Design a Dual-Band High-Impedance Surface Structure for Electromagnetic Protection in WLAN Applications

Ming-Shing Lin 1, Yi-Han Huang 2, and Chung-I G. Hsu 3 Department of Electrical Engineering, NYUST,

No.123, Sec. 3, University Rd., Douliou, Yunlin 64002, Taiwan, R.O.C. 1starlin@yuntech.edu.tw 2duemei@hotmail.com

3cghsu@yuntech.edu.tw

Abstract— In this study, a dual-band high-impedance surface (HIS) structure and a dual-band monopole antenna were designed for 2.45 GHz and 5.25 GHz WLAN applications. This HIS structure was proposed as an electromagnetic (EM) protective sheet to reduce undesirable EM radiation of antenna. The effects of EM protection sheets including metallic plates, R-cards, and HIS on the specific absorption rate (SAR) and on the antenna performance were then numerically and experimentally investigated. It was found that that the HIS can serve as an EM protective sheet without affecting the forward radiation power of the antenna.

Keywords— electromagnetic compatibility, electromagnetic

protection sheet, high-impedance surface (HIS), total radiated power (TRP), specific absorption rate (SAR)

I. INTRODUCTION In recent years, many wireless communication systems

have been rapidly developed. They include wireless local area network (WLAN), Bluetooth, and long-term evolution (LTE), to name only a few. In all these systems, the effects of electromagnetic (EM) radiation on human bodies have become an important research topic in the area of electromagnetic compatibility (EMC). In IEEE Std C95.1 [1], ICNIRP Guidelines [2], FCC OET Bulletin 65C [3], IEC62209-1 [4], and IEEE Std 1528 [5], recommendations (or restrictions) and measurement procedures have been made available to ensure the EM radiation safety of wireless communication devices and to protect human bodies against adverse health effects resulting from EM exposure. In order to pass the requirements of existing standards or regulations for wireless devices, a designer should consider the radiated power density, electromagnetic compatibility (EMC), and specific absorption rate (SAR) of these devices [6]-[7]. In addition to EMC and radiation hazard issues, the designer should also minimize the influence of any nearby user’s body on the performance of the antennas mounted on theses devices and promote the quality of communications [8]-[9].

In previous studies [10]-[15], many techniques have been used to reduce the SAR of a wireless device. It is found that metallic plates [10], R-cards 11], EBG structures [12], metamaterials [13], and high-impedance surface (HIS) [14]-[15] can be designed as EM protective sheets to reduce the SAR of a wireless device. However, the properties of a

wireless antenna may be affected by the presence of an EM protective sheet. Therefore, the performance of EM protective sheets should be evaluated in terms of their effects on the SAR and the performance of the communication system.

The performance of a wireless product can be evaluated in terms of the total radiated power (TRP), total isotropic sensitivity (TIS), and other antenna parameters [16]. Radiation hazards can be specified by the maximum permissible exposure (MPE) and SAR [1]-[3]. Limits for the MPE are quoted in terms of the power density of a plane wave (or the maximum electric or magnetic fields). Limits for the SAR are related to the power absorbed by a user’s body. In previous study [15], the forward horizon radiated power (FHRP), backward horizon radiated power (BHRP), forward radiated power (FRP), and backward radiated power (BRP) for the antennas of a wireless device were defined to evaluate the performance of an EM protective sheet. Among these four terms, the FHRP and FRP are related to the maximum communication distance between a wireless device and a base station, and the BHRP and BRP pertain to the SAR of the wireless device.

In this study, a dual-band EM protection sheet using HIS structures was designed to reduce undesirable EM radiation of antenna in the WLAN application. The dual-band monopole antenna and dual-band HIS were both designed using the simulator Ansoft HFSS. The reflection coefficient, radiation pattern, TRP, and NHRP were all simulated with and without the presence of the HIS. To verify that the designed dual-band monopole antenna in the presence of the dual-band HIS can indeed reduce adverse effects on human bodies, we established a human-head model to simulate the SAR. In comparison with other types of EM protection sheets, it is also found that HISs are more valuable in practical applications.

II. CONFIGURATION OF ELECTROMAGNETIC PROTECTIVE SHEETS AND ANTENNA

In this study, metallic plates, R-cards, and HISs were used as EM protective sheets. First, an HIS was designed as EM protective sheets for dual-band (2.45 GHz and 5.25 GHz) WLAN antenna [15]. Fig. 1 shows the geometries of dual-band HIS structures. The dimensions of the HIS are designed as follows: the width of cell, L= 25 mm; the inner and outer

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radius of annular, r= 8.5 mm and R= 12 mm; the length and width of the Criss-cross, l=7.5 mm and w = 2 mm; the thickness of the substrate, T = 1.6 mm.

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Fig. 1. Geometries of HIS structures designed for 2.45GHz and 5.25 GHz: (a) top and side view for unit cell, (b) periodic array

The reflection phase ( 11S! ) can be used to characterize the performance of HIS structures. The performance of the HIS was simulated by using the software Ansoft HFSS incorporating with a periodic boundary condition [17], and verified by arc-horns test system in anechoic chamber (see the photos in Fig. 2). Measurement and simulation results of the reflection phase of the designed dual-band HIS are shown in Fig. 3. The measurement results can verify the simulation results. From the results, the resonant frequency of the HIS is estimated to be 2.45 GHz and 5.25 GHz,

Fig. 2. The photos of arc-horns test system for evaluating the reflection phase of HISs

Fig. 3. Measurement and simulation results of the reflection phase of the designed dual-band HIS

Fig. 4 shows the configuration of a dual-band monopole antenna with an HIS below. The monopole antenna designed for the 2.45 GHz and 5.25 GHz bands consists of a fork shape monopole and a 50-" CPW feed line. The antenna has a length (L3) of 17 mm and a width (W2) of 3 mm; the feed line is printed on a grounded FR4 substrate with a dielectric

constant of 4.4, a loss tangent of 0.02, a thickness (T) of 1.6 mm, a width (W1) of 25 mm, and a length (L1) of 50 mm. For convenience, refer to this grounded substrate as a PCB. Below the PCB is another grounded FR4 substrate on which the HIS of 3 3# cells resides. When projected onto the same plane, the PCB and the HIS have a gap of 0.5 mm in between.

In the study of the cases with other EM protective sheets (e.g., metallic plates, R-cards,), one can replace the HIS by an EM protective sheet as shown in Figure 4(b). The width and length of the considered EM protective sheets are 75 and 75 mm. The metallic plate is made by copper with conductivity 75.8 10$ = # S/m. The surface resistance ( 1sR t$= ) of the R-card is 131 " .

Fig. 4. Configuration of a dual-band monopole antenna with an HIS: (a) top view, (b) side view

III. EFFECTS OF EM PROTECTIVE SHEETS ON THE ANTENNA PARAMETERS

To investigate the influence of EM protective sheets on the performance of the antenna, the return loss ( 11 (dB)S ) of the antenna with EM protective sheets (as shown in Fig. 4) were analysed by numerical simulation and measurement methods. Simulated and measured 11S of the monopole antenna, the metal-plate-added antenna, the R-card-added antenna, and the HIS-added antenna are shown in Fig. 5. Although the results of measurement and simulation are not entirely consistence. It was found that the return loss of the antenna is affected by nearby EM protective sheets. As far as the operating band for the 2.45 GHz and 5.25 GHz (determined by 11 10 dBS % & ) is concerned, the HIS-added antenna has the simulated return loss of -13.20 dB (at 2.45 GHz) with a bandwidth from 2.4 to 2.49 GHz, and -18.71 dB (at 5.25 GHz) with a bandwidth from 5.11 to 5.27 GHz, and has the measured return loss of –12.63 dB (at 2.45 GHz), with a bandwidth from 2.38 to 2.48 GHz, and -14.07 dB (at 5.25 GHz) with a bandwidth from 4.57 to 5.25 GHz. Clearly, the impedance bandwidths of HIS-added antennas are significantly reduced as compared with that of the HIS-less monopole antenna. However, the HIS-added antenna can provide a reasonable operating band to

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cover the desired WLAN band of 2.4-2.484 GHz and 5.15- 5.35 GHz, and the operating bands of the metal-plate-added antenna do not meet the requirement.

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Fig. 5. (a) Simulated and (b) measured 11S of the monopole antenna, the metal-plate-added antenna, the R-card-added antenna, and the HIS-added antennas

IV. EFFECTS OF EM PROTECTIVE SHEETS ON RADIATED POWER

Using the configuration of the monopole antenna with an HIS in Fig. 4, the radiation pattern of the monopole antenna, the metal-plate-added antenna, the R-card-added antenna, and the HIS-added antenna were simulated and measured. For conciseness, we only show in Fig 6 for the simulated and measured co-polarized radiation patterns (x-y plane) of the monopole antenna and the HIS-added antenna at 2.45 GHz. One can find that the forward radiations (i.e., the radiations paraxial to the +x direction) of the HIS-added antenna remain similar to that of the HIS-less monopole antenna itself. In contrast, the backward radiations (i.e., the radiations paraxial to the -x direction) of the HIS-added antenna are significantly reduced from that of the HIS-less monopole antenna alone, implying that the SAR of the former can be greatly reduced from that of the latter. The simulated and measured radiation patterns of the antenna with other EM protective sheets also revealed that those EM protective sheets can also reduce the power in the backward radiations.

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Fig. 6. (a) Simulated and (b) measured radiation patterns (2.45GHz) of the monopole antenna and the HIS-added antennas

To further illustrate what is claimed above, the TRP, FRP, 030FHRP

±, BRP, and 030BHRP

±defined in Lin [15] were used to

evaluate the performance of these EM protective sheets. These definitions follow the TRP and near-horizon radiated power (NHRP) defined CTIA Test Plan [16] for the purpose of

evaluating the radiation performance of the wireless devices. The NHRP is used to determine the power radiated near the azimuth axis. Note that the FHRP and FRP are related to the maximum communication distance between the wireless device and the base station, and the BHRP and BRP pertain to the SAR of the wireless device.

Let the power delivered to the antenna ( TP ) be set to be 1 W for the 2.45 GHz WLAN band. With this TP and the simulated (or measured) antenna power gains ( TG ), the

30FHRP± , 30BHRP± , FRP, and BRP of the monopole antenna and EM-protective-sheet-added antennas can be computed. The results are summarized in Table 1. Clearly, the FRP and

30FHRP± of the HIS-added antennas are close to those of the monopole antenna alone. On the other hand, the BRP and

30BHRP± of the HIS-added antennas are approximately 10 dB smaller than those of the monopole antenna alone. This indicates that the HISs can serve as an EM protective sheet without affecting the forward radiation power of the antenna. However, metallic plates and R-cards cannot provide this property.

TABLE 1 SIMULATED AND MEASURED 30FHRP± , 30BHRP± , FRP, AND BRP

V. EFFECTS OF EM PROTECTIVE SHEETS ON SAR

Fig.7 shows a human head model proposed to simulate the effects of EM protective sheets on the SAR of a wireless devices. The simulation model includes the monopole antenna with or without an EM protective sheet and a head model (SAM phantom). The properties of the head tissue-equivalent dielectric [5] for the 2.45 GHz (or 5.25 GHz) WLAN band are 39.2r' = and 1.8$ = S/m (or 35.8r' = and 3.5$ = S/m) . The spatial- peak SAR averaging over 10g was simulated by using Ansoft HFSS. For computation of the SAR, the head tissue density is assumed to be 1000 kg/m3, and the power delivered to antenna is 1 W. The simulated peak 10g-SARs for the monopole antenna, the metal-plate-added antenna, the R-card-added antenna, and the HIS-added antenna are shown in Table 2. It was found that metal plates, R-cards, and HIS can be used to reduce the SAR. However, the SAR reductions owing to these HISs are larger than those owing to R-cards.

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Fig.7 A human-head model for SAR simulation

TABLE 2 HEAD MODEL FOR 10G- SAR (W/KG) SIMULATION

VI. CONCLUSIONS In this study, a dual-band HIS structure and a dual-band

monopole antenna were designed for 2.45 GHz and 5.25 GHz WLAN band. This HIS was used as EM protective sheets to reduce the SAR. It was found that all EM protective sheets (HIS, Metal, R-card) can reduce the SAR. However, only the HIS have less impact on the FRP, 30FHRP± , and other antenna parameters. This indicates that the HIS can serve as an EM protective sheet without affecting the forward radiation power of the antenna. The results of this study will provide useful information for the design of EM protective sheets and wireless terminal antennas.

ACKNOWLEDGMENT The authors thank the National Science Council, Taiwan,

ROC, for financial support under Grant No. NSC 101-2221-E-224 -063.

REFERENCES [1] ANSI/IEEE Std. C95.1-2005, “IEEE standard for safety levels with

respect to human exposure to radio frequency electromagnetic fields, 3 kHz to 300 GHz,” IEEE, New York, 2005.

[2] ICNIRP, “International commission on non-ionizing radiation protection guidelines for limiting exposure to time-varying electric, magnetic, and electromagnetic fields (up to 300 GHz),” Health Phys., Vol. 74, No. 4, 494-522, Apr. 1998.

[3] FCC OET Bulletin 65, Supplement C, “Evaluating compliance with FCC guidelines for human exposure to radiofrequency electromagnetic fields,” June 2001.

[4] BS EN 62209-1:2006, “Human exposure to radio frequency fields from hand-held and body-mounted wireless communication devices – Human models, instrumentation, and procedures –Part 1: Procedure to determine the specific absorption rate (SAR) for hand-held devices used in close proximity to the ear (frequency range of 300 MHz to 3 GHz) ,” Sep. 2006.

[5] IEEE Std. 1528-2003, “IEEE recommended practice for determining the peak spatial-average specific absorption rate (SAR) in the human head from wireless communications devices: measurement techniques,” Dec. 2003.

[6] Kivekas, O., J. Ollikainen, T. Lehtiniemi, and P. Vainikainen, “Bandwidth, SAR, and efficiency of internal mobile phone antennas”, IEEE Trans. on Electromagnetic Compatibility, Vol. 46, No. 1, 71-86, Feb. 2004.

[7] Kusuma, A. H., A.-F. Sheta, I. Elshafiey, Z. Siddiqui, M. A. S. Alkanhal, S. Aldosari, S. A. Alshebeili, and S. F. Mahmoud, “A new low SAR antenna structure for wireless handset applications,” Progress In Electromagnetics Research, Vol. 112, 23-40, 2011.

[8] Juho Poutanen, Interaction between mobile terminal antenna and user, Master Thesis, Department of Electrical and Communications Engineering, Helsinki University of Technology, Espoo, Otamedia, 2007.

[9] M. A. Jensen andY. Rahmat-Samii, “EM interaction of handset antennas and a human in personal communications,” Proc. IEEE, Vol. 83, pp.7–17, Jan. 1995.

[10] Sager, M., M. Forcucci, T. Kristensen, “A novel technique to increase the realized efficiency of a mobile phone antenna placed beside a head-phantom,” IEEE Antennas and Propagation Society International Symposium, Vol. 2, 1013-1016, Columbus, Ohio, USA, Jun. 2003.

[11] Chou, H.-H., H.-T. Hsu, H.-T. Chou, K.-H. Liu, and F.-Y. Kuo, “Reduction of peak SAR in human head for handset applications with resistive sheets (r-cards) ,” Progress In Electromagnetics Research, Vol. 94, 281-296, 2009.

[12] Kwak, S. I., D-U Sim, J. H. Kwon, H. D. Choi, “Comparison of the SAR in the human head using the EBG structures applied to a mobile handset,” European Microwave Conference, 937-940, Munich, European, Oct. 2007.

[13] Jiun-Nan Hwang and Fu-Chiarng Chen, "Reduction of the Peak SAR in the Human Head With Metamaterial" , IEEE Trans. Antennas and Propagation, Vol.54, No.12, Dec. 2006.

[14] M.-S. Lin, C.-H. Huang, and C.-N. Chiu, “Use of high-impedance screens for enhancing antenna performance with electromagnetic compatibility,” Progress In Electromagnetics Research, Vol.116, pp.137-157, May 2011.

[15] Ming Shing Lin, Chuang Hao Huang, Chung I G. Hsu, “Performance Study of Electromagnetic Protective Sheets for Wireless Communication Systems,” 2012 Asia-Pacific International Symposium on Electromagnetic Compatibility, pp. TH-AM-BIO3-2, Sentosa , Singapore, May 21 - 24, 2012

[16] CTIA Certification Program Management Document, “Test Plan for Wireless Device Over the Air Performance, Method of Measurement for Radiated RF Power and Receiver Performance,” Revision 3.2.1, March 2013.

[17] Remski, R. T., “Analysis of photonic bandgap surfaces using ansoft HFSS,” Microwave Journal, Vol. 53, 190-198, Sep. 2000.

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