Koch Fractal Boundary Single feed Circularly Polarized Microstrip Antenna

Transcription

1 1 Journal of Microwavs, Optolctronics and Elctromagntic Applications, Vol. 6, No. 2, Dcmbr Koch Fractal Boundary Singl fd Circularly Polarizd Microstrip Antnna P. Nagswara Rao and N. V. S.N Sarma and Dpartmnt of Elctronics and Communication Enginring National Institut of Tchnology, Warangal, India Abstract: Koch fractal boundary circularly polarizd microstrip antnna of stag 1 and stag 2 is proposd. This antnna uss Koch curv of stag 1 and stag 2 as boundary with indntation angl of 30 o. It givs minimum axial ratio of 0. 8 db at th cntr frquncy, 6 db axial ratio bandwidth of about % and an impdanc bandwidth of about 3 %. With a singl fd, Circular polarization is obtaind. It is compact in siz compard to its Euclidan countrpart and provids a gain of approximatly 9dBi at th cntr frquncy. It has bn shown that an improvmnt in th Axial ratio and Impdanc Bandwidth can b obtaind by changing th Fractal Dimnsion of th boundary. Indx trms : Axial ratio, Fractal dimnsion, Indntation angl, Koch fractal boundary I. INTRODUCTION Circular polarization with singl fd by diffrnt mthods, lik loading triangular patch by a stub ithr at th vrtx or at th bottom dg, with a crossd slot, a small notch on dg, hav bn rportd [1]. Circular polarization can also b obtaind by a squar patch with truncatd cornrs and with diagonal slots [2]. It is always advantagous to us singl fd to gt circular polarization, bcaus this procdur dos not rquir any additional circuit to provid two orthogonal mods of th sam amplitud. A novl concpt of using fractal boundary for obtaining circular polarization is proposd in th prsnt papr probably for th first tim to th bst of th knowldg of th authors. In th prsnt papr Koch fractal boundary stag 1 and stag 2 fractal boundary cp antnnas with an indntation angl 30 o ar considrd. Th proposd antnna givs a minimum axial ratio of 0.8 db at th cntr frquncy. It has bn shown that fractal dimnsion can b modifid by changing th indntation angl of th boundary [3]. An improvmnt in th both impdanc and axial ratio bandwidth is obtaind by changing th fractal dimnsion of fractal boundary i.. by changing th indntation angl of th boundary.

2 2 Journal of Microwavs, Optolctronics and Elctromagntic Applications, Vol. 6, No. 2, Dcmbr II. ANTENNS DESIGN AND SIMULATION RESULTS of 1 st, 2 nd Th proposd antnnas ar obtaind by rplacing ach sid of quilatral triangl by Koch curv stag with an indntation angl of 30 o to gt corrsponding fractal boundary CP antnnas. In ordr to gt circular polarization it is rquird to xcit two orthogonal nar dgnrat mods. This is accomplishd by having a rctangular slot of 20 mm X 1 mm at th cntr of th patch. It is fd slightly off-symmtric to produc two mods. Th antnnas ar printd on a substrat with ε r of 4.17 and thicknss of 1.6 mm. It is invrtd and kpt ovr a foam of thicknss of 3.4 mm. Th initial sid lngth of th triangular patch is takn as mm. Th antnnas ar fd at a point (-10mm,-15mm) with rspct to th cntr of th patch, to gt right handd circular polarization. This antnna provids almost a flat gain of approximatly 9dBi ovr th frquncy of opration. Th major advantags of th proposd CP antnnas ar xcllnt polarization purity, good radiation fficincy, compact siz and high gain. Th gain of th antnna can furthr b incrasd by oprating th antnna at highr ordr mods. It has bn shown that a gain of 13 dbi is possibl with Koch curv - basd fractal boundary antnna whn it is oprating at highr ordr mods [4]. As th antnna is tchd on a substrat and is invrtd, th dilctric acts as a Radom or suprstrat. This suprstrat is mainly usful to provid protction against hat, physical damag and nvironmntal ffcts. This is rquird mainly for antnnas mployd in Airborn and spaccraft applications. Whn microstrip antnna is coatd with dilctric layr, th rsonant frquncy is altrd. This chang in th rsonant frquncy can b calculatd [5] using th quation (1). (a) (b) Fig 1: Fractal boundary CP antnnas with an indntation angl of 30 o (a) Stag 1 (b) stag 2

3 3 Journal of Microwavs, Optolctronics and Elctromagntic Applications, Vol. 6, No. 2, Dcmbr Frquncy ( GHz) S11 (db) Stag 1 Stag Fig 2: S11 Charactristics of stag-1 and 2 of fractal boundary CP Antnnas shown in fig 1. f f r r = ε 1 ε o 2 1 ε 1+ 2 ε o ε = Th ffctiv dilctric constant without covr o (1) ε = ε o ε +, with dilctric covr f r = chang in rsonant frquncy f r = Rsonant frquncy with dilctric covr Th rsonant frquncy of th patch can b calculatd using th formula [6]: f r 2c = (2) 3l ε t whr l t = Equivalnt sid lngth of quilatral triangl of sam ara as that of fractal boundary antnna and c = vlocity of light in fr spac Th calculatd and simulatd rsonant frquncis ar shown in Tabl I. TABLE I : RESONANT FREQUENCY CALCULATION USING DIFFERENT TECHNIQUES Antnna Rsonant frquncy using Eq. Rsonant frquncy (1) and (2) obtaind using IE3D Diffrnc Stag GHz 1.51 GHz 0.6 % Stag GHz GHz 0.2 %

4 4 Journal of Microwavs, Optolctronics and Elctromagntic Applications, Vol. 6, No. 2, Dcmbr Axial Ratio(dB) Frquncy (GHz) Stag 1 Stag 2 Fig 3: Axial ratio of th antnnas shown in fig (1) Fig 4 (a) : Variation of input impdanc of fractal boundary antnna of stag 1 shown in fig 1(a).

5 5 Journal of Microwavs, Optolctronics and Elctromagntic Applications, Vol. 6, No. 2, Dcmbr Fig 4 (b) : Variation of input impdanc of fractal boundary antnna stag 2 shown in fig 1(b). Fig 1 shows th first two stags of fractal boundary circularly polarizd microstrip antnna. Figs 2 and 3 show th S11 and Axial Ratio charactristics of antnnas shown in fig 1. It is obsrvd that th rsonant frquncy is shiftd lft toward th origin with ach itration. Th shift in rsonant frquncy is du to incras of primtr of th patch in givn spac. In this antnna th frquncy at which th occurrnc of minimum VSWR and minimum axial ratio is at th sam frquncy. So th frquncy rang ovr which th antnna is matchd will fit into th frquncy rang of 6db axial ratio. This may not happn gnrally in Euclidan shapd patch antnnas. By changing th indntation angl of fractal boundary i.. changing fractal dimnsion, a marginal improvmnt in th bandwidth can b achivd [9]. Th fractal dimnsion can b found in trms of its Indntation angl [3], givn by log 4 D = (3) log 2(1 + cosθ )

6 6 Journal of Microwavs, Optolctronics and Elctromagntic Applications, Vol. 6, No. 2, Dcmbr whr D is th fractal dimnsion and θ is th Indntation angl of Koch fractal boundary. Whn th indntation angl of th fractal boundary is takn as 60 o th axial ratio bandwidth is lss compard to that of 30 o. It is around 1.3 % for indntation angl of 60 o and 1.65 % for 30 o. Howvr, 6dB axial ratio bandwidth of mor than 2 % is rportd with stackd patch configuration [7]. Co-Polarization Co-Polarization Cross-Polarization Cross-Polarization Fig 5: Radiation pattrns of Right and lft hand circular polarization for th proposd fractal boundary stag 1 and stag 2 antnnas. Th prcntags of rduction of ara of stag-2 fractal boundary patch antnna compard to squar and circular patch antnnas oprating at th sam frquncy ar 13 % and 26 % rspctivly. Th siz rduction occurs mainly bcaus of rduction in ratio of ara to primtr of th gomtry. Th comparison is shown in tabl II. Th ara and primtr of Koch fractal boundary patch antnna can b found using th rlation (4) and (5) for th first stag with a as th initial sid lngth of th quilatral triangl and θ is th indntation angl. 12a Primtr ( L) = (4) 2(1 + cosθ ) Ara (A) = a 2 2 sinθ cosθ + 3 a (5) [2(1 + cosθ )] It has alrady bn stablishd that th Koch curv boundary fractal antnnas will hav smallr siz compard to th circular and hxagonal shapd patch antnnas [4]. Th siz rduction is always advantagous whn matrial cost is involvd in th dsign. Th polarization purity of circularly polarizd microstrip antnnas can b bst visualizd using Smith chart as shown in fig 4. Th curv showing th input impdanc on th chart will hav a loop if two dgnrat mods ar xcitd in th patch. Th ara of th loop givs th ida how clos th two mods ar, and it is rducd to a point in a prfctly circularly polarizd patch antnna at th rsonant frquncy [8]. This antnna also provids almost no cross polarization at th frquncy of opration. Th radiation pattrns of th antnnas for right and lft handd circular polarization ar shown in fig 5. Comparison of various paramtrs of

7 7 Journal of Microwavs, Optolctronics and Elctromagntic Applications, Vol. 6, No. 2, Dcmbr Koch fractal boundary with indntation angl 30 o and 60 o is shown in Tabl III. All th proposd antnnas ar simulatd using IE3D Elctromagntic Simulator which is basd on MoM cod. TABLE II : COMPARISON OF AREAS OF DIFFERENT PATCHES OPERATING AT GHZ Gomtrical Paramtr Fractal boundary Patch Squar Patch Circular Patch Ara 6648 mm mm mm 2 Primtr 378 mm 350 mm 339 mm TABLE III : COMPARISON OF PARAMETERS OF KOCH FRACTAL BOUNDARY CP ANTENNA WITH INDENTATION ANGLE OF 30 O AND 60 O Paramtr Indntation angl Stag 1 antnna Stag 2 antnna Rsonant frquncy Impdanc Bandwidth Minimum Axial Ratio 6 db Axial ratio Bandwidth 30 o 1510 MHz 1447 MHz 60 o 1379 MHz 1268 Mhz 30 o 2.93 % 2.93 % 60 o 2.9 % 2.5 % 30 o 1.26 db 0.82 db 60 o 0.36 db 0.29 db 30 o 1.62 % 1.6 % 60 o 1.52 % 1.3 % III. CONCLUSION Th proposd circularly polarizd antnna with Koch fractal boundary givs a minimum axial ratio of 0.8 db at cntr frquncy for th stag-2 antnna, apart from giving a gain of almost 9dBi ovr th ntir band of opration. Th siz is also considrably rducd compard to its Euclidan countrpart. Furthr, th frquncy corrsponding to minimum VSWR and minimum axial ratio is xactly th sam. Chang in th fractal dimnsion of th boundary ld to an improvmnt in impdanc and axial ratio bandwidth. Rfrncs [1]. Jui-Han Lu and Kin Lu Wong, Singl fd Circularly Polarizd Equilatral Triangular Microstrip Antnna with a tuning stub, IEEE Transactions on Antnnas and Propagation. vol. 48, No.12, Dc [2]. P.C Sharma and K C Gupta, Analysis and optimizd dsign of singl Fd Circularly Polarizd Microstrip Antnnas, IEEE Transactions on Antnnas and Propagation., Vol-31 pp , Nov [3]. K.J Vinoy and K Abraham, On th Rlationship Btwn Fractal Dimnsion and th Prformanc of Multi- Rsonant Dipol Antnnas using Koch Curvs,IEEE Transactions on Antnnas and Propagation, vol-51, No-9, pp , Sp

8 8 Journal of Microwavs, Optolctronics and Elctromagntic Applications, Vol. 6, No. 2, Dcmbr [4]. Carmn Borja, Jordi Romu, On th bhaviour of Koch Island Fractal Boundary Microstrip Patch Antnna, IEEE Transactions on Antnnas and Propagation, Vol-51. No.6, pp , Jun [5]. David M Pozar and Danil H. Schubrt, Microstrip Antnnas Th Analysis and Dsign of Microstrip Antnnas and Arrays IEEE Prss, NY [6]. Nasimuddin and A K Vrma, Fast and accurat modl for analysis of Equilatral Triangular patch Antnna. Journal of Microwavs and Optolctronics, Vol-3, No.4, pp , April [7]. Choon Sa L and Vahakn Nalbandian,. Planar circularly polarizd microstrip Antnna with Singl fd, IEEE Transactions on Antnnas and Propagation, Vol.47, No. 6, Jun [8]. J.R Jams and P S Hall, Hand Book of Microstrip Antnnas, Vol-I, Ptr Prgrins Ltd, UK [9]. P Nagswara Rao and N V S N Sarma Th Significanc of Indntation angl of fractal boundary on th prformanc of microstrip antnna, IEEE 2007 Intrnational Symposium on Microwav, Antnna, Propagation and EMC Tchnologis For Wirlss Communications (MAPE 2007) to b hld in August 2007 ( Accptd for Prsntation)