Design of an Absorptive Structure for WCDMA Band

Dong-Uk Sim*, Jong-Myun Kim*, Young-Jun Chong*, and Seong-Ook Park** *Radio Technology Research Department, Electronics and Telecommunications Research Institute (ETRI),

218 Gajeonno, Yuseong-gu, Daejeon, Korea **Department of Electrical Engineering, Korea Advanced Institute of Science & Technology,

291 Daehak-ro, Yuseong-gu, Daejeon, Korea Email of Corresponding Author: simdonguk@etri.re.kr

Abstract— In this paper, an absorptive structure based on a periodic structure is presented and its absorption performance is examined. This is a single-layer planar resonant structure, and a lossy periodic surface is used for enhancing the absorption characteristic instead of the conventional resistive layer. Using well known structure, we propose a design technique to determine the important design parameter by using a simple equivalent circuit and analytical method. In addition, the method for accurately measuring the reflectivity of the structure is presented, and through measured result the design is verified. The measured result for the proposed absorptive structure, showing a broadband reflectivity response with a fractional bandwidth of approximately 91% below -10 dB, is presented along with other computational results to confirm the function of the structure.

Keywords— Periodic structure, absorptive device, fading, multipath, microwave absorber.

I. INTRODUCTION Generally, it is well known that producing a new absorbing

material possessing the matching frequency characteristic that we want is hard. The reason is that the absorbing materials are manufactured through the complex process of controlling the manufacturing conditions. Usually, these have been done by trial and error method. Therefore, it has been demanded to establish a simple method of realizing the matching characteristic that we desire. From this view point, a planar resonant-type absorbing structure such as the Salisbury screen [1] has been reported. It is one of the simplest absorbers, and it doesn’t lead to the complexity of the manufacturing. In addition, this type of absorber is suitable for wireless security, radar cross-section (RCS) reduction, and enhancing the in-room electromagnetic wave environment within a building such as indoor wireless local area network (LAN) region. By making up for the main disadvantage of Salisbury screen, in which the thickness of the absorbing structure requires at least a quarter-wavelength above the conducting plane, several planar resonant absorbers using a period structure have been reported [2].

In this paper, an absorptive structure based on a periodic structure is presented and its absorption performance is examined. This periodic structure acts as lossy artificial magnetic conductor (AMC) over a finite frequency bandwidth. Using well known unit cell structure, a design technique to determine the important design parameter by using a simple

Figure 1. Cross section of the proposed absorptive structure and unit cell configuration.

equivalent circuit and analytical method is proposed. In addition, the method for accurately measuring the reflectivity of the structure is presented, and through measured result our design is verified. By utilizing the proposed design technique for embodying absorptive structure and the proposed structure, we can apply it to EM wave measurement facility such as a small anechoic chamber for measuring WCDMA mobile phone.

II. DESIGN AND RESULTS The geometry of the proposed absorptive structure and unit

cell are shown in Figure 1. The structure is made up of a thin transparent sheet, upon which resistive unit cells are periodically arranged, a dielectric spacer with a thickness of d, and a conducting back-plane. The square patch is a well-known structure as a unit cell of a fundamental FSS [3], which is under consideration for the analysis of the relation between the sheet resistance of a periodic structure and the performance of the absorber. The absorptive structure can be represented as a lossy AMC comprising a resistive periodic surface over a thin grounded dielectric slab. Considering the normal incidence, the equivalent circuit for the overall absorbing structure is equal to the parallel connection between the parallel LC resonant circuit (ZS = jX1) and the series RLC circuit (ZF = R+jX2). From equation (1), logical values of R and the input impedance satisfying the resonance condition can be computed. Also, all the lumped elements can be calculated analytically from

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1 2 3 4-200

0

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R Re(Z

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Figure 2. The lumped resistance and impedance of the absorptive structure computed by the resonance condition.

1 2 3 4-40

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full wave sim. equiv. circuit measured

Figure 3. Calculated and measured results for the proposed absorptive structure.

2 21 1 2 1 2

2 2 2 21 2 1 2

{ ( ) }+ ( ) ( )AX R X X X X RZ j

R X X R X X� �

�� � � �

. (1)

equations or computed by a full wave simulation. For WCDMA band application, the unit cell with the size g = 9 mm and w = 45mm and the grounded dielectric slab with the parameters of d =15 mm and �r = 1 were considered. Fig. 2 shows three results computed by the resonance condition, in which zero values mean non-existing solutions. It is clearly seen that the minimum reflection can be generated at the frequency of around 2 GHz in which the corresponding input impedance of the absorber and lumped resistance, R, is approximately 376 � and 135 �, respectively. By a good estimate [4], the corresponding sheet resistance, Rs, can be found to be approximately 94 �/sq. Considering the error of the estimation, it can be found by full wave simulation that the optimum value of Rs is 80 �/sq at 2 GHz, by which the absorptive structure covers WCDMA band.

Figure 3 shows the simulated and the measured results of the fabricated absorptive structure. The absorber sample for the measurement was fabricated by attaching a thin resistive sheet with a sheet resistance of 80 �/sq to a low-loss foam dielectric spacer (�r = 1), the back-plane of which serves as a metal ground. It can be seen that the measured result shows a fractional bandwidth of 91% below -10 dB, which covers WCDMA band, and the result is in fair agreement with the

10 15 20 25 30 35

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Figure 4. The measured transmission result of the absorptive structure in the time domain.

simulated result in terms of the changing trend of the performance including the position of the absorption null. The difference between the measurement and calculation results can be attributed to non-uniform spacing between the sides and the center due to a sag occurring when positioning a low-loss foam dielectric spacer and a thin resistive sheet. Based on the measurement method in [6], the method for accurately measuring the reflectivity of the structure is to perform time-gated measurement. Figure 4 shows the measured transmission result of the absorptive structure in the time domain, in which there are many reflections from several scatters including the absorptive structure. In the figure, time-gated section is marked, which means reflection block resulted from only absorptive structure. After the time-gating and inverse FFT, the final reflectivity of the structure as a function of frequency can be accurately measured in the frequency domain as shown in Figure 3.

III. CONCLUSION An absorptive structure based on a periodic structure has

been presented and its absorption performance has been examined. Also, a design technique to determine the important design parameter by using a simple equivalent circuit and analytical method has been proposed.

REFERENCES [1] Ronald L. Fante and T. McCormack, “Reflection properties of the

Salisbury screen,” IEEE Trans. Antenna Propagat., vol. 36, no. 10, pp. 1443-1454, October, 1988.

[2] A. Tennant and B Chambers, “A Single-Layer Tunable microwave absorber using an active FSS,” IEEE Microwave and Wireless Components Letters, vol. 14, no.1, pp. 46-47, January, 2004.

[3] B. A. Munk, Frequency selective Surfaces: Theory and Design, John Wiley & Sons, 2000.

[4] K. W. Whites and R. Mittra, “An equivalent boundary-condition model for lossy planar periodic structures at low frequencies,” IEEE Trans. Antennas Propag., vol. 44, no. 12, pp. 1617-1628, 1996.

[5] Jong-Hwa Kwon, Sang-Il Kwak, Dong-Uk Sim, and Jong-Gwan Yook, “Partial EBG structure with decap for ultra-wideband suppression of simultaneous switching noise in a high-speed system,” ETRI Journal, vol. 32, no. 2, pp. 265-272, Apr. 2010.

[6] K.A. Jose, Vasundara V. Varadan, and Vijay K. Varadan, “Free-space vs. one-horn interferometer techniques for radar absorber measurements,” Microwave Journal, vol. 14, no. 9, pp. 148-154, Sept. 1998.

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