Design and Simulation of Ultra Wide-band Antenna for High Data Rate Applications

Transcription

1 Design and Simulatin f Ultra Wide-band Antenna fr High Data Rate Applicatins Ramaswamy Karthikeyan B1, Gvind R.Kadambi2 1 M.Sc (Engg.) student, 2 Prfessr and Dean-Research, Wireless and Optical Cmmunicatin M.S.Ramaiah Schl f Advanced Studies, Bangalre Abstract The UWE radi applicatin cvering the frequency spectrum f GHz ffers a ptentially attractive slutin fr wireless system applicatins. The assciated benefits f UWE technlgy are bandwidth. cst. pwer cnsumptin and cnsistent high data rates. Hwever fr system implementatin f UWE technlgy. design and develpment f antennas cnstitute a critical rle in determining the viability and engineeringfeasibility fuwb. One f the primary bjectives f this paper is t frmulate a design cncept f utilizatin f planar mnple f cmpact size and relatively simpler structural cnfiguratins fr UWB systems. This paper presents design and simulatin studies n UWB planar mnple cnfiguratin cmprising the gemetric prfiles f cannical shapes such as rectangular. ellipse and circular. This paper als cvers the nvelties and the design advantages f a slt n the radiating structure f planar mnple fr the design implementatin t accmplish UWEperfrmance. The cncept f tuning stubs lcated n the slt cnturs f the planar mnple has been illustrated t realize the ultra wide bandwidth f the antenna. This paper als brings ut a relative perfrmance cmparisn f varius mnple prfiles. The sizes f the radiating element as well as that f the grund plane f the prpsed design are significantly smaller than that presented in earlier reprted designs. The cncept f pen slt n a planar mnple and the design f the tuning stubs are majr cntributins f this paper t antenna engineering. With a view t place emphasis n the nature f the radiatin characteristics f UWB antenna. thus paper als deals with a typical study n the develpment f Vivaldi antenna. Vivaldi antenna has the distinct feature f exhibiting end-fire radiatin pattern. Key Wrds: UWB Antenna, Planar Antenna and Vivaldi Antenna 1. Intrductin UWB is an unique and new usage f recently legalized frequency spectrum. UWB radis can use frequencies frm 3.1 GHz t 10.6 GHz - a band f7.5 GHz wide. Each radi channel f UWB system can have a bandwidth f mre than 500 MHz, depending n its center frequency [1-3]. The antenna design faces new challenges due t the distributin f the wide span f UWB band with special emissin masks. The requirements fr cnventinal antennas used fr UWB r impulse systems include a brad impedance bandwidth, a stable transmit-receive transfer respnse, and high radiatin efficiency. In the past, many designers and researchers have studied / analyzed such antennas in great detail. Transverse electrmagnetic mde hrns and self-similar spiral antennas are typical bradband slutins but suffer frm the severely frequency-dependent changes in their phase centers. Bicnical and disk-cnical antennas feature stable phase centers acrss brad bandwidths by using resistive lads t suppress the reflectin ccurring at their ends. Hwever, the cnventinal bradband antennas mentined abve usually are t bulky t be suitable fr prtable UWB devices [4-5]. The antenna structures like spiral, lg peridic antennas are difficult t be mdeled using cmmercial EM sftwares. Design ptimizatin invlving the significant dimensins is nt a straightfrward prpsitin [6-7]. In the design f micrstrip antenna prpsed by Sek H. Chi et al. [8], the realized bandwidth was partially due t nn existence f grund plane under a sectin f micrstrip antenna. The design presented by Takashi Arita et. al. [9], was lacking the cmpactness as well as design versatility. In the analysis f Jhnna Pwell [10], the Hrn, bicnical and helical antennas are suitable fr UWB but they are 3D and nn planar in structure. The planar gemetries like bwtie, diamnd diple and rectangular lp antennas d nt have bandwidths brad enugh fr UWB. In rectangular planar mnple design f Girish Kumar et al. [II], the authrs are nt successful in realizing the bandwidth sufficient fr UWB applicatins. Further it is als bserved that the dimensins required fr the achievement f bandwidth fr UWB with certain cnfiguratins are significantly large. The Vivaldi antenna prpsed by Raviprakash Rajaraman [12] des nt psses the UWB bandwidth fr its single element. In view f the abve bservatins there is a need t lk int the alternative design slutins fr UWB antennas, which are characterized with desirable features like cmpactness, design versatility and prvisin fr relatively easy tuning. This paper attempts t undertake a design methdlgy f UWB antennas, which vercmes sme f the shrtcmings f the existing slutins. SAsTECH 102 Vl. VI, NO.1, April 2007

2 2. GEOMETRIC MODEL Planar mnple antenna can be designed in many radiatr gemetric prfiles like Rectangular, Circular, Elliptical, Triangular and unsymmetrical shapes. Since the antenna is designed fr UWB system, its cmpactness and mbility are f majr cncern while designing the antenna. The radiatr is mdified with the different cannical shapes like Rectangular, Circular, Elliptical s and the length f the grund plane is fixed at 25 mm. Hwever the surface area f radiatr is identical fr all the gemetric shapes. The Dielectric substrate is chsen as FR-4 f Dielectric cnstant 1:,=4.3. The thickness f the metallizatin ver the substrate is 0.02mm. Slts Tuning Stubs Radiatr Substrate Strip Cnnectr t the base f radiatr Lumped Prt at Optinnun Pint Grund plane 2.1 Rectangular The rectangular mnple is designed with radiatr dimensin f 20 x 21 mm. The radiatr is mdified by the intrductin f slts and the feed pint is varied fr its ptimum impedance matching, which is explained in detail later. 2.3 Elliptical Fig 2:Circular T1I1linc Stub. Substrate The design f the elliptical mnple has been ptimized with the radiatr dimensins f majr axis 16mm and minr axis 10.4mm. The elliptical slts are intrduced fr better impedance bandwidth. The effects f slts, tuning stubs and change in the feed pint are als described in detail in later sectins. Radiatr --_. L1IIUped Pri at OptUnwn Pint Grund plane Slts - Tuning Stubs (I1 and T2) Radiatr Strip Cnnectr t the base f radiatr Fig I: Rectangular - Substrate Lumped Prt at Optimum Pint Grund plane The ptimized design gemetry f the rectangular mnple is shwn in the Fig 1. The rectangular mnple antenna shws a return lss better than -10 db acrss the UWB spectrum f GHz. Fig 3: Elliptical 2.2 Circular The circular mnple antenna has been ptimized fr the radiatr dimensin f radius 11.4mm. The intrductin f slts and matching stubs are imprtant t cntrl the bandwidth and the resnance. The Fig 2 shws the ptimized design f circular mnple fr UWB applicatins. 2.4 Vivaldi Antenna The Vivaldi antenna illustrated in Fig 4 cnsists f tapered slt etched n ne side f a duble-sided PCB. The slt is narrwer twards ne end. The traveling wave, which prpagates alng the tapered slt, radiates in the end-fire directin. It has a very wide pattern bandwidth and has the capacity t generate a symmetric main beam despite its planar gemetry. SASTECH 103 Vl. VI, NO.1, April 2007

3 The very imprtant aspect f the Vivaldi antenna is that the radiatr is the etched part f the grund plane. Theretically the bandwidth f the Vivaldi antenna can be infinite, but is limited by its physical size and the fabricatin capabilities 1 41mm j mm- Fig 4: Vivaldi Antenna Dielectric Substrate Tp Metallizatin Tapered Slt Slt line Transitin Feeding Stub Micrstrip 3. SIMULA non OF ANTENNA DESIGNS Prt The basic design dimensins f the antenna are arrived at frm theretical as well as empirical basis and drawn in the Ganymede Graphical editr, which cmes alng with the Empire 3D Electrmagnetic field slver. Fig 5 shws the return lss plt f a rectangular planar mnple antenna withut slt and tuning stub. As can be seen frm the plt, the bandwidth f the rectangular planar mnple antenna is nt very wide despite many resnant frequencies with in the UWB spectrum. The primary reasn fr decreased bandwidth can be attributed t intermediate peaks f the return lss plt with magnitude higher than 10dB. The presence f the intermediate peaks makes the return lss plt a sequence f discrete narrw bands with separatin between them. One f the alternatives t increase the bandwidth f the rectangular planar mnple antenna is t reduce the magnitude f the intermediate peaks t be lwer than -10 db. In rder t btain the cntinuus bandwidth fr ultra wideband spectrum, the experimentatin is carried ut further e+9 4e+9 6e+9 frequency in Hz 8e+9 le+lo Fig 5: SII plt fr Rectangular mnple withut slts and tuning stubs. There are many techniques t ptimize the planar antenna perfrmance and sme f the designs have been reviewed. Here the experimentatin is carried ut with the variatin f feed pint, intrductin f slts with pen end and tuning stubs n the slt cntur. 3.1 Feed pint Variatin The determinatin f the feed pint is an imprtant parameter in the antenna design. In the initial design simulatin, the feed pint is assumed t be at the mid pint alng the width f the planar mnple. In subsequent design iteratins, the feed pint is mved incrementally alng the width f the planar mnple t determine the ptimum impedance match Feed pint variatin in Rectangular mnple If the antenna is fed at the extreme left edge f the radiatr, there will be tw resnant frequencies but the bandwidth is nt enugh fr UWB applicatins. If the feed pint is in the center, it exhibits three resnant frequencies. Yet it des nt shw UWB perfrmance. The precise lcatin f the feed pint is determined by varying the psitin f the feed pint by a step f 0.5 mm and repeating the simulatin. The ptimum feed pint fr return lss f -I OdB between GHz, is lcated in between the left edge and the mid pint alng the width fthe mnple Feed pint variatin in Circular mnple The feed pint f the circular mnple cannt be changed in view f its circular shape. S a small f strip f height 0.25 mrn is cnnected at the base f the radiatr and the lumped feed prt is cnnected t the strip shwn in Fig 2. The feed prt can be mved alng the strip t lcate the ptimum feed pint and it is placed at 1.5mm twards right frm the center f the radiatr Feed pint variatin in Elliptical mnple Like in circular mnple, the elliptical mnple design als invlves the attachment f a strip f height 0.25mrn at the base f the radiatr as shwn in Fig 3. In the specific design ptimizatin, the feed pint is lcated at 2.4mm twards the right with respect t center f the elliptical mnple. 3.2 Effect f slt t shift the resnance frequency When the Radiatr f the chsen dimensins des nt yield the bandwidth fr UWB peratin, the slt can be intrduced n the radiatr, which increases the perimeter f the radiatr, i.e. increased length fr the current flw path, which in turn will shift the resnant frequency f the antenna. The width f the slt als affects the design. The dimensins f the slt are ptimized fr desired results Slts n the Rectangular mnple In the rectangular mnple gemetry, bth the vertical and hrizntal slts f width 1.5mm are intrduced as shwn in Fig I. Trimming the length f the vertical slt imprves the SAsTECH 104 Vl. VI, NO.1, April 2007

4 impedance match at the higher frequencies. Trimming the length f the hrizntal slt reduces the number f resnant frequencies. The simulatins are carried ut with the pen end f the slt at the center; If that slt pening is mved twards right it prduces mre number f resnances in the first half f the UWB spectrum and the peaks f the return lss plt f the rectangular mnple appearing in the upper half f the UWB spectrum are fund t be abve -IOdB. If the pen end f the slt is mved t the left, all the peaks f the return lss plt f the rectangular mnple are f magnitude less than -lodb and thereby yielding very wide bandwidth Slts in the Circular In circular mnple gemetry, circular slt f 1.5mm width is intrduced with the slt pening f 3 mm as shwn in the Fig 2. The antenna radiated well even withut slts and cvered the UWB band fr -8.5dB f return lss. In rder t reduce the peaks f the return lss plt at 7.5GHz, intrduced Slts in the Elliptical the slt is Similar t the circular mnple gemetry, the elliptical slt f width 1.5mm is intrduced n the radiatr shwn in Fig 3. The psitin and the width f the pen end f the slt are ptimized t btain the return lss better than -lodb cvering the UWB spectrum. 3.3 Tuning stubs fr imprved perfrmance Small stubs are used fr tuning the perfrmance f the antenna t realize the return lss f -lodb in the entire UWB spectrum. The stubs used are, metallized strip, which are placed n the slts as shwn in Figs 1,2 and 3. The lcatin f the stubs alng the slt cntur helps t tune the antenna fr the desired perfrmance. In the designed UWB antennas, tw tuning stubs dented as TI and T2 are used. In the rectangular mnple antenna, T1 is placed n the vertical slt and T2 is placed n the hrizntal slt. 3.4 Design Optimisatin f Vivaldi Antenna The Vivaldi antenna is designed with slt prfile f expnential taper. Initially, the minimum width f the slt is 1.5mm and the maximum width f the slt is 12.8mm. The lcatin f maximum width f the slt is alng the edge f the grund plane. The chice f these parameters results in the three resnant frequencies at 3.4, 5.5, and 8 GHz. Then minimum width f the slt is reduced t O.5mm, the maximum width is increased and the tapering is als adjusted based n the required results. The micrstrip feed line is mved up and dwn t imprve the impedance matching. The structure f the feed stub design is als changed and ptimized fr imprved perfrmance. 4. RESULTS AND DISCUSSION The return lss (SII) plt f the planar mnple antennas and Vivaldi antennas btained thrugh simulatin are discussed belw. 4.1 Rectangular The return lss plt f the designed rectangular planar mnple is shwn in Fig 6. The hrizantal and vertical slts as well as the tuning stubs intrduced n the radiatr have the advantageus feature t realize very wide bandwidth (2.3 - II GHz) fr return lss better than -I OdB. S e+9 2e+9 4e+9 6e+9 8e+9 frequency 4e+9 6e+9 8e+9 frequency in Hz Fig 6: SII plt f Rectangular in Hz Fig 7: SII plt f Circular 1e e Circular The design ptimizatin f the circular mnple antenna invlving the determinatin f precise feed pint as well as the lcatins and sizes f the tuning stubs helped t realize a return lss better than -10 db cvering GHz frequency range. 1e e+10 The feed pint is mved alng a strip cnnected at the base (bttm) f the radiatr. The ptimized feed pint psitin is determined by successive cmparisn f iteratins 4.3 Elliptical The elliptical mnple gemetry is ptimized withut changing the lcatin f pen end f the slt. Similar t the SAsTECH 105 Vl. VI, NO.1, April 2007

5 circular mnple the feed pint is mved alng the strip at the base f the radiatr and the ptimized feed pint is identified SU e+9 0 half f the UWB spectrum. The return lss plt shws three deep resnances, as shwn in the Fig VALIDATION AND COMPARISON Table 1 summarizes the results f all the planar mnple antennas prpsed in this paper. The mnple antennas are with slts and tuning stubs n their radiating structures. mnple Cnfiguratin Rectangular Circular (db) Elliptical Gain Peak Bandwidth Minr 2.3 Majr II (GHz) axis Dimensins Radius 20 Radiatr x axis=16 = = (mm) Fig 8: SIt plt f Elliptical The elliptical slts and the tuning stubs n the slt cntur f the radiatr significantly reduced the amplitude f peaks f the return lss respnse t belw -10 db. The realized bandwidth f elliptical planar mnple antenna fr -10 db return lss is GHz. 4.4 Vivaldi Antenna The fllwing are the techniques used t imprve the impedance bandwidth f the Vivaldi antenna. Cnfiguratin (mm) Changing the psitin f the micrstrip Rectangular feed Widening Flare Opening Increase/ Decrease the linear slt width Varying the length f the Feed stub Changing the shape fthe Feed stub Table 1: Summary f simulated planar mnple Results In [I I], the authrs had utilized a grund plane f size 300 x 300 mm in their design. In the prpsed paper, the size f the grund plane is nly a small fractin (= I/I th) f that in [11]. Further the design f the rectangular mnple f[ii] des nt appear t be intended fr UWB. Circular Elliptical Minr Dimensins Radiatr Bandwidth Majr axis Radius (GHz) x 42 = = Table 2:Results f Wide band Planar [II] In fact a bandwidth f nly 2 GHz was realized fr rectangular mnple f [I I]. In the design f this paper, UWB perfrmance has been realized even thugh the radiatr and grund plane dimensins are significantly smaller than that f [11]. This substantiates the nvelty f the slt and tuning stubs prpsed in this paper. 6. CONCLUSIONS Fig 9: S\I plt f Vivaldi Antenna The Vivaldi antenna f Fig 4 prvides the bandwidth f 5.7 GHz frm GHz, fr the return lss < -lodb, which can be useful with the UWB devices perating at the secnd The antenna design fr ultra wideband spectrum is nt a new tpic t the antenna designers, but the gemetries, cmplexities and the technical apprach fr the desired design slutins vary with individual applicatins. One f the primary bjectives f this paper is t evlve a design cncept f utilizatin f planar mnple f cmpact size and relatively simpler structural cnfiguratins fr UWB SAsTECH 106 Vl. VI, N. I, April 2007