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

AbstractA 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 00 to 450. 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

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Proceedings of the 5th European Conference on Antennas and Propagation (EUCAP)978-88-8202-074-3/11/$26.00 2011 IEEE 872

Fig. 2. Simulation results of the conventional conducting FSS layer.

Fig. 3. Simulation transmission and reflection coefficients results of the dual-layer FSS showing absorption in the stop band.

using Ansoft HFSS, a commercially available electromagnetic software [15]. The simulation transmission and reflection results of this conducting FSS are presented in Fig. 2 for normal incidence. At 5.21 GHz, the transmission and reflection coefficients are -35.7 dB and -0.1 dB, respectively. The stop-band -10 dB transmission bandwidth is 1.33 GHz which can easily cover IEEE 802.11a bandwidth requirement.

B. Absorbing FSS (Normal Incidence) The second resistive layer was designed to absorb

reflections caused by the first layer at the resonance frequency. The dual-layer FSS was simulated and the results are shown in Fig. 3. At 5.09 GHz, the transmission and reflection coefficients are -30 dB and -55 dB, respectively. In this case, the stop-band -10 dB bandwidth is 1.96 GHz. The transmission of the useful microwave signals outside the stop-band is unaffected in this case as well.

Fig. 4. Simulation transmission/reflection coefficients of the conducting FSS layer for perpendicular polarization (TE).

Fig. 5. Simulation transmission and reflection coefficients results of the conducting FSS layer for parallel polarization (TM).

C. Stable Reflecting FSS (Oblique Incidence) As FSSs are spatial filters, there frequency response should

be stable when the angle of incidence is changed from normal to oblique incidence [16]. This section describes frequency stability of the proposed FSS for perpendicular (TE) and parallel (TM) polarization.

Fig. 4 shows the oblique incidence performance of the conducting circular loop FSS layer for perpendicular polarization (TE). For 00 and 450 angles of incidence, the resonance frequencies are 5.15 GHz and 5.02 GHz and the transmission coefficients are -34.8 dB and -35.7 dB, respectively. The reflection coefficient at the resonance frequencies is almost 0 dB for all angles of incidence. The -10 dB stop-band bandwidths for 00 and 450 angles of incidence are 1.16 GHz and 1.26 GHz, respectively.

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Fig. 6. Simulation transmission and reflection coefficients results of the dual-layer FSS for perpendicular polarization (TE).

Fig. 5 shows the oblique incidence performance of conducting circular loop FSS layer for parallel polarization (TM). For 00 and 450 angles of incidence, the resonance frequencies are 5.11 GHz and 5.14 GHz and the transmission coefficients are -34.7 dB and -31.7 dB, respectively. The reflection coefficient is almost 0 dB for all angles of incidence. The -10 dB stop-band bandwidths for 00 and 450 angles of incidence are 1.19 GHz and 0.82 GHz, respectively.

D. Stable Absorbing FSS (Oblique Incidence) In this section, the simulation results of the dual-layer FSS

(as shown in Fig. 1) are presented for perpendicular (TE) and parallel (TM) polarization.

Fig. 6 shows the simulation results of the dual-layer FSS for perpendicular polarization (TE) for 00 and 450 angles of incidence. The resonance frequencies in this case are 5.14 GHz and 5.03 GHz, respectively. The transmission coefficients at the resonance frequencies are -30.1 dB and -31.3 dB, and the reflection coefficients are -44.9 dB and -13.5 dB, respectively. The -10 dB stop-band bandwidths for 00 and 450 angles of incidence are 2 GHz and 1.77 GHz.

Fig. 7 shows the dual-layer FSS performance for parallel polarization (TM) for 00 and 450 angles of incidence. The resonance frequencies in this case are 5.08 GHz and 5.11 GHz. The transmission coefficient at the resonance frequencies are -29.9 dB and -24.7 dB and the reflection coefficients are -44.3 dB and -8.4 dB, respectively. The -10 dB stop-band bandwidths for 00 and 450 incidence angles are 1.94 GHz and 1.03 GHz, respectively.

IV. DISCUSSION The simulation results for the dual-layer FSS absorber are

presented in this paper to give an overview of the improvement in the FSS design. It is clear from the results that

Fig. 7. Simulation transmission and reflection coefficients results of the dual-layer FSS for parallel polarization (TM).

FSS design has a stable frequency response for both perpendicular (TE) and parallel (TM) polarization when the angle of incident wave changed from normal to oblique incidence. Furthermore, the level of absorption achieved from this FSS design is much greater than the previous research [9]. The advantages of this FSS design are: (1) it is a simple design easy to manufacture than the previous research where the conducting cross dipole FSS is sandwiched between two dielectrics; (2) it provides maximum absorption and stability in the 5 GHz band and maintain the transmission of other useful microwave signals. However, in this dual-layer FSS design, the distance between the conducting and resistive sheet is 0/6, which is more than what is presented in [9].

V. CONCLUSION A dual-layer circular loop FSS absorber has been designed

and its performance has been investigated. It has a stable frequency response for both polarizations at normal and oblique incidence. Improved frequency stability of this design ensures that the FSS will absorb over a wide range of incidence angles, not only at normal incidence. Research is underway to reduce the distance between two FSS layers to make it more compact.

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