Frequency Selective Surface Absorber for WLAN Security - IEEE Xplore

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Abstract—A frequency selective surface (FSS) with absorber characteristics is presented for 5 GHz wireless local area network (WLAN) security. The proposed ...

Proceedings of the 5th European Conference on Antennas and Propagation (EUCAP)

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: [email protected], [email protected] + CSIRO, ICT Centre, P O Box 76, Epping, NSW 1710, Australia Email: [email protected] # Department of Electronic Engineering, ISRA University, WR Plaza, I-10 Markaz, Islamabad, Pakistan Email: [email protected]

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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 00 to 450. Preliminary simulation results are presented.

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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.

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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 The use of FSS absorber has been investigated by different consisting of resistive circular loop in front of the conducting researchers to make the design more compact and simple [6-9]. FSS layer having a distance of 9mm. This concept follows the One of the main challenges behind the designing of FSS is to principle of conventional Salisbury screen and Jaumann reduce the distance between the conducting FSS and resistive absorbers. The thickness of the FR-4 used for resistive FSS sheet [9]. This is useful in making more compact, simple and layer 0.8mm while the surface resistance is chosen as practical designs by following the conventional Salisbury and 50Ω/square. Jaumann absorbers technique [10-14]. In this paper, a dualIII. SIMULATION RESULTS layer FSS filter with absorber characteristics is presented to The simulation results for the dual-layer FSS are presented block 5 GHz WLAN signals. It provides WLAN security by absorbing its signals while other useful microwave and to give an overview of the improvement in the FSS design as millimetre wave signals such as mobile phone, VHF/UHF TV compared to [9]. and the rest can pass through. Also it does not cause additional A. Reflecting FSS (Normal Incidence) multipath, delay spread and resultant fading. First, the conventional conducting FSS layer was simulated

978-88-8202-074-3/11/$26.00 ©2011 IEEE

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

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

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

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

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.

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.

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 stopband is unaffected in this case as well.

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 duallayer FSS for perpendicular polarization (TE).

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

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.

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 λ0/6, which is more than what is presented in [9].

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.

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.

Fig. 7 shows the dual-layer FSS performance for parallel REFERENCES 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 - [1] B. A. Munk, Frequency Selective Surfaces: Theory and Design. New York: Wiley, 2000. 29.9 dB and -24.7 dB and the reflection coefficients are -44.3 H. Li, B. Z. Wang, G. Zheng, and W. Shao, “A reflect array antenna dB and -8.4 dB, respectively. The -10 dB stop-band [2] backed on FSS for low RCS and high radiation performances,” Progress bandwidths for 00 and 450 incidence angles are 1.94 GHz and in Electromagnetics Research C, Vol.15, pp. 145-155, 2010. 1.03 GHz, respectively. [3] C. Mias, C. Tsakonas, and C. Oswald, “An Investigation Into the 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

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