Journal of Microwaves, Optoelectronics and Electromagnetic Applications, Vol. 9, No. 1, June 2010 10 Design of CPW Fed Ultra wideband Fractal Antenna and Backscattering Reduction Raj Kumar and P. Malathi Microwave and Millimeter Wave Antenna Lab., Department of Electronics Engineering Defence Institute of Advanced Technology (Deemed University) Girinagar, Pune-411 025, Maharashtra, India Email: raj34_shivani@yahoo.co.in Abstract This paper presents the design of ultra-wideband (UWB) circular shaped Fractal antenna and its backscattering reduction. The fourth iterative Circular fractal antenna have been designed and fabricated on ε r = 4.3 and h = 1.53 mm with radius 41 mm. The antenna offers excellent ultra wideband performance ranging from 0.80 GHz to 10.68 GHz. The antenna exhibits impedance bandwidth of 9.88 GHz corresponding to 172.13 % at 10 db return loss points. The lower end frequency of fractal antenna shifted to 0.8 GHz in comparison to simple patch resonant frequency 1.446 GHz of same dimension. This indicates the size reduction of antenna. The experimental radiation pattern of fractal antenna has been observed nearly omni-directional. The backscattering reduction of fractal antenna has also been studied. The fourth iterative fractal antenna exhibits low backscattering. The effect of superstrate on antenna metallization can be helpful to tune the low RCS minima in operating band. Such type of antenna can be used for UWB system, radar and EW applications. Index Terms - Fractal antenna, Resonant Frequency, Substrate, Superstrate and backscattering reduction. I. INTRODUCTION Modern telecommunication system requires antennas with wider bandwidth and smaller dimensions than conventionally possible. Now days, the size of electronics systems has decreased drastically, whereas their functionality has increased. The antennas have not experienced the same evolution. The antenna size with respect to the wavelength is the parameter that will have influence on the radiation characteristics. For efficient radiation, the size should be of the order of a λ/2 or larger. But as antenna size reduces, the bandwidth, gain and efficiency of antenna deteriorate [1]. Several researchers studied [2]-[5] and suggested that incorporation of fractal geometry in conventional antenna is advantageous to have multi-bands / ultra wideband performance and miniaturization of antenna. The multi band and ultra wide band properties of antenna are due to their self-similarity of fractal geometry [2]-[3] while the space filling properties [4]-[5] of antenna leads to the miniaturization of antenna.
Journal of Microwaves, Optoelectronics and Electromagnetic Applications, Vol. 9, No. 1, June 2010 11 On other hand, the study of backscattering RCS is of great important as it used in modeling of many portions of aircrafts and missiles. Recent interest in RCS is to reduce the detectability of target or platform. Antennas on it are one of the main contributions to the total radar cross section. There are several technique for RCS reduction of microstrip antenna [6]-[7]. The RCS reduction of microstrip fractal antennas varies with respect to number of iterations, substrate thicknesses, dielectric constant of substrate and superstrate on metallic portion of fractal antenna. The low RCS antennas are useful for electronic warfare. This paper presents the design of fractal antenna of ultra wide bandwidth, compact size and good radiation characteristics. This antenna exhibits low backscattering RCS. This can be useful for UWB wireless communication system and military applications. II. FRACTAL GEOMETRY OF ANTENNA The Simple circular solid microstrip antenna has been designed on substrate dielectric constant ε r = 4.3 and thickness h = 1.53 mm with radius 41 mm called initiator or zeroth iteration as shown in Fig. 1(a). The fourth iterative fractal circular antenna has been designed on the same substrate with same dimension. The fourth iterative fractal antenna structure has been generated from the solid circular patch. In the first iteration, an equilateral triangle of 71.016 mm side length has been subtracted from simple solid circular patch shown in Fig. 1(b). In the second iteration, a circle of half radius of first has been taken and an equilateral triangle of 35.508 mm side length has been subtracted. In the third and fourth iteration, an equilateral triangle of the side length 17.754 mm and 8.877 mm have been subtracted from the circle of radius 20.5 mm and 10.25 mm respectively as illustrated in Fig.1(c) and Fig.1(d). Like this, the process can be repeated for infinite time. The infinite iterative structure is impossible to fabricate. In this paper, a fourth iterative fractal antenna has been finalized because this structure exhibits low backscattering. Fig. 1a, Zeroth Iterative Simple Circular Patch Antenna Fig. 1b, First Iterative Fractal Antenna
Journal of Microwaves, Optoelectronics and Electromagnetic Applications, Vol. 9, No. 1, June 2010 12 Fig. 1c, Second Iterative Fractal Antenna Fig. 1d, Photograph of Fourth Iterative Fractal Antenna III. DESIGN OF CIRCULAR MICROSTRIP ANTENNA The design expression of simple circular microstrip antenna for calculating the resonant frequency is given as (1) Where v o is the velocity of light. The effective radius r eff can be calculated by following expression (2) Where r o is radius of the circular patch. The dimension of the simple solid circular patch is taken as radius 41 mm. This patch has been designed on FR4 substrate dielectric constant of ε r = 4.3 and thickness h = 1.43 mm. The fourth iterative fractal antenna has been fed with CPW-fed. The antenna is fed with 50 Ω coplanar feed line of 2.2 mm width, length of 15 mm and gap of 0.25 mm between ground and CPW feed. The advantage of coplanar feed is that the feed of the antenna, ground and radiating elements all are printed on the same side of the substrate. In the Coplanar technology, no via (plated through hole) is required for ground purpose. So, this technique is less costly than microstrip circuit. The CPW - fed antenna not only performs better in respect of bandwidth and the radiation pattern is also good. IV. RESULTS AND DISCUSSION A. Fractal Antenna The solid circular patch antenna and fourth iterative fractal antenna have been fabricated and tested using VNA ZVA40. The experimental center frequencies of solid circular patch with coaxial have been observed at 1.446 GHz as shown in Fig. 2. The experimental result of the fourth iterative fractal antenna exhibits the ultra wide band from frequency 0.80 GHz to 10.68 GHz as shown in Fig. 3. The impedance bandwidth of this circular fractal antenna is 9.88 GHz corresponding to 172.13 % bandwidth. It is observed from Fig. 2 and Fig. 3, that the lower end frequency of fractal antenna has
Journal of Microwaves, Optoelectronics and Electromagnetic Applications, Vol. 9, No. 1, June 2010 13 been shifted to 0.8 GHz in comparison to solid circular patch antenna resonant frequency 1.446 GHz. This indicates the size reduction of the antenna due fractal characteristics. The experimental gain and radiation pattern of the antenna has been measured in anechoic chamber using antenna measurement system at resonant frequencies 0.858 GHz, 1.49 GHz, 5.325 GHz and 10.2 GHz; as shown in Fig. 4 to 7. The radiation pattern is nearly omni-directional. The gain of the antenna is less 5 dbi. This antenna is useful for ultra wide band system, networking, and radar, imaging and positioning systems. Fig. 2. Experimental Result of Conventional Microstrip Antenna Fig. 3. Experimental Result of Fourth Iterative Fractal Antenna with CPW Fed
Journal of Microwaves, Optoelectronics and Electromagnetic Applications, Vol. 9, No. 1, June 2010 14 Fig. 4. Experimental Radiation Pattern of Fourth Iterative Fractal Antenna at Frequency 0.858 GHz Fig. 5. Experimental Radiation Pattern of Fourth Iterative Fractal Antenna at Frequency 1.49 GHz
Journal of Microwaves, Optoelectronics and Electromagnetic Applications, Vol. 9, No. 1, June 2010 15 Fig. 6. Experimental Radiation Pattern of Fourth Iterative Fractal Antenna at Frequency 5.325 GHz Fig. 7. Experimental Radiation Pattern of Fourth Iterative Fractal Antenna at Frequency 10.2 GHz
Journal of Microwaves, Optoelectronics and Electromagnetic Applications, Vol. 9, No. 1, June 2010 16 B. Backscattering Reduction Backscattering reduction of fourth iterative fractal antenna structure has been analyzed with respect to various parameters like substrate thickness, substrate dielectric constant, superstrate thickness and superstrate dielectric constant against frequency. The method of moments has been used to analyze the backscattering behavior. The accuracy of this method has been verified with the published results [9]. Fig. 8 shows the comparison of the backscattering RCS of a rectangular plate of size 101.6mm by 101.6mm with the calculated results by [8] at frequency 9.23 GHz. Both results are in good agreement. Fig. 8. Backscattered RCS of a rectangular flat plate at f = 9.23 GHz C. Effect of Iterations on Backscattering Reduction The Backscattering reduction of fractal antenna has been investigated with respect to iterations shown in Fig. 8. The backscattering of fractal antenna with respect to iteration has been calculated for substrate dielectric constant ε r = 2.2 and h = 0.787 mm at frequency 1.8 GHz. The backscattering of solid patch called zeroth iteration (i.e. it_0 shown in Fig. 9) is around 32.5 db. The aspect angle has been taken from 90 to +90 for vertical - vertical polarization (incident and scattered wave are vertically polarized i.e (VV). It is clear from the Fig. 8, as the iteration increases the RCS reduces but beyond fourth iteration much reduction in RCS has not been observed. So, it concluded that fourth iterative (it_4) fractal antenna exhibits the maximum backscattering reduction. Based on this investigation, fourth iterative fractal antenna structure has been studied for antenna characteristics as well as backscattering behavior.
Journal of Microwaves, Optoelectronics and Electromagnetic Applications, Vol. 9, No. 1, June 2010 17 Fig. 9. Monostatic Backscattering RCS with respect to number of iterations at 1.8 GHz D. Effect of Substrate Thickness on Backscattering Reduction The effect of thickness of the substrate on backscattering RCS of fourth iterative fractal antenna has been studied for ε r = 2.2 as shown in Fig. 10. As the thickness of the substrate increases from 1 mm to 9 mm, the monostatic RCS of this structure reduces and beyond this thickness, backscattering RCS increases for both vertical-vertical (VV) and horizontalhorizontal (i.e. HH; means incident and scattered wave both are horizontal polarized) polarization. But bistatic backscattering RCS gives the minimum backscattering at thickness around 5mm for vertical polarization and around 7mm for horizontal polarization as shown in Fig. 10. RCS Vs SubstrateThickness Plot at 1.8 GHz RCS in dbsm -16.5 1 3 5 7 9 11 Monostatic VV Monostatic HH -21.5 Bistatic VV Bistatic HH -26.5-31.5-36.5-41.5 Substrate Thickness in mm Fig. 10. Backscattering RCS Reduction vs. substrate thickness for є r = 2.2 at 1.8 GHz
Journal of Microwaves, Optoelectronics and Electromagnetic Applications, Vol. 9, No. 1, June 2010 18 E. Effect of Substrate Dielectric Constant on Backscattering Reduction The effect of substrate dielectric constant has also been analyzed for thickness 9mm and dielectric constant ε r = 2.2, 4.3 and 6.15. It can be seen clearly in Fig. 11, as the dielectric constant of substrate increases, RCS minima are shifted towards lower frequency side and RCS bandwidth is also shrunk. So, it is predicted from the results that as the substrate dielectric constant increases, the bandwidth of backscattering reduction decreases. The RCS reduction varies with respect to frequency. Through out the band the RCS reduction is 15 db or better. It varies with frequencies at some of frequency RCS is better than 25 db. It is observed as superstrate thickness increases the backscattering RCS minima shifted towards lower frequency side. If superstrate dielectric constant increases, the RCS minima shifted again towards lower side. The shift in backscattering RCS minima is more due to substrate thickness than superstrate thickness. Superstrate loading on the metallic pattern can be used for frequency tuning for low RCS. Fig. 11. Effect of substrate dielectric constant on RCS Reduction Vs Frequency F. Bistatic Backscattering RCS Reduction The bistatic backscattering RCS reduction for various substrate thickness 3 mm, 5mm and 9mm for dielectric constant ε r = 2.2 has been analyzed at angle 60 0. Fig. 12 shows the bistatic backscattering reduction with respect to the frequency. It is clear that RCS reduction is good throughout the band around 18 db. The RCS minima vary with respect to frequencies. At some of the frequency RCS is very low even better than 30 db. So, the RCS reduction in case of bistatic is better than monostatic.
Journal of Microwaves, Optoelectronics and Electromagnetic Applications, Vol. 9, No. 1, June 2010 19 Fig. 12 Bistatic backscattering RCS for various substrate thickness Vs frequency V. CONCLUSION Circular Microstrip Fractal antenna has been designed and demonstrated for ultra wide bandwidth and size reduction using self-similarity and space filling properties. This antenna exhibits the impedance bandwidth from 0.8 GHz to 10.68 GHz which is more than the Federal Commission Communication (FCC) bandwidth 3.1 GHz to 10.6 GHz for wireless communication system. The radiation pattern of this antenna is nearly Omni - directional. The backscattering behavior of this antenna reveals that fourth iterative fractal structure exhibits the low backscattering. backscattered RCS can be controlled by iteration number, substrate and superstrate thicknesses, and dielectric constant. Such type of fractal antenna is useful to model the target for low backscattering and UWB wireless communication System for civil and military applications. REFERENCES [1] J. Bahl and P. Bhartia, Microstrip Antennas, Dedham, Ma, Artech House, 1981. [2] C. Puente, J. Romeu, R. Pous, and A. Cardama, On the behavior of the Sierpinski multiband antenna, IEEE Trans. on Antennas and Propagation Vol. 46, No. 4, pp. 517-524, 1998. [3] S. N. Khan, J. Hu, J. Xiong, and S. He, Circular fractal monopole antenna for low VSWR UWB applications, Progress in Electromagnetics Research Letters, Vol. 1, pp. 19 25, 2008. [4] Edward Lule, Tadeusz Babij, and Time Derivative, Koch island fractal ultra wideband dipole antenna, IEEE Antennas and Propagation Society International Symposium, Vol. 3, pp. 2516 2519, June 2004. [5] J. P. Gianviffwb and Y. Rahmat-Samii, Fractal antennas: A novel antenna miniaturization technique, and applications, Antennas Propag. Mag. Vol. 44, pp. 20 36, 2002. [6] G. Cui, Y. Liu, and S. Gong, A novel fractal patch antenna with low RCS, Journal of Electromagnetic Waves and Applications, Vol. 21, No. 15, pp. 2403 2411, 2007. [7] D. R. Jackson, The RCS of a rectangular microstrip patch in a substrate-superstrate geometry, IEEE Transaction on Antennas and Propagation, Vol. 38, pp. 2-8, 1990. [8] 3D - EM Simulator, Concerto 6.5, Vector Fields Limited, U.K. 2006. The