VOL. 3, NO. 7, July 2012 ISSN 2079-8407 Journal of Emerging Trends in Computing and Information Sciences ©2009-2012 CIS Journal. All rights reserved. http://www.cisjournal.org 1013 Design and Simulation of Miniaturized Minkowski Fractal Aperture-Coupled Antenna for 5.8 GHz RFID Applications 1 D. K. Naji, 2 J. S. Aziz, 3 R. S. Fyath Department of Electronic and Communications Engineering., College of Engineering, Al-Nahrain University, Baghdad, Iraq 1 dknaji73@yahoo.com , 2 jsaziz53@yahoo.com , 3 rsfyath@yahoo.com ABSTRACT A miniaturized 3rd-order Minkowski fractal aperture-coupled antenna is designed for 5.8 GHz RFID applications using particle swarm optimization (PSO) technique. The PSO technique runs on MATLAB environment and synchronously coupled with an electromagnetic simulator (CST Microwave Studio) to estimate the radiation pattern parameters at each optimization iteration. Two objective functions are used to optimize the geometry of the antenna: return loss and the relative antenna size with respect to the reference (non-fractal) antenna. The simulated results show that the optimized fractal antenna has less than -37 dB return loss, more than 65% reduction in patch area and, more than 4 dB gains. Keywords: Aperture-coupled antenna, Minkowski fractal antenna, Particle swarm optimization, RFID. 1. INTRODUCTION Radio frequency identification (RFID) has excelled in automatic identification, bioengineering applications and data collection industry through its speed, agility, and endurance [1]. Recently, the operating frequency of RFID systems moves towards higher frequencies such as ISM band (5.8 GHz) to achieve higher identification range with higher data transfer rate [2]. This motivates researchers to apply micro strip antenna technology for RFID systems to gain the advantages of low profile, light weight, small volume, and mass production [3, 4]. For micro strip patch antennas, aperture coupling is preferred to other feeding mechanisms as it offers greater design flexibility [5, 6]. Aperture coupling has considerable advantage as a feeding mechanism particularly in fractal designs where identifying a suitable feed position on such complex geometrical shapes is particularly difficult [7]. Miniaturization of micro strip patch antenna has been typically accomplished by a suitable loading. One common technique in loading is to modify basic patch shapes. Applying fractals to antenna elements allows for smaller size, multiband, and broad-band properties. This is the cause of widespread research on fractal antennas in recent years [8-10]. Fractal geometries have self-similarity and space-filling nature when applied to antenna design [11-13]; and they can realize multi-frequency and size- reducing features. Several fractal geometries have been explored for antennas with special characteristics, such as the Sierpinski monopole [14], Koch curves [15] and the tree monopole [16]. These fractal geometries verify that the fractal antenna has size-reducing features within limited space. The design of RFID antenna is still a challenge for miniaturization system due to the limited available area. This paper presents feasibility study to design miniaturized aperture-coupled micro strip antenna for 5.8 GHz RFID applications. The antenna miniaturization is achieved through two phases. The first phase applies fractal geometry while the second phase adopts particle swarm optimization (PSO) technique to get further area reduction. The CST Microwave Studio is used during the optimization process as an electromagnetic (EM) simulator to extract the antenna performance parameters that enter the objective function. 2. DESIGN METHODOLOGY Particle swarm optimization algorithm has been tested by different research groups to different benchmark functions and results show that it is an excellent global optimizer that can be used for different electromagnetic problems especially antenna miniaturization [17, 18]. To calculate the antenna fitness function associated with the PSO algorithm, a comprehensive numerical modeling must be carried out to simulate the EM performance of the antenna at each iteration of optimization. The EM model should be very efficient in both speed of computation and accuracy since the geometry of the fractal antenna is relatively more complicated than the conventional counterpart and the dimensions of some structure parameters are much smaller than the operating wavelength. The required EM model features are recovered in this paper by using a commercial EM simulator namely CST MWS. This simulator uses finite integration time domain (FITD) method to assign the EM properties of antennas and has been proven in the literature as a powerful and very accurate tool for this purpose. In this work, the fractal RFID antenna is optimized using PSO technique while the FITD method is used in parallel with it to compute the EM part of the fitness function (see Fig. 1). The PSO technique runs under MATLAB environment and the FITD method is offered by CST MWS software package. For each generation of the PSO algorithm, the antenna geometrical parameters are updated and mapped to CST MWS to simulate the EM properties of the antenna. According to the EM simulator