International Journal of Advances in Engineering, 2015, 1(4), 557-561 ISSN: 2394-9260 (printed version); ISSN: 2394-9279 (online version); url:http://www.ijae.in Octagonal Fractal Antenna Design using Koch Curve S.Suresh and T.Usha Dept. of ECE, A.K.T MCET, Kallakurichi, Tamilnadu, India ushaparames92@gmail.com Received 16 March 2015 / Accepted 10 April 2015 RESEARCH ARTICLE Abstract In this paper an octagonal fractal antenna using Koch curve is proposed. Self-similarity and space-filling are the unique properties of fractals. The fractal geometry is used in antenna design for achieving the desired miniaturization and multiband properties. In this proposed antenna iterations are performed by applying Koch curve in each side of the octagonal geometry. The material used for substrate is FR4 with relative permittivity of 4.4 and thickness is about 1.6 mm. Coaxial probe is used to feed the antenna. This antenna is designed and simulated by using HFSS software. The results show that the proposed antenna offers good performance in dual band frequencies (5 GHz and 6.8 GHz) which is suitable for wireless applications. Keywords Antenna,fractal, Koch curve,coaxial probe. I. INTRODUCTION In modern wireless communication systems antennas are needed with smaller size and wider bandwidth. This has initiated antenna research in various directions; one of them is using fractal shaped antenna elements. The microstrip patch antenna consists of a radiating patch on one side of a dielectric substrate and a ground plane on the other side of the substrate. The patch can take any possible shape and is made of conducting material such as copper or gold. Fractals have self-similarity and space-filling properties which provide design of antennas with smaller size. Fractal geometry has unique geometrical features occurring in nature. It can be used to describe the branching of tree leaves and plants, lightning, coastline, snowflake and many more examples in nature. Fractal antenna design has two things such as initiator and generator. Initiator is the basic shape of the geometry and it can be any shape either triangle, rectangle or any other quadrilateral. Generator is the shape which is obtained by scaling the initiator and will be repeated either inside or outside on the initiator to obtain subsequent stages to reach final fractal geometry. So generator is obtained from the initiator itself. The selfsimilarity and space-filling properties of the fractals are useful in designing multiband antenna and for miniaturization of an antenna by increasing the electrical length into a compact physical volume. Sharp edges, corners and discontinuities help to make antenna to radiate efficiently. By increasing number of iterations in antenna design the resonant frequency is decreased while electrical length is increased. Many fractal geometries have been found to be useful in developing new and innovative design for antennas. It includes Koch curve, Sierpinski gasket geometry, Sierpinski carpet geometry, Hilbert curve and Minkowski loop. Koch curve is one of the self-similar and space-filling fractals which is used to obtain wideband/multiband and /or miniaturized antennas. It has highly rough and uneven shape which helps to work as a very efficient radiator. A relation exists between antenna dimensions and wavelength. It states that antenna size should be greater than quarter of wavelength unless antenna will not be efficient. Since antenna size is increased because of gain, radiation resistance, and bandwidth are reduced. Hexagonal geometry is designed with substrate of having relative permittivity of 2.3 and thickness is about 2 mm and iterations have done to improve the gain of the antenna. Coax feed is placed on the patches which is located 1 mm from each side at the corner [1]. In [2], octagonal geometry structure has proposed to obtain good performances in bandwidth and gain. By adding octagon inside the base shape iterations are done. Rogers TMM substrate with relative permittivity of 4.5 and thickness is about 1.524 mm has chosen. The coax feed is placed which is 26.5 mm from the center to match 50 ohm input impedance. The octagon has 2 cm side length. Sierpinski fractal antenna [3] is designed by three iterations in the base shape to display a multiband behavior with three bands. FR4 substrate is chosen with an aluminium ground plane of 300 mm x 300 mm dimension. A novel fractal design of a multiband antenna is presented and iterations are performed by inscribing a slot of a hexagonal geometry in a circle. The fabrication of antenna is done with substrate of 4.4 relative permittivity and 1.6 mm thickness. Coplanar waveguide is used to feed the antenna [4]. In [5], the overview of fractal antenna engineering is studied and described about the construction of various fractal geometries used in antenna design which includes Sierpinski gasket, Koch snowflake, Hilbert curve, Sierpinski carpet. The fractal antenna is designed with heptagonal geometry and three iterations are performed. The antenna is fed by coplanar waveguide. The design and fabrication of antenna is done on FR4 substrate which has 4.4 relative permittivity and 1.6 mm thickness [6]. In this paper a new fractal antenna which is designed by applying Koch curve in an octagonal geometry is proposed. FR4 material is chosen for the dielectric substrate which has relative permittivity of 4.4 and thickness is about 1.6 mm. This antenna is designed and simulated by HFSS software. It is observed that in the second iteration of the antenna design has good return loss than the base shape and first iteration.
558 Int. J. Adv. Eng., 2015, 1(4), 557-561 II. ANTENNA DESIGN The proposed antenna is designed by applying Koch curve in each side of the octagonal geometry. The material used for substrate is FR4 with relative permittivity of 4.4 and thickness is about 1.6 mm. Coaxial probe is used to feed the fractal antenna. The dimensions of the substrate and ground plane are 55 mm x 55 mm. The iterations are done by only doing modifications in radiating patch and the dimensions of the substrate and ground plane are kept constant in all the iterations. According to the octagonal properties, the interior angle of the octagon is 135 and the exterior angle is 45. Thus we can conclude, cos (135 /2) = d/2/r r = 1.306d Where d is the side length of the patch and r is the radius of the octagonal patch. The base shape of the proposed fractal antenna is constructed by applying Koch curve to the each eight sides of the octagonal geometry. And then one more octagon is subtracted from the radiating patch. By this way base shape is designed. Koch curve is one of the self-similar and space-filling fractals which is used to obtain wideband/multiband and /or miniaturized antennas. It has highly rough and uneven shape which helps to work as a very efficient radiator. The Koch curve is formed by starting with a straight line. Divide the line in three parts. Replace the center part by an equilateral triangle with the base removed. Figure.1Base shape The radius of the octagonal geometry of the proposed fractal antenna is 13.06 mm. It has side length of 10 mm. After applying the Koch curve to the each side of the octagonal geometry, the side length of the radiating patch is about 3.3318970 mm. Figure.2 First iteration Coaxial probe is used to feed the fractal antenna. The location of the coax feed is placed on the radiating patch which is 10.5 mm from the center at the corner. Fig. 1 shows the base shape of the proposed fractal antenna. Then the first iteration of the proposed fractal antenna is done by applying Koch curve to the each side of the one more octagonal geometry which has radius of 7.06 mm. As done in the base shape, here also one more octagon is subtracted from the radiating patch. The location of the coax feed is placed on the radiating patch which is 10.5 mm from the center at the corner. The dimensions of the substrate and ground plane are kept constant as in the base shape. The Fig. 2 shows the first iteration of the proposed fractal antenna. This process is repeated to obtain the structure of second iteration of the fractal antenna. As done in the base shape and first iteration, the Koch curve is applied to the each side of the octagonal geometry which has radius of 4.1
559 Int. J. Adv. Eng., 2015, 1(4), 557-561 mm and then one more octagon is subtracted from the radiating patch to obtain the second iteration of the proposed fractal antenna. Figure.3 Second iteration The location of the coax feed is placed on the radiating patch which is 10.5 mm from the center at the corner. The dimensions of the substrate and ground plane are kept constant as in the base shape and the first iteration. The Fig. 3 shows the second iteration of the proposed fractal antenna. The Table I shows the dimensions of the proposed fractal antenna. TABLE I ANTENNA DIMENSIONS Parameters Substrate length 55 Substrate width 55 Radius of initial octagon 13.06 Initial octagonal side length 10 Ground length 55 Ground width 55 feed location from center 10.5 Values (mm) III.SIMULATION RESULTS The proposed fractal antenna is designed and simulated using HFSS software. The return loss should be below -10 db (S11 < -10 db) and VSWR should be below 2 (VSWR < 2). Fig. 4 shows the return loss of the base shape of the antenna. The graph is plotted between the return loss in db and frequency in GHz. Fig. 5 shows the graph of the return loss vs frequency for the first iteration of the fractal antenna. Fig. 6 shows the graph of the return loss vs frequency for the second iteration of the proposed fractal antenna. Figure.4 S11 graph for base shape
560 Int. J. Adv. Eng., 2015, 1(4), 557-561 Figure.5 S11 graph for first iteration Figure.6 S11 graph for second iteration The simulated results of the antenna (base shape, first iteration and second iteration) are given by following tabulations respectively. The Table II, III and IV shows the simulated results of the base shape, first iteration and second iteration of the octagonal fractal antenna respectively. TABLE II SIMULATED RESULTS OF BASE SHAPE 6.8-19.1797 8.1-14.5913 TABLE III SIMULATED RESULTS OF FIRST ITERATION 4.9-11.5723 6.8-21.7280 TABLE IV SIMULATED RESULTS OF SECOND ITERATION 5-13.9260 6.8-37.4878
561 Int. J. Adv. Eng., 2015, 1(4), 557-561 According to the results it is observed that the second iteration of the octagonal fractal antenna has good return loss than the base shape and the first iteration. In the second iteration the return loss is obtained -37.4878 db at 6.8 GHz frequency range. CONCLUSION A new octagonal fractal antenna using Koch curve is presented in this paper. The proposed structure has a dimension of 55 mm x 55 mm. The dimensions of the substrate and ground plane are kept constant and iterations are done in radiating patch only. The simulated results are obtained using HFSS software. The proposed antenna exhibits good performance in dual band frequencies (5 GHz and 6.8 GHz) which is suitable for wireless applications such as media streaming and STM-1 (Synchronous Transport Module level 1). REFERENCES 1. A.Azari, J.Rowhani, Ultra wideband fractal microstrip antenna design, progress In Electromagnetics Research, vol 2, pp.7-12, 2008. 2. Abolfazl Azari, A New Super Wideband Fractal Microstrip Antenna, IEEE transactions on antennas and propagation, vol.59, no.5, pp.1724-1727, 2011. 3. Muhammad Waqas, Zubair Ahmed, Mojeeb Bin Ihsan, Multiband Sierpinski Fractal Antenna, IEEE, 2009. 4. Saira Joseph, Binu Paul, Shanta Mridula, Pezholil Mohanan, A Novel Planar Fractal Antenna with CPW-Feed for Multiband Applications, a novel planar fractal antenna with cpw feed for radio engineering, vol.22, no.4, pp.1262-1266,2013. 5. Douglas H. Werner, Suman Ganguly, An Overview of Fractal Antenna Engineering Research, IEEE Antennas and Propagation Magazine, Vol.45, No.1, 2003. 6. Chakkrit Kamtongdee1 and Nantakan Wongkasem, a Novel Design of Compact 2.4 GHz Microstrip Antennas, IEEE, pp.766-769, 2009. 7. Deepti Das Krishna, Student Member, IEEE, M. Gopikrishna, Student Member, IEEE, C. K. Anandan, P. Mohanan, Senior Member, IEEE, and K. Vasudevan, Senior Member, IEEE, CPW-Fed Koch Fractal Slot Antenna for WLAN/WiMAX Applications,IEEE antennas and wireless propagation letters, vol. 7,pp.389-392,2008. 8. A.A.Lotfi-Neyestanak, M.R.Azadi and A.Emami-Forooshani, Compact Size Ultra Wideband Hexagonal Fractal Antenna, IEEE, pp.387-390, 2010. 9. Muhammad Naeem Iqbal, Hamood-Ur-Rahman, and Syeda Fizzah Jilani, an Ultra wideband Monopole Fractal Antenna with Coplanar Waveguide Feed, International Journal of Antennas and Propagation Volume 2014, Article ID 510913, 7 pages, 2014. 10. S. Suganthi, Member IACSIT, D. Kumar, and S. Raghavan, Design and Simulation of Miniaturized Multiband Fractal Antennas for Microwave Applications, International Journal of Information and Electronics Engineering, Vol. 2, No. 5, pp.825-830,2012.