FRACTAL KOCH DIPOLE ANTENNA FOR UHF BAND APPLICATION Mohd Nazri A. Karim, Mohamad Kamal A. Rahim, and Thelaha Masri Department of Radio Communication Engineering (RaCED), Faculty of Electrical Engineering, Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia; Corresponding author: ithelaha@yahoo.com.my Received 19 February 2009 ABSTRACT: The fractal Koch curve is one of the popular fractals shape. This article discussed the analysis of the fractal Koch dipole antenna (FKDA) using log periodic technique. The effect of flare angle of the an- tenna has been described to reduce the antenna size. The antenna is oper- ated at ultrahigh frequency (UHF) band frequency between 460 MHz and 4 GHz. The analysis is based on the number of elements, degree of flare an- gle, current distribution, and radiation pattern of the antennas. The antenna has been designed and simulated using computer simulation technology (CST) and fabricated using FR4 board. The performance of the antennas has been measured, analyzed, and discussed in this article. © 2009 Wiley Periodicals, Inc. Microwave Opt Technol Lett 51: 2612–2614, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24702 Key words: fractal Koch antenna; UHF frequency; radiation pattern; current distribution; wideband 1. INTRODUCTION In recent years, the demands of portable systems have in- creased; low-profile systems have drawn and bring much inter- est to researchers. In making such low-profile communication systems, the size of the antenna is critical. Therefore, many techniques, such as using higher dielectric substrate, multiple layers, and by optimizing the shape of the antennas, have been proposed and applied to the microstrip and planar antennas. The application of fractal geometry to conventional antenna struc- tures optimizes the shape of the antennas to increase their electrical length, which thus reduces their overall size. This is because fractal geometries have two main features in common, space-filling and self-similar properties. Fractal shape antenna elements present various advantages: multiband frequency and reduced antenna size [1]. Fractal shapes become more enticing among researchers because of their capabilities. In applications where spaces are constraints, alternative smaller designs need to be considered. In this article, fractal Koch dipoles arrays with 15 elements are introduced and discussed. The design proce- dures of the antennas are elaborated, and the simulated return loss, current distribution, and size reduction have been ana- lyzed. 2. DESIGN CONSIDERATION Fractal Koch dipole antenna (FKDA) array with 15 elements has been designed using computer simulation technology software as shown in Figure 1. The planar fractal dipoles are placed on the top and bottom layer of the substrate. The FR4 board has been used as a substrate material, which has a dielectric constant of 4.6 and a tangent loss of 0.019. The antenna operates at UHF band frequen- cies covering from 460 MHz to 4 GHz, which can be used for digital TV (DTV), digital video broadcasting (DVB), and wireless regional area network (WRAN). A straight dipole array antenna and fractal Koch dipole array antenna with different flare angles have been designed and compared in terms of size reduction, return loss, and current distribution. The antenna was designed using log periodic technique. This technique has been used for VHF and UHF band applications. It is capable to produce constant gain over a wideband frequency. The design procedures for log periodic antenna are elaborated in more detail in [2]. Figure 1 shows the layout of fractal Koch dipole array. The parameters that were involved such as w n represents the width of the dipole, d n represents the distance between each element, l represents the total length of the structure, and w and h represent the total width and the height of the structure, respectively. The relation between the parameters is given by [2]:  = d n d n+1 = l n l n+1 = W n W n+1 . (1) where d n is the distance between each element, l n is the length of the dipole, and w n is the width of the element. The length of each element depends on the resonant frequen- cies. The formulas to design dipole antenna are given in many books and papers [3–5]. The antenna was iterated one-third of length of the dipole. Figure 2 shows the fractal Koch dipole array structures with 15 elements. The antenna was fed using 50- SMA connector. It is connected to a central transmission line with a phase reversal between the dipole. This is required so that radiation is in backfire direction (toward smaller element). If the phase reversal is not used, radiation will occur in end-fire direction (toward larger element). This will cause scalloping in amplitude pattern and leads to unpredictable impedance behavior. This is referred as end effect [2]. In simulation, the SMA connector needs to be designed to match with the line of the dipole. The excitation of the antenna can be fed using coaxial feeding technique or direct feeding technique. Figure 1 Layout of fractal Koch dipole array 2612 MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 51, No. 11, November 2009 DOI 10.1002/mop