Variations on the fractal Sierpinski antenna flare angle

Variations on the Fractal Sierpinski Antenna Flare Angle C.PUEN1E'. M~NAVARRO. J.ROMEU. R POUS. E1.ECTROMAGNFTICS & PHOTONICS ENGINEERING CROUP (EEF) clGran CapitA Jn. dui D3 Campus Nord. 08034 Barcclonn. puenicGhc.up'.es, Tel:34-3-4017210. Fax:34-34017232 DEFT TSC. UNIVERSITAT YOLIT~CNICA DE CATAWNYA. Abstract.-:A further Investigation on the fractal multiband Sierpinski Antenna Is Introduced here. it is shown that a variation on the antenna's flare angle Is translated Into a shift of the operating bands, as well 8s into a change In the impedance level and radiation patterns. The contribution of those variations to the truncation eflecl is outlined. 1. Inlroducfion.- The fractal Sierpinski antenna based on an equilateral triangle generator IS] was the first reported example of a multiband fractal antenna 1 1]-[7]. In this work, a new degree of freedom to the antenna design is introduced: the variation on the anienna flare angle. Experimental results , ,, , cacm . .. . . . . are shown to describe the variations on the impedance levels and absolute band positions introduced when changing the flare angle. Basically it is shown that broader angles shift the operating bands to lower frequencies, which can be useful to reduce the antenna height. multiband behavior is broken and the antenna approaches the behavior of a classical monopole antenna. This is basically due to the truncation effect. 2. lnput Parameters.- Brown & Woodward described in [SI the input impedance behavior of triangular and conical antennas. Basically. resistance and reactance variations were smoother when opening the nare angle. A 4.77 C" similar performance was observed on the triangular antennas, although in this case the variations became stronger with respect to the conical one. Since the Sierpinski antenna has an overall triangular shape, it appears natural to investigate whether those effects on the ao im When the angle is made too narrow the KO ~m their experimental results evinced that the input 811 cm - performance of triangular antennas are translated into the behavior of the fractal one. SPK30fhotlom) Therefore, the SPK-90. SPK-60. y SPK-30 antennas (Fig.1) were constructed by following Flg.1 The SPKW (top). SPK60(center). Sierpinbki rracca, 0-7803-4478-2/98/$10.00 0 1998 IEEE 2340 the s~me pocedun U described in 171. The fdng flue angles we. mpcctively M, e=# y 5w. The plots in Fig. 2 describe the input para" (input refkction coefficient relative to 50+8) input resistance and rcaccanCe) for the lhree antennas as a function of hequency. In gml. it can be stated that: 1) The behavior of the lhrre antennas is basically log-periodic, being the log-pwiod t2, which is precisely the scale factor that relates thc scvcral fractal iterations in 2) The number of bands or log-periods (5) matches the number of fractal iterations as demibedin(I]. 3) Broader flare angles introduce a shift on Ihc "nt fqucncics toward longa wavelengths. 4) The variations on Ihc impedance plots (uc stronger for the narrower M~~IN at kas( at the first band. Tbis result is comparabk to thc bipng~lar mha behavior described in IS]. 5) The impedance minimum arc rcduced at odd R~(WUIICCS am rcduad for broader M~~CS, which changes the matching levels with mpcct(0 509. the MkMa body. It should be stressed tha~ the multiband behavior of the ~ltnlll~ must be related IO its fmctal shape, as it is extensively discussed in 161. The absolute drift on the opcdng band position (not ita relative spacing) with respect to the Ti vmion of thc M~MB (SPK 60). must be relakd to the rizc of the isosceles triangk twin edges. cumnts propagating along the M~CMSI cdga U it is described in 161 and will be explained somcwhm clr. ~i-cforc. longer edger hosl longa resonant wavelengths yielding (0thcshiftofthcspcctd~porue. E L plrvrl-- uu CII Basically. the behavior of thcsc 9 can be modc~cd with L' .,, lo IU 1 It is interesting to notice that the narrower antenna kndr IO deviate input reflection coefficient displaya 8 doubk match featwe which co"Ja IO a double nsonrna in E behavior of the SPK30 MLCIIM is closcr to that of a classical monopole which holds a clear harmonic (periodic) non multiband bchavior. ph3 ,-A ra I~CSPKC)~ (-I SPKW-.) from the log-periodic behavior. Ita im Ihc impcdam PI&. %&w, Ihc 3. d SPK30(-) aalunms. 234 I 3. Kudiution Pufferns.- The radiation patterns for the SPK90 and SPK30 antennas (main polarization component along the Qdirection, Bto" cut) are shown in figures 3 and 4 respectively. Those corresponding to the SPK60 antenna are described in [2),[6],17). Again. it is observed that the broader angle antenna features a multihand behavior, with a clear pattern similarity 'p=O" among bands. Contrarily, significant deviations among patterns are stated in the SPK30 case; the number of grating 5 X; $<:i?..,,,,.,\T c. rf., ,?a,' 5 ," lobes tends to increase with frequency, . , .) (I ' !;., i which makes this antenna comparable to I ixoiiic I IGOW'T the classical monopole. Once more, such a phenomenon must be related to the 4 X; d,,2., 1; .k.$ -0 :,: >7. :-; 2 shorter length of the triangle edges. a" = ( ' ..*' ,' . Somehow, the current active region that J-~~IiX, p d y lilt> characterizes most multifrequency antennas 161 has less room to expand and U: to become attenuated by the radiation ~ process before reaching the antenna tips. I"11~or.H~ I-~.1~~~H~ In fractal theory terms, such a truncation effect can be explained by the lack of Flg.3 Radiation pallern (w=W. E, comp)nenO larger characteristic scales lo keep the perfect symmetry of the ideal fractal set. I :' m I\ ~ *\., Y 'I " 3 a "\ I . : . . e r%; pwL' ,-?? I) '. . ; I lor the SPKW anrcnna. This should not be taken as a basis to argue that fractal antennas are not multiband antennas; the suitability of fractals to become multiband antennas directly comes out from the classical scaling propeny of Maxwell equations 161. Therefore, one should expect a multiband behavior even for the SPK30 antenna. provided that the antenna size and the number of fractal iterations is made large enough. 4. Conclusions.. The behavior of three Sierpinski fractal antennas has been y described through several experimental X; - ~'&,~ -&-;yT)- .* It is shown that such a change introduces significant variations on the antenna input parameters and radiation 2 pattems. Although the designer can cp"0" ." '? " .. -' ' ' results. The antennas had the same basic 2 shape with a change on the flare angle. ,c '. L',' : , , .) , !.* J I 80 rx, _, J J ?rriH$ I 4 !'p r;Hz freely use those variations for P, .Y..,' . .m .: I into account that by namowing the 2 engineering purposes, it must be taken antenna angle a deviation from the multiband behavior is obtained. Except q for those cases, where the truncation effect is important. it can be concluded 2 ~ ~ i . ,/ . again that fractals can be successfully 1-11 WGH: pl6Wr;lh r-a.o)I;nt . .. , * ", used to construct simple multiband antennas. Ftg.4. Radiation pattern (er' =W" cut. E, Acknowledgments,- This project has compnenr) for Ihe SPK30 antenna. 2342 heen partially supported by the grant TIC-96-0724-C06-04 of the Spanish goveminent and by Ihe companies SISTEMAS RADIANTS F.MOYAN0 and FRACTUS. Fractal and Mullifractal antennas are parent pending. References Ill C.Puentc. J.Romcu. R. Banolud. y R. POUS. "Pcnurtcation of Ihc Sicrpinski antcnna to allwatc operaling bunds". Etrrronicr Lffrrn. vol. 32. no.24. pp.2186-2188. November 1996. 121 C.Pucntc. J.Romcu. R.Pws. X.Garcia. F. Bcnltcz. "Fractal multiband antcnna based on the Sierpinski ga5ket". Elecminin Lrrm. vol. 32, no. I. pp. I .2. January 19%. 131 N.Cohcn. R.G. Iluhfeld. "Fractal loops and the SIMII Iwp appmaiimtion". C~m"n. Quanrdy. 77-81. Winter 19%. 141 C.Pucnfc. I.Cluct. F. Ssguts. J.Runru. M.Q.Ldpcz Salvans. R.Pws. "Multiband popertics of fractal ITCC antenna generaled by claclwhcmical dcprition". Elrcrronirs kfrrrr. vol. 32. no.25. pp. 2298.2299, Deccmbcr 1996. 15) H.O.Peitgcn. ll.Jiirgenr. D. Saupc, Cham onJ Jracruls, new Jronrirrs o/scirnrr. New York : Springer Verlag. 1992. 161 C.Pucnte. "Fractal Anlcnnu". Ph.D. Disrcflation. Dept. TSC. Univenilu Polittcnics de Caelunya. June 1997. [7] C.Pucnte. J.Romeu, R.Pws. A.Cardntna, 'On the Behaviur of the Multiband Sicrpinski Fractal Anlcnna". acccpld lor publication in IEEE 'Tronr. on Anrmwr ondPruprgorion. 1998. [E] G.ff.BNwn. 0.M.Wdward. "8rpcrimnlally Dctcmlinut Radialion ChPralcristkr ol Conical and Triangular Anlcnw': RCA Rcvicw. pp 425.4S2. Deccmbcr 19S2. 2343