A MULTINOTCHED OCTAGONAL SHAPED FRACTAL UWB ANTENNA Shrivishal Tripathi, 1 Akhilesh Mohan, 2 and Sandeep Yadav 1 1 CoE in ICT, IIT Jodhpur, Rajasthan, India; Corresponding author: shrivishal@iitj.ac.in 2 Department of E&EC, IIT Kharagpur, West Bengal, India Received 4 March 2014 ABSTRACT: In this article, a simple design approach for small ultra- wideband (UWB) antenna is presented. The small size and wideband operability is obtained by introducing Minkowski like fractal geometry in the design. It is observed that addition of fractal elements at the edges of octagonal shape leads to wideband phenomena. Furthermore, the insertion of multiple notches in the ground plane improves return loss characteristic in the desired UWB operational bandwidth of 3.1– 10.6 GHz. The compact 16.5 3 13.5 mm 2 antenna geometry size exhibits nearly omnidirectional radiation pattern, with VSWR < 2 over the entire UWB frequency range. The optimized antenna is fabricated and meas- ured using vector network analyzer. The measured characteristics are in good agreement with the simulated one. V C 2014 Wiley Periodicals, Inc. Microwave Opt Technol Lett 56:2469–2473, 2014; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.28629 Key words: UWB; multiband notch; fractal geometry; IFS; slot antenna; octagonal 1. INTRODUCTION The rapid growth in modern wireless system put several chal- lenges to designers such as smaller size, high data transmission capacity, low power consumption, ease of fabrication, low cost and wideband capabilities, and so forth. Hence, design a compact small wideband antenna is extremely tedious task. These prob- lems are resolved up to some extent by applying fractal geometry concept in ultrawideband (UWB) antenna designing. The UWB antennas are gaining attention due to their potential of very high data transmission rate above 200 Mbps in the few meters range at very low power levels, by spreading the transmitted signal over the UWB operating bandwidth 3.1–10.6 GHz [1]. Such developments in antenna designing may provide advantage over conventional data transmission in case of indoor communication. To design the geometrically miniaturized UWB antennas systems, capable of serving both as transmitter and receiver and exhibiting consistent and predictable behavior over the entire UWB fre- quency range, is a tedious task and demands cautiousness. Fractal geometry concept is applied in the antenna design to miniaturize the antenna size without hindering the antenna per- formance. The application of fractal geometry in wideband antenna designing exhibits not only the miniaturization and bandwidth enhancement properties but also provides the wide- band operational bandwidth, due to its self-similarity and space filling properties [2–5]. The space filling properties of fractal geometry is used in antenna designing to increases the effective electrical path length of the antenna in a given small area [4, 5]. This provides a possibility to achieve the desired antenna char- acteristics similar to larger antenna. The fractal geometries such as Koch snowflake [4], hexagonal shaped [3], Sierpinski triangle [3, 4, 6], and some irregular geometry [7, 8] are used to design UWB antenna. In all the above different geometrical shaped antennas, a small part of the geometry is repeated in a self- similar pattern representing a class of electromagnetic radiators, which helps in improvement of the antenna characteristic [4, 9]. However, most of them have large dimension and complex structure. Therefore, fractal antenna is a promising topic and needs to be genuinely investigated and developed. In this article, we have proposed a small novel UWB antenna based on fractal geometry. The Minkowski like fractal structure is combined with octagonal shaped geometry to increase the effective length of the antenna. The proposed antenna has a compact size of 13.5 3 16.5 mm 2 . It exhibits the return loss (S 11 ) better than 10 dB over the frequency range of 3.1– 10.6 GHz. This article, further discusses the effect of multiple notches in the ground plane and other bandwidth enhancing con- straints to obtain the entire UWB frequency range. Using this type of design technique many other wideband antennas can be designed in smaller size. 2. THEORY AND DESIGN APPROACH The proposed antenna is based on a combination of the Min- kowski like fractal and the octagonal geometry. The first itera- tion divides the initial length in five equal parts, and using that segment the Minkowski like structure is generated. Figure 1 shows the recursive procedure of the Minkowski like fractal geometry, which is further applied to the octagonal geometry as a generator as shown in Figure 2. Here, this octagonal shape works as an initiator, and Minkowski like fractal works as a generator in the development of initial antenna design. The proposed antenna is examined and designed by simulation software program Ansoft HFSS v.13, based on a three- dimensional (3D) full-wave finite element method. The proposed antenna design is simulated on a rectangular substrate (W 3 L), Rogers RT/Duroid 5880, having a dielectric constant of e r 5 2.2, loss tangent tan d 5 0.0009, and thickness of 0.787 mm. A rec- tangular substrate is chosen as a basic structure of the proposed antenna due to its characteristic of wideband operability and good radiation property [10]. The patch is connected to a 50 X micro- strip line of width (W m ) and length (L m ) for impedance matching as shown in Figure 3. Ground plane is positioned on the other side of the substrate of length (L g ) and width (W). 3. RESULTS AND DISCUSSION To achieve the desired characteristic and compact size of the proposed octagonal fractal antenna (OFA), an optimization of the different bandwidth enhancement design parameters has Figure 1 Generation of a Minkowski like structure Figure 2 Minkowski like fractal as applied to the edges of Octagonal geometry DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 56, No. 11, November 2014 2469