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Compact Multisize Electromagnetic Bandgap Structures with Wide Stopband K Kamardin, M. K. A. Rahim, N. A. Samsuri and M. A. R Osman Abstract— Electromagnetic Band Gap (EBG) has become one of the most promising technologies to be applied for antenna applications. Research in EBG field has recently received remarkable interest and potentially destined as the future wireless technology. EBG offers the potential for novel and unique capabilities to manipulate electromagnetic waves. In this paper, Multisize EBG structures were investigated. Studies were carried with the goal to explore the possibility of achieving a wide stopband region by employing a multisize arrangement. Investigations were carried for four types of shapes including Circular, Square, Koch-square and Fractal-minkowski. Promising results show that multisize EBG design with equal edge gap offers a wide stopband region with deep attenuation, while sustaining the compact size. Arrangement of the multisize EBG patch is a significant factor in achieving the desired wide stopband as well as the patch’s and via’s size. Index Terms— Band Stop EBG, EBG, EBG for antenna, Multisize EBG, Wide Stopband
—————————— ——————————
1 INTRODUCTION
I
N recent years, Electromagnetic Band Gap (EBG) structures have attracted many microwave circuits and antenna researchers as the new focus area of interest. EBG consists of a plane with metallic patches in periodic arrangement. Figure 1 illustrates the basic mushroom-like EBG structure. The structure contains periodic metal patches that are connected to ground with vias. Such EBG acts as a two-dimensional band-stop filter which can block electromagnetic wave propagation within certain desired frequencies [1-2]. EBG structure can be applied widely in improving microwave applications’ performance since it offers unique properties such as wideband rejection for millimeter and microwave range [3].
Wide rejection bandwidth is typically achieved by increasing the dimension or the number of patches which results bigger physical size. In this study, multisize circular patch EBG structure is proposed to achieve a wide bandstop with high attenuation to maintain a compact structure. EBG structure can be described as a lumped circuit [4] that depicts an electric filter that consists of parallel capacitor and inductor (Figure 1). The patch width and gap represent the capacitance’s characteristic while the inductance effect is associated with the vias’ dimensions. The centre stopband frequency of an EBG structure can be determined from the following formulae in (1) [5]. C indicates the capacitance while L denotes the inductance effect. On the other hand, the bandwidth of the stopband frequency can be deduced from Equation in (2), where is the free space impedance [6].
fc =
1 2π LC 1
BW =
η
L C
(1)
(2)
To date, there is no accurate close form equation that can analytically determine the stopband frequency for a specific EBG structure. The equations presented earlier ———————————————— will serve as a starting point in designing the desired EBG • K. Kamardin is withFaculty of Electrical Engineering, Universiti Teknologi structure. Malaysia, 81310 Skudai, Johor Bahru. Help from Finite Different Time Domain (FDTD) cal• M. K. A. Rahim is withFaculty of Electrical Engineering, Universiti Tekculation is needed to complete the analysis. In this study, nologi Malaysia, 81310 Skudai, Johor Bahru. • N.A.Samsuri and M. A. R. osman are withFaculty of Electrical Engineer- the design will be solved using CST simulation software ing, Universiti Teknologi Malaysia, 81310 Skudai, Johor Bahru. that utilises FDTD simulation. Fig. 1. Mushroom EBG layout and Equivalent Lumped Circuit [5]
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2 SINGLE PERIOD AND MULTISIZE EBG STRUCTURES
This study was carried with the goal to explore the possibility of achieving a wide stopband region by employing a multisize arrangement. It is predicted that an optimum multisize EBG structure design can establish an ultra
Fig. 2. Simulation set-up based on method of suspended transmission line
wide stopband region due to the cascading of individuals’ stopband of different patch size. The investigation starts from single period EBG pattern before extending to multisize arrangement. Arrangement of the multize structures was investigated by comparing structures with equal edge gap between patches; and structures with equal centre spacing between patches. Method of suspended transmission line (MOSTL) was used to determine the S21 value to analyse the stopband region. Transmission line of 3mm width is proposed to be placed on top of a dielectric board (Figure 2). Such line matches well with 50 ohm SMA connector. The material used for the EBG structure is a 1.6mm thick FR4 board with relative permittivity of 4.5 and tangent loss of 0.019. The design and analysis was conducted using CST software that applies Finite Different Time Domain (FDTD). Four types of EBG shapes were used in this study, which are circular, square, koch-square and fractalminkowski shapes.
(a)
(a)
(b) (b)
(c) (c) Fig. 3. Schematic representation of Circular EBG (a) Single Period (b) Multisize EBG with equal edge gap (c) Multisize EBG with equal centre spacing distance
(i)
(ii)
Fig. 4. Schematic representation of square, koch-square, fractalminkowski (i) Single Period (ii) Multisize EBG with equal edge gap
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Figure 3 (a) illustrates the schematic representation of single period Circular EBG structures with 30 elements of 50x50mm dimension. The EBG structure in this study consists of periodic circular patches grounded with vias. The patch are separated with a gap, g. The radius, r of the patch is 3mm and via radius is 0.5mm. Comparison was conducted between single period structure (Figure 3(a)) and multisize structure (Figure 3(b, c)). Arrangement of the multisize structures was also investigated by comparing multisize design with equal edge gap (30 elements, edge gap= 3mm) (Figure 3(b)), with multisize structure with equal centre spacing (25 elements, centre spacing = 10mm) design (Figure 3(c)). Following the previous investigation, the study was extended to explore the square, koch-square and fractalminkowski shape arrangements with similar approach (Figure 4).
(a) Circular
3 RESULTS AND DISCUSSION The EBG structures of single and multisize arrangements have been simulated using method of suspended microstrip line. When a multisize arrangement was proposed, a significant wide bandstop has been established. Comparison between multisize with equal edge gap; versus multisize with equal centre spacing was observed for circular EBG structure, as an initial investigation. From the results (Figure 5), multisize structure with equal edge gap shows a wider stopband region with bandwidth of 5.79GHz (3.05-8.84GHz) compared to the multisize structure with equal centre spacing with bandwidth equals to 3.79GHz (3.04-6.83GHz).
(b) Square
(c) Koch-square
Fig. 5. Simulated transmission coefficient S21 for Multisize Circular EBG with equal edge gap versus equal centre spacing distance
The investigation was then continued to analyse the results between single period and multisize structure with equal edge gap arrangement. Figure 6 shows S21 results for single and multisize EBG structures for four different shapes which include circular, square, kochsquare and fractal-minkowski EBG structures. Referring to Figure 6 (a), multisize arrangement for circular patch EBG, shows a huge improvement on the stopband region with bandwidth of 5.79GHz (3.05– 8.84GHz), compared with the single period of bandwidth equals to 2.67GHz (3.38-6.05GHz). A very deep attenuation can also be observed.
(d) Fractal-minkowski Fig. 6. S21 for single and multisize EBG with equal edge gap for four different shapes
Multisize arrangement of square shapes improvised the stopband bandwidth from 2.8-8.03 GHz, a 55.8% improvement from the single arrangement as shown in Figure 6 (b).
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TABLE 1 BANDGAP FREQUENCIES AND BANDWIDTH BETWEEN SINGLE PERIOD AND MULTISIZE ARRANGEMENT FOR CIRCULAR, SQUARE, KOCH-SQUARE AND FRACTAL-MINKOWSKI EBG STRUCTURES Single Period
Type of EBG structure
Fig. 7. Fabricated Multisize Circular Patch EBG
Multisize Circular Patch EBG Simulation vs Measurement
Bandgap frequencies (GHz)
Circular
3.38 – 6.05
Square
3.11 - 5.33
Kochsquare
3.59 – 6.47
Fractalminkowski
3.39 – 5.53 9.67 – 10.33
Multisize
Band width (%)
59 54.5 59.8
49.4
Bandwidth Enhance -ment (%)
Bandgap frequencies (GHz)
Bandwidth (%)
3.05 – 8.84 2.8 - 8.03
111.5
52.5
110.3 119.1
55.8 59.3
117.5
68.1
3.36– 10.4 3.01– 9.19 17.14 – 19.25
0
S21 (dB)
-20
-40
-60
-80
Measurement Simulation
-100 2
4
6
8
10
Frequency (GHz)
Fig. 8. Measurement versus Simulation of S21 Multisize Circular Patch EBG
Similar trend was also obtained for koch-square ebg structure with stopband region from 3.36-10.4GHz for multisize arrangement; an improvement from 3.596.47GHz of the single period design (Figure 6 (c)). As for fractal-minkowski EBG, the single period arrangement exhibits a dual narrow stopbands at 3.39 – 5.53 GHz and 9.67 – 10.33 GHz (Figure 6 (d)). As expected, due to the nature of fractal-minkowski shape, multibands are exhibited by fractal structure. When a multisize arrangement was deployed, dual bands were obtained with an ultra wide first stopband at 3.01 – 9.19 GHz and the second band from 17.14 – 19.25 GHz. In parallel with multibands characteristics of fractal shape, wider stopbands region was also observed with multisize arrangement as opposed to the typical narrowbands. Measurement was then conducted to validate the simulation results (Figure 7). Figure 8 show the comparison between measurement and simulation of the Multisize Circular with Equal Edge gap EBG arrangement. From the graph, similar trend between measurement and simulation can be seen. Measurement results shows a bandstop region from 3.4-12.83 GHz while 3.1-6.04GHz for simulation. The frequency shift is predicted due to the dielectric loss of the FR4 material used as well as the inconsistency of dielectric constant value of such material for different frequencies. Despite the small shift, the wide stopband
trend is obtained hence validating the simulations results. Table 1 summarizes the comparison between single period and multisize structure for all circular, square, kochsquare and fractal-minkowski shapes. Promising simulation and measurement results show that multisize EBG design with equal edge gap offers a wide stopband region with deep attenuation, while sustaining the compact size. The bandwidths have been improved to more than 50% when multisize arrangements were deployed for all shapes.
4 CONCLUSION In this study, the possibility of multisize EBG structure to achieve a wide stopband region was investigated for four types of shapes i.e. circular, square, koch-square and fractal-minkowski. Promising results show that multisize design with equal edge gap EBG structure exhibits a stopband with high attenuation, while maintaining a compact physical size. The patch parameters including the width or radius; and the gap between the elements plays important roles as they denotes the L and C values that determine the operating frequencies of the structures. Arrangement of the EBG patch is also an important factor to obtain a broad stopband region in the multisize EBG structure. It is predicted that an optimum multisize EBG structure design can establish an ultra wide stopband region due to the cascading of individuals’ stopband of different patch size. Such design can be utilised as a bandstop filter that can be integrated in many applications such as compact high performance antennas. In addition, the bandstop property can also be applied as a band rejecter in antenna system. This new application of EBG is appealing in antenna designs especially as a tunable band rejecter.
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ACKNOWLEDGMENT The authors wish to thank the Ministry of Science Technology and Innovation (MOSTI) and Ministry of Higher Education (MOHE) Malaysia for supporting the research work.
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K. Kamardin received her B.Eng. in Electronic (Communications) from University of Sheffield, United Kingdom in 2004 and obtained her MSc from Universiti Teknologi Mara (UiTM) in 2007. She had previously served as a Senior Assistant Researcher in TM Research & Development for three years. Currently, she is pursuing her PhD in Electrical Engineering at the Faculty of Electrical Engineering, Universiti Teknologi Malaysia. Her research interests are in the area of Antenna & Propagation and Metamaterials Enginering. She is focusing in the field of Electromagnetic Band Gap Structure for Antenna Engineering. M. K. A. Rahim received his B.Eng. in Electrical and Electronics from University of Strathclyde U.K. in 1987, Master in Electrical Communication Engineering from University of New South Wales Australia in 1992 and PhD in Electrical Engineering from University of Brimingham U.K. in 2003. Currently, he is an Associate Professor at the Faculty of Electrical Engineering, Universiti Teknologi Malaysia, Skudai, 81310, Johor, Malaysia. His current research interests are in the area of Planar and Dielectric antenna, Active Antenna, Array antenna, Electromagnetic bad gap, (EBG), Left handed metamaterial (LHM), Frequency Selective Surface (FSS) Artificial magnetic conductor (AMC). N. A. Samsuri received her B.Eng. in Electrical Engineering (Telecommunication) for Universiti Teknologi Malaysia in 2001. She obtained her MSc and PhD from Loughborough Univeristy, United Kindgom in 2004 and 2009 respectively. She is currently serve as a lecturer in Faculty of Electrical Engineering, Universiti Teknologi Malaysia. M. A. R. Osman received her B.Sc. in Electronic Engineering from Sudan University of Science and Technology, Khartoum, Sudan in 2002 and MEng in Electrical (Electronics & Telecommunication) from Universiti Teknologi Malaysia in 2007. Currently, she is pursuing her PhD in Electrical Engineering at the Faculty of Electrical Engineering, Universiti Teknologi Malaysia. Her current research interest is in the area of Antennas and Propagations Systems. More focus is on the antenna designs and performances for wearable applications.
