Frequency Selective Surface Absorber for WLAN Security Umair Rafique * , Ghaffer I. Kiani + , M. M. Ahmed * and Shahid Habib # *Department of Electronic Engineering, Mohammad Ali Jinnah University, Islamabad, Pakistan Emails: umair@jinnah.edu.pk, mansoor@jinnah.edu.pk + CSIRO, ICT Centre, P O Box 76, Epping, NSW 1710, Australia Email: ghaffer.kiani@csiro.au # Department of Electronic Engineering, ISRA University, WR Plaza, I-10 Markaz, Islamabad, Pakistan Email: shahid.habib@isra.edu.pk Abstract—A frequency selective surface (FSS) with absorber characteristics is presented for 5 GHz wireless local area network (WLAN) security. The proposed FSS has great potential to absorb WLAN signals by reducing multipath fading effects while allowing the transmission of other useful RF/microwave signals such as mobile phones, VHF/UHF TV etc. It consists of two layers, one with resistive FSS and other with conducting FSS. It has a stable frequency response for both TE and TM polarizations when the angle of incident wave is varied from 0 0 to 45 0 . Preliminary simulation results are presented. I. INTRODUCTION Frequency selective surfaces (FSSs) are used as spatial filters for microwave and millimetre wave electromagnetic signals [1]. An FSS could be used in many engineering applications such as RCS reduction [2], telecommunication [3], and WLAN security [4-5]. With the advancement in telecommunication, the use of wireless technology for information system has significantly increased. It provides an advantage of getting free of physical cabling but demands several issues to be addressed as well. The issue is to provide security for information flow in wireless local area networks (WLANs). Since WLANs are based on radio frequency (RF), the information can be hacked by intruders. A band-stop FSS which could be posted on walls of the buildings can provide solution for wireless security. The selective nature of FSS allows other useful RF/microwave signals to pass through while blocking WLAN signals. The use of FSS absorber has been investigated by different researchers to make the design more compact and simple [6-9]. One of the main challenges behind the designing of FSS is to reduce the distance between the conducting FSS and resistive sheet [9]. This is useful in making more compact, simple and practical designs by following the conventional Salisbury and Jaumann absorbers technique [10-14]. In this paper, a dual- layer FSS filter with absorber characteristics is presented to block 5 GHz WLAN signals. It provides WLAN security by absorbing its signals while other useful microwave and millimetre wave signals such as mobile phone, VHF/UHF TV and the rest can pass through. Also it does not cause additional multipath, delay spread and resultant fading. Fig. 1. The dimensions of the unit cell of the dual-layer FSS absorber. II. DESIGN The configuration of the dual-layer FSS is shown in Fig. 1. The band-stop characteristics are achieved by incorporating a conducting circular loop FSS on one side of the FR-4 sheet, having a thickness of 1.6mm. The function of this conducting FSS layer is to act as a reflector for WLAN signals while passing other useful signals. Then, the absorption characteristics are achieved by placing a second FSS layer consisting of resistive circular loop in front of the conducting FSS layer having a distance of 9mm. This concept follows the principle of conventional Salisbury screen and Jaumann absorbers. The thickness of the FR-4 used for resistive FSS layer 0.8mm while the surface resistance is chosen as 50/square. III. SIMULATION RESULTS The simulation results for the dual-layer FSS are presented to give an overview of the improvement in the FSS design as compared to [9]. A. Reflecting FSS (Normal Incidence) First, the conventional conducting FSS layer was simulated Resistive Circular Loop 20 mm 7.8 mm Conductive Circular Loop 9 mm 7.8 mm 1.6 mm 0.8 mm 2.8 mm 0.75 mm 20 mm FR4 FR4 Proceedings of the 5th European Conference on Antennas and Propagation (EUCAP) 978-88-8202-074-3/11/$26.00 ©2011 IEEE 872