting the length of theradiating strip, while the matching could be adjusted byvarying the position of the feed point relative to the short-cir-Žcuited side. This position is relatively critical a few tenths of.a millimeter .Finally, the antenna was fully integrated within the box.Figure 2 shows the evolution from the ‘‘classical’’ PIFA to theversion integrated with the box.ŽThis version has been called SMILA smart monoblock-.integrated L-antenna , and is depicted more in detail inFigure 3.The final prototype has been drawn with the CAD pro-gram SolidWorks, and machined by CNC out of a massivebrass block.MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 27, No. 3, November 5 2000 155Figure 2 From PIFA to SMILA3. MEASURED PERFORMANCESThe performances of the SMILA have been measured accu-rately. Figure 4 shows the measured SWR. The bandwidth foran SWR 2 is 77.3 MHz, corresponding to 4.1%. It must benoted that the S -measurement using a network analyzer is11influenced by surface waves propagating around the cable.ŽThis is due to the fact that the antenna is very small about.5 . To make proper measurements, absorbing material hadto be put around the cable.To measure the maximum gain and radiation patterns, anyconnection to the outside world should be avoided as this would completely modify the results 2 . The technique de- scribed in 3 has been used: a VCO and a miniature batteryhave been integrated into the SMILA box. Figure 5 shows theprototype used for measurements.The measured peak gain is 0.8 dBi, very close to the  Žtheoretical limit 4 , which is 2.2 dBi for a perfect antennacompletely filling a sphere having the diameter of our cylin-.drical case . This good result is due to the nature of theŽantenna: no dielectric, relatively thick metallic parts 0.5 mm.thickness , and is also due to the fact that the antenna sizeFigure 3 Detailed view of the SMILA showing the importantparametersoccupies a maximum of volume, and that the case itselfparticipates in the radiation.The radiation patterns have been measured, and show nodeep null: the patterns are relatively omnidirectional, as canbe expected from an electrically small antenna.It has to be noted that, for such a small antenna, thepolarization is not well defined. This is not a drawback since,in mobile communications applications, there are always manyreflections which randomly degrade the polarization purity.One limitation of such an antenna is the lower frequencylimit as a function of the box dimensions. For instance, for a35 mm diameter box, the lower limit is about 800 MHz: atthis frequency, the antenna would occupy the entire circum-ference of the box. Another limitation is the bandwidth: itcould be increased by increasing the antenna height H, butthis is not always possible due to design considerations. Butthese limitations are common to all antennas which are smallwith respect to the wavelength.4. CONCLUSIONSThe SMILA described here is a simple and efficient antenna,which can easily be manufactured as an integral part of themetallic box enclosing the electronic, reducing the costs topractically zero. Its frequency and impedance tuning are verysimple. In the described application, its shape is round, butany other shape can be envisaged, depending on the applica-tion. To the authors’ knowledge, it has the maximum possiblepeak gain within the available volume, and its bandwidth issufficient for many applications.ŽThis antenna has been patented Swiss Patent 1999.184999 and European Patent 99120230.0MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 27, No. 3, November 5 2000156Figure 4 Measured SWR and Smith chart of the SMILAFigure 5 SMILA prototype used for the measurements, with inte-grated VCO and batteriesREFERENCES1. T. Taga, ‘‘Analysis of planar inverted-F antennas and antennadesign for portable radio equipment,’’ Analysis, design and mea-surement of small and low-profile antennas, K. Hirasawa and M.Ž .Haneishi Editors , Artech House, Norwood, MA, 1992, pp.161180.2. O. Staub, J.-F. Zurcher, A. Skrivervik, Some considerations on the¨correct measurement of the gain and bandwidth of electricallyŽ .small antennas, Microwave Opt Technol Lett 17 1998 , 156160.3. J.-F. Zurcher, O. Staub, A.K. Skrivervik, and M. Hermanjat,¨Accurate measurement of the maximum gain of electrically smallŽ .antennas, Microwave Opt Technol Lett 23 1999 , 328331.4. R.F. Harrington, Effect of antenna size on gain, bandwidth andefficiency, J Res Nat Bureau StandardsD, Radio Propagat 64DŽ .1960 , 112. 2000 John Wiley & Sons, Inc.MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 27, No. 3, November 5 2000 157

SMILA: a compact and efficient antenna for mobile communications

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SMILA: a compact and efficient antenna for mobile communications

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