A Wideband Wearable Antenna for ISM and WLAN Applications
Kavisha Singh1 , Dr. Nand Kishore21M. Tech Scholar, Dept. of Electronics Engineering, Harcourt Butler Technical University, Uttar Pradesh, India
2Assistant Professor, Dept. of Electronics Engineering, Harcourt Butler Technical University, Uttar Pradesh, India ***
Abstract - A wideband slotted patch antenna with a partial ground for wearable applications is presented in this article. The dimensions have been optimized to obtain a compact structure of 30x20x0.8 on FR4 epoxy substrate. A triple band operation with resonant frequencies at 2.4 GHz, 3.8 GHz and 5.8 GHz is observed. A peak gain of 3.7 dB is found for the center frequency of 5.8 GHz with 97% efficiency. The average value of SAR simulated on a three-layer human body phantom for an input power of 1 mW is 0.0175 W/kg. The antenna is appropriate for implementation in Wireless Body Area Networks.
Key Words: Wearable, Complementary split ring resonator, Return loss, SAR
1.INTRODUCTION
Theworldofwearableshasgainedatremendousincreasein the last decade owing to their relevance in the areas of healthmonitoring,diagnosis,trackingandlocationsensing [1].Wearableantennasaremostcommonlydevelopedfor operationatspecificfrequencieskeepinginviewtheareaof application. These antennas are most widely used in WirelessBodyAreaNetworks(WBAN)duetotheirpromise invarioushealthcareapplications.Asetoffrequencybands are designated to commercial WBAN systems including frequencies in the range of 402 MHz to 10.6 GHz [2]. The realizationofwearableantennashasstrongdependenceon the lossy nature of body tissues. The close proximity of human body, structural deformation, robustness and wearer’scomfortarethecrucialaspectsinwearableantenna configuration. These antennas also tend to suffer from frequencydetuning[1]. Astheseantennasformapartofon bodycommunicationsystem,theyneedtobelightweight, compactandwaterresistant.
Microstrip patch antennas form a viable solution for wearableantennadesignastheyproduceomnidirectional radiationpatternandcanbeminiaturizedbythechoiceof suitablesubstrates.Theeaseoffabricationandlowcostadd totheadvantagesofmicrostrippatchantennas[3].
In[2]authorshavepresentedaphotopaperbasedflexible antennaoperatingin2.5GHzand5.8GHzfrequencybands. Thoughphotopaperisflexibleandlowcostbutitisproneto degradation.In[4]anantennafabricatedonafeltsubstrate with dual band operation is proposed for on/off body wearable applications. The antenna spans the ISM and WLANfrequencies.Inreference[5]authorshavepresented
a triband electronic bandgap array antenna operating in WLAN and WiMAX bands. In [6] three forms of dual band (2.45GHzand5.2GHz)antennaswithelectronicbandgap structure have been discussed. Though the use of EBG structure beneath the antenna makes them bulky. The authorsof[7]havepresentedasemiflexiblehybridMoore fractalantennaoperatingin1.38-1.8GHzand2.45-4.88GHz bands.Theproposedantennahasacompactgeometryand lowervaluesofSAR.Ameanderlinepatchantennaspanning frequenciesbetween2.36-2.46GHzisdiscussedin[8].In[9] theauthorshaveproposedanantennaincorporatingafoam spaceroperatingat2.45GHz.Awidebandminkowskifractal geometry-basedantennaispresentedin[10]withoperation infrequencyrangeof700MHzto4.71GHz.Theauthorsof [11]havepresentedanIDEbandgapunitcell-basedantenna foruseinMBANapplications.In[12]a2.45GHzISMband antennawithdoubleflexiblesubstratesisdiscussed In[13] anISMbandantennawithdenimasasubstateispresented. However, textile antennas are susceptible to water and moisturecontent.
Thisworkpresentsatriplebandcompactwearableantenna with an acceptable value of SAR. The antenna shows a widebandoperationandissuitableforWBANapplications. The comparatively simple design and smaller size as comparedtopreviousworksmakesitagoodalternativefor wearableapplications.Freespaceandonbodyanalysishave been performed for the proposed antenna. The reflection coefficient,acceptablegainandonbodySARsimulationsalso showthefeasibilityoftheproposedwork.
2. GEOMETRY
TheantennaisstructuredonaFR-4epoxysubstratewitha relativepermittivityof4.4andadissipationfactorof0.02.A highdielectricconstantsubstratewasutilizedtobringdown the measurements of the proposed antenna [14]. The resulting geometry has compact dimensions of 30mm x 20mm x 0.8mm. The design process started with the modellingofasquareshapedpatchandapartialground.The limited size of ground plane helps in broadening the bandwidthofantenna[15].Foursquareshapedslotsatthe corners of the patch and a 0.5 mm wide single complimentary split ring resonator were incorporated to achieve resonances at the desired frequencies. The dimensions of slots have been optimized to obtain the desiredoperation.Inordertoimprovethereturnloss,three slotsofequalsizewereetchedfromthelateralextremitiesof thepatch.AnupturnedSshapedgroovewasetchedtorefine
theoperationoftheantenna.TheSshapedslotalsohelped in achieving operation in 3.8 GHz band. A microstrip line feedof50Ωisappliedtotheantenna.Fig.1showsthelayout oftheplannedantenna.
3. RESULTS
Acomputationalanalysishasbeenperformedtodetermine thereturnlossoftheintendedantenna.Thesimulated findings reveal the triband operation of the antenna. Resonanceisachievedatthefrequenciesof2.4GHz,3.8GHz and5.8GHz.Therelatedreturnlossfiguresare-16.18dB,13.03dBand-34.93dBrespectively.Theantennashowsa widebandoperationfrom4.5GHz-6.4GHz.Fig.2depictsthe reflection coefficient curve of the planned antenna. The monitored E plane and H plane radiation patterns for the resonantfrequenciesaredisplayedinFig.3.
Theobservedgainatfrequenciesof2.4GHz,3.8GHzand5.8 GHzare0.6155dB,2.7226dBand3.7205dBrespectively. TheVSWRcharacteristicsrepresentedinFig.4exhibitfair levelsofimpedancematchingoftheantenna.
Wearableantennasfunctioninpropinquityofhumanbody therefore the antenna behavior is dependent on the dielectricpropertiesofbodytissues.Thevaryingelectrical propertiesofdifferentlayersoftissuesrequiretheantenna tobeoptimizedwithahumanbodyphantominthedesign process.Ahumanbodyphantomcomprisingthreelayersof tissues was utilized to study the impact of human body reciprocity with the antenna. The dimensions of the phantom are 50mmx50mmx33 mm. The three-layered phantom is made up of skin, fat and muscle tissues of thickness2mm,8mmand23mmrespectively.Therelative permittivity,conductivityandlosstangentatfrequencies2.4 GHzand5.8GHzhavebeenspecifiedintable2.
The antenna reflection coefficient characteristics when placedonthephantomareshowninfig.6.Theresultsshowa slight detuning at the frequencies of resonance. The resultsshowoperationinthedesiredbands.Theradiation patternforonbodysimulationshavebeendepictedinfigure 7.
5. SAR ANALYSIS
Themagnitudeofradiation thatcanbesustainedperunit mass of human body is called SAR. According to InternationalStandardsthestipulatedvaluesofSARare1.6 W/kgaggregatedover1gmoftissueand2W/kgover10gm oftissuerespectively[1].TheamountofSARwasmeasured forthefrequenciesof2.4GHzand5.8GHz.Radiationpattern analysis of on body measurements reveal the reduced amountofbackloberadiationswhenantennaisemplaced onphantom.Thereducedbackloberadiationscorrespondto lesseramountofSAR.
ThemeasuredSARfor1mWinputpowernormalizedover1 gm of tissue at 2.4 GHz is 0.0175 W/kg and at 5.8 GHz is 0.0879 W/kg. The SAR analysis also reveals that the penetration depth of electromagnetic waves is lesser at higherfrequencies.
REFERENCES
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6. CONCLUSIONS
:SARAnalysis
The proposed antenna has a tri band operation in ISM, WiMAX and WLAN frequency bands. The impedance bandwidths measured across the three bands are 4.61%, 13.33% and 34.86% respectively. The range of operation shows that the proposed work is acceptable for WBAN applications.Theon-bodyreflectioncoefficientandradiation patternresultsalsoshowtheefficacyoftheproposeddesign. TheantennahasfairvaluesofgainandSARmeasurement whichmakeitsuitableforonbodycommunication.
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