ENHANCED WIDEBAND AND COMPACT SIZE FRACTAL KOKH ANTENNA Babak Mirzapour and Abbas Ali Lotfi Neyestanak 1 Electrical Engineering Department, Shahed University, Tehran, I. R. Iran 2 Electrical Engineering Department, Islamic Azad University, Branch of Rey, Tehran, I. R. Iran Received 30 September 2006 Figure 8 Comparison of 3D EM-simulated and measured performance characteristics of the fully embedded circular-stacked spiral inductor with two windings and an inner diameter of 1000 ␮m. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley. com] ABSTRACT: A novel wideband and compact size fractal Kokh antenna is described in this article. The return-loss, E-plane and H-plane radiation pattern of the proposed antenna are measured and simulated by Finite Element method. Both experimental and numerical results show that the proposed antenna has smaller size and wideband behavior respect to similar fractal Kokh antenna designs. The Proposed antenna is able to achieve an impedance bandwidth of 19% for VSWR less than 2. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 1077–1080, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop. 22349 Key words: microstrip fractal kokh antennas; small size antenna; wideband antenna density. The measured performance characteristics are well matched with the 3D EM simulated ones. Thus, the proposed circular-stacked spiral inductors can be useful for the miniaturized SOP with various functionalities, low cost, low profile, small size and volume. ACKNOWLEDGMENTS This research work was partially supported by the Seoul Research and Business Development Program (Grant no.10583). Fabrication was carried out at Daeduck Electronics in Korea. The authors acknowledge Mr. H. S. Lee and Mr. Y. J. Ko of Daeduck Electronics and MiNDaP group members of Kwangwoon University for their technical supports and discussions. 1. INTRODUCTION With the advance of wireless communication systems and increasing importance of other wireless applications, wideband and low profile antennas are in great demand for both commercial and military applications. Multiband and wideband antennas are desirable in personal communication systems, small satellite communication terminals, and other wireless applications. Wideband antennas also find applications in Unmanned Aerial Vehicles (UAVs), Counter Camouflage, Concealment and Deception (CC&D), Synthetic Aperture Radar (SAR), and Ground Moving Target Indicators (GMTI). REFERENCES 1. J. Prymark, S. Bhattacharya, and K. Paik, Fundamentals of passives: Discrete, integrated, and embedded, In: R.R. Tummala (Eds.), Fundamentals of Microsystems Packaging, McGraw-Hill, New York, 2001. Chapter 11. 2. K. Lim, et al., RF-system-on-package (SOP) for wireless communications, IEEE Microwave Mag 3 (2002), 88 –99. 3. A. Sutono, D. Heo, E. Chen, K. Lim, and J. Laskar, High-Q LTCCbased passive library for wireless system-on-package (SOP) module development, IEEE Trans Microwave Theory Tech 49 (2001), 1715– 1724. 4. S.H. Lee, et al., High performance spiral inductors embedded on organic substrates for SOP applications, IEEE MTT-S Int Microwave Symp Dig 3 (2002), 2229 –2232. 5. M.F. Davis, et al., Integrated RF architectures in fully organic SOP technology, IEEE Trans Adv Packag 25 (2002), 136 –142. 6. C.P. Yue and S.S. Wong, On-chip spiral inductors with patterned ground shields for Si-based RF ICs, IEEE J Solid State Circ 33 (1998), 743–752. 7. A. Sutono, et al., RF/microwave characterization of multilayer ceramicbased MCM technology, IEEE Trans Adv Packag 22 (1999), 326 –331. 8. S. Chaki, S. Aono, N. Andoh, Y. Sasaki, N. Tanino, and O. Ishihara, Experimental study on spiral inductors, In: IEEE MTT-S Digest, Orlando, FL, May 16 –20, 1995. pp. 753–756. © 2007 Wiley Periodicals, Inc. DOI 10.1002/mop Figure 1 Geometry of enhanced Koch island antenna MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 49, No. 5, May 2007 1077 Figure 4 Return Loss of Ordinary and enhanced Fractal Kokh antenna at second iteration Figure 2 Geometry of the enhanced Koch Island or “snowflake” pre fractal for increasing number of iterations Recent efforts by several researchers around the world to combine fractal geometry with electromagnetic theory have led to a plethora of new and innovative antenna designs. Fractals have been successfully used for modeling such complex natural objects as galaxies, cloud boundaries, mountain ranges, coastlines, snowflakes, trees, leaves, ferns, and much more. Since the pioneering work of Mandelbrot and others, a wide variety of applications for fractals continue to be found in many branches of science and engineering. One such area is fractal electrodynamics [1–11], in which fractal geometry is combined with electromagnetic theory for the purpose of investigating a new class of radiation, propagation, and scattering problems. One of the most promising areas of fractal electrodynamics research is in its application to antenna theory and design. The general concepts of fractals can be applied to develop various antenna elements and arrays. Applying fractals to antenna elements allows for smaller, resonant antennas that are multiband/ broadband and may be optimized for gain. Figure 3 Return Loss of Ordinary and enhanced Fractal Kokh antenna at first iteration 1078 The fact that most fractals have infinite complexity and detail can be used to reduce antenna size and develop low profile antennas. When antenna elements or arrays are designed with the concept of self-similarity for most fractals, they can achieve multiple frequency bands because different parts of the antenna are similar to each other at different scales. Application of the fractional dimension of fractal structure leads to the gain optimization of wire antennas. The combination of the infinite complexity and detail and the self-similarity makes it possible to design antennas with very wideband performance. Fractal shaped antennas exhibit some interesting features that stem from their geometrical properties such as multiband behavior of self-similar fractal antennas and frequency selective surfaces (FSS) [11–12]. The fractal concept can be used to reduce antenna size, such as the Koch dipole, Koch monopole, Koch loop, and Minkowski loop. Or, it can be used to achieve multiple bandwidths and increase bandwidth of each single band because of the self-similarity in the geometry, such as the Sierpinski dipole, Cantor slot patch, and fractal tree dipole. In other designs, fractal structures are used to achieve a single very wideband response, e.g., the printed circuit fractal loop antenna. 2. GEOMETERY Kokh snowflake fractal antenna is a popular known fractal antenna. This fractal starts out as a solid equilateral triangle in the Figure 5 Return Loss of Ordinary and enhanced Fractal Kokh antenna at third iteration MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 49, No. 5, May 2007 DOI 10.1002/mop Figure 8 Radiation Pattern for enhanced wideband Kokh antenna at 6 GHz for (a) H-Plane pattern and (b) E-Plane pattern Figure 6 Simulated Return Loss for enhanced Fractal Kokh antenna at three iterations plane. Unlike the Sierpinski gasket, which was formed by systematically removing smaller and smaller triangles from the original structure, the kokh snowflake is constructed by adding smaller triangles to the structure in an iterative fashion. The enhanced Kokh antenna is six Kokh island microstrip antennas which are loaded by a fractal shaped slot on a complete Kokh patch antenna fed by a 50 Ohm coax feed that has been derived on an air substrate layer as illustrated in Figure 1. Geometry of the enhanced Koch Island or “snowflake” pre fractal for increasing number of iterations is shown in Figure 2. As can be seen the desired Kokh antennas are smaller than Ordinary fractal Kokh antennas. It is obvious from Figure 3 that for ⫺10 dB return loss, the bandwidth is ⬃19% for the first iteration. The impedance bandwidth about 19% for second iteration and 17% for third iteration are achievable as can be seen in Figures 4 and 5. Figures 6 and 7 show the frequency shift procedure of return loss toward low frequencies by increasing of enhanced Kokh iterations in simulation and measurement. The radiation properties of the ordinary and enhanced fractal Koch antenna at third iteration are studied. Figures 8–10 show the measurement of the E Plane and H plane radiation pattern in 6, 6.375, and 6.6 GHz at the antenna radiation band. Table 1 summarizes the performance comparison. 3. RESULTS The simulation results of the return loss for the proposed antennas are shown in Figures 3–5. The return loss result of enhanced fractal Kokh antenna and return loss of Ordinary fractal Kokh antenna comparing once with each other are shown at a single figure. The result for first iteration is shown at Figure 3, second iteration is shown at Figure 4 and Third iteration is shown at Figure 5. Figure 9 Radiation Pattern for enhanced wideband Kokh antenna at 6.37 GHz for (a) H-Plane pattern and (b) E-Plane pattern Figure 7 Measured and simulated return loss for third iteration of enhanced wideband fractal Kokh antenna DOI 10.1002/mop Figure 10 Radiation Pattern for enhanced wideband Kokh antenna at 6.6GHz for (a) H-Plane pattern and (b) E-Plane pattern MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 49, No. 5, May 2007 1079 TABLE 1 Illustrates a Comparison Between Ordinary and Enhanced Fractal Kokh Antenna Antenna BW Ordinary Kokh (%) BW Enhanced Kokh (%) Percent of Size Reduction (%) Kokh 1 Kokh 2 Kokh 3 19.1 17.7 16.4 19.5 19 17 17 16 19.5 cated between them. The filter has the advantages of low insertion loss, compact size, and good selectivity. The simulated and measured results are presented and show good agreements. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 1080 –1081, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ mop.22348 Key words: bandpass filter; dual-band; slotline resonator; microstrip resonator 4. CONCLUSION 1. INTRODUCTION The enhanced Kokh fractal antenna with wide bandwidth and compact size is presented in this article. Measured results on fabricated antenna were used to confirm the simulation results. There is increasing demand for dual-band wireless communication systems to provide users with various services. The transceivers of these wireless systems require dual-band filters to suppress unwanted signals. To meet the requirement, much work has been done and various configurations have been proposed [1– 8]. Dualband filter can be realized by combining two independent bandpass filters using the common input and output ports [1–3]. Extra impedance-matching network was used in Ref. 1 to design the input and output structure. In Refs. 2 and 3, coupled structure was adopted to eliminate the impedance-matching network. These filters have the advantage that the specifications of the two passbands can be easily met. In Ref. 4, dual-band operation was obtained by cascading a broadband filter with a bandstop filter. Using this structure, the two passbands was separated by a bandstop filter with properly arranged central frequency. However, the specifications are not easy to satisfy. By employing the properties of step impedance microstrip resonator, the first-harmonic frequency is adjusted by changing the impedance ratio to construct the higher passband, meanwhile the total length is fixed to maintain the central frequency of the lower passband [5– 8]. However, it is hard to satisfy the specifications of both passbands, especially that of higher passband. In this paper, a dual-band filter using two types of resonators is proposed. By introducing slot resonators on the ground of conventional microstrip resonator, the filter provides two transmission paths for RF signals. One path consists of split-ring slotline resonators, the other one is composed of a half-wavelength microstrip resonator. Each path generates its own passband. Both microstrip resonator and slotline resonators are designed with common input and output ports without any external impedance-matching network. Finally, a dual-band filter is optimally designed and fabricated. Predicted dual-band frequency response is well confirmed by experiment of a fabricated filter. REFERENCES 1. C. Puente, J. Romeu, R. Pous, and A. Cardama, On the behavior of the Sierpinski multiband antenna, IEEE Trans Antennas Propag 46 (1998), 517–524. 2. J. Soler and J. Romeu, Generalized Sierpinski fractal antenna, IEEE Trans Antennas Propag 49 (2001), 1237–1239. 3. J. Romeu and Y. Rahmat-Samii, Fractal FSS: A novel multiband frequency selective surface, IEEE Trans Antennas Propag 48 (2000), 713–719. 4. E. Parker and A.N.A. El Sheikh, Convoluted array elements and reduced size unit cells for frequency selective surfaces, in IEE Proc Part H: Microwaves, Opt Antennas, 138 (1991), 19 –22. 5. C. Puente, J. Romeu, and A. Cardama, The Koch monopole: A small fractal antenna, IEEE Trans Antennas Propag 48 (2000), 1773–1781. 6. C. Puente and R. Pous, Fractal design of multiband and low side-lobe arrays, IEEE Trans Antennas Propag 44 (1996), 1–10. 7. P.E. Mayes, Frequency-independent antennas and broad-band derivatives thereof, Proc IEEE 80 (1992), 103–112. 8. P.E. Mayes, G.A. Deschamps, and W.T. Patton, Backward-wave radiation from periodic structures and application to the design of frequency-independent antennas, Proc IRE 49 (1961), 962–963. 9. G.A. Deschamps and J.D. Dyson, The logarithmic spiral in a singleaperture multimode antenna system, IEEE Trans Antennas Propag AP-19 (1971), 90 –96. 10. V.H. Rumsey, Frequency Independent Antennas. New York: Academic, 1966. 11. R.L. Carrel, Analysis and design of the log-periodic dipole antenna, Doctoral dissertation, Department Electrical Engineering, University of Illinois, Urbana-Champaign, 1961. 12. H. Jasik, Antenna Engineering Handbook, McGraw-Hill, New York, 1961, pp. 2.10 –2.13. © 2007 Wiley Periodicals, Inc. NOVEL DUAL-BAND FILTER USING SLOTLINE AND MICROSTRIP RESONATORS Xiu Yin Zhang, Jian-Xin Chen, and Quan Xue Wireless Communications Research Center, Department of Electronic Engineering, City University of Hong Kong, Hong Kong, China Received 30 September 2006 ABSTRACT: A novel dual-band filter using slotline and microstrip resonators is proposed. By introducing slotline resonators on the ground of conventional microstrip resonator, the filter provides two transmission paths for RF signals. Each path generates its own passband. Both microstrip resonator and slotline resonators share the input and output ports without any external impedance-matching network. Deep rejection between the two passbands is obtained due to a transmission zero lo- 1080 2. FILTER DESIGN The configuration of the proposed filter is shown in Figure 1, which is a double-layer configuration. The filter consists of two types of resonators. The microstrip resonator is fabricated on the top side of the substrate, which is employed to generate higher passband. The length of the resonator is about half guided wavelength at the center frequency of higher passband. The slotline resonators are located on the bottom side of the substrate. They are used to obtain lower passband. Two coupled split-ring slotline resonators are employed. The slotline length of the resonator is about half guided wavelength at the center frequency of lower passband. Contrary to microstrip open-loop resonators [9], the proposed split-ring slotline resonator has maximum magnetic field density near the slit and maximum electrical field density at the opposite side of the split-ring. Thus, the interstage coupling is magnetic coupling. The coupling intensity depends primarily on the separation distance between these two resonators. Both the slotline resonators and microstrip resonator are fed by two microstrip lines through coupling. MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 49, No. 5, May 2007 DOI 10.1002/mop