ADVANCES in NATURAL and APPLIED SCIENCES ISSN: 1995-0772 Published BYAENSI Publication EISSN: 1998-1090 http://www.aensiweb.com/anas 2017 May 11(7):pages 104-110 Open Access Journal Surface Wave Bandgap Analysis of Modified Mushroom like EBG Structure 1 D.Helena Margaret, 2 B.Manimegalai, 3 V.Thilaga 1,3 Alagappa Chettiar College of Engineering and Technology,Karaikudi, Tamil Nadu, India. 2 Thiagarajar College of Engineering, Madurai, Tamil Nadu, India. Received 28 February 2017; Accepted 29 April 2017; Available online 2 May 2017 Address For Correspondence: D. Helena Margaret,Department of Electronics and Communication Engineering Copyright 2017 by authors and American-Eurasian Network for Scientific Information (AENSI Publication). This work is licensed under the Creative Commons Attribution International License (CC BY). http://creativecommons.org/licenses/by/4.0/ ABSTRACT The Mushroom like Electromagnetic Band Gap (EBG)s can be integrated into antenna to minimize surface wave coupling effects. This increases radiation efficiency of the antenna due to its compact size and low loss. This paper presents the band gap analysis of a modified mushroom like electromagnetic band gap structure with bandgap frequency from 4-6GHz. The dispersion diagrams are extracted using eigen mode solver in Ansoft HFSS and the simulation results verify its characteristics of suppressing surface wave. The parametric analysis is done on surface wave band gap by varying the EBG parameters. KEYWORDS: Band gap, dispersion diagram, Electromagnetic Band Gap, surface wave suppression INTRODUCTION In broadband antenna application, the antenna cavity is generally loaded with absorbers to eradicate the backward radiation, but in doing so the radiation efficiency of the antenna is decreased. More over most of the energy radiated by an antenna is formed into surface waves and leads to distorted radiation pattern hence very poor front to back ratio [1]. Accordingly to get better performance of antenna in broadband application, the Electromagnetic band Gap structures also called as artificial magnetic conductors or high impedance surfaces are widely used. To attain that so many EBG structures are investigated. Among that the mushroom like Electromagnetic Band Gap structure is widely used. This has two distinct characteristics. They are surface wave suppression and in-phase reflection. Suppressing the surface wave in an antenna provide good antenna performance such as gain of the antenna, reducing mutual coupling and radiation in backside [2].One of the applications in Mushroom like Electromagnetic Band Gap structure is the elimination of scan blindness in phased array antennas [3]. Scanblindness is caused by interference between the Floquet modes of the array and surface wave modes of the same propagation constant. This limits the useful scan range of the phased array and results in a lower efficiency. This scan blindness can be eliminated by incorporating mushroom like EBG in the array and hence suppressing the surface waves. In another way, the surface wave band gap characteristics of the mushroom like electromagnetic band gap structure is used to implement a rejection filter in an Ultra Wide band (UWB) monopole antenna [4]. Interference from undesired frequencies can be rejected by designing the surface wave band gap frequency to lie within the appropriate frequency range [5].The various types of electromagnetic bandgap structure are available for the antenna applications. In this paper we propose a modified mushroom like Electromagnetic Bandgap structure to enhance the antenna radiation by suppressing the surface wave. The dispersion characteristic of the modified mushroom ToCite ThisArticle: D. Helena Margaret, B. Manimegalai, V. Thilaga., Surface Wave Bandgap Analysis of Modified Mushroom like EBG Structure. Advances in Natural and Applied Sciences. 11(7);Pages: 104-110
105 D. Helena Margaret et al., 2017/Advances in Natural and Applied Sciences. 11(7) May2017, Pages: 104-110 Electromagnetic Band Gap structure is studied using the eigen mode solver. This EBG has the surface wave band gap frequency of 4-6 GHz. The parametric analysis on bandgap is done by varying parameters of EBG structure. Configuration Of Modified Mushroom Ebg Structure: The mushroom EBG consists of a top and bottom patch in a dielectric substrate connected through a metallic via. This via acts as the inductance and the gap between adjacent top patches acts as capacitance. Hence the mushroom EBG can be modeled as an LC circuit with inductance and capacitance given by the following equations: L = μ 0 μ r h (1) C = wε 0 (1+ε r ) 1 (2w+g) cosh (2) π g The mushroom EBG is modified by adding four slots in the top patch. The modified mushroom like EBG Fig. 1: Configuration of Modified Mushroom EBG structure is shown in Fig.1.The modified mushroom EBG structure has 0.8mm thickness of substrate with the relative permittivity of 4.4 (FR4). The unit cell parameters are: patch w=7.5 X 7.5 mm, gap between the patches, g=0.5mm, radius of via, r= 0.2mm, slot length a=2 mm, slot width b=0.5 mm. Bandgap Characterization: The Eigen mode solver is used to determine natural resonances and dispersion properties of the EBG unit cell. Simulations are carried out using Ansoft High Frequency Structure Simulator (HFSS) software. It is highly time consuming compared to other methods but gives accurate results. The values of width, radius of via, gap g of the EBG structure are varied to obtain the optimized result. The size reduction will increase its structural fitness for compact devices and the possibility to provide more EBG cells in a limited area. Since an open structure is considered, the air box located above the structure must be appropriately terminated. Fig. 2:(a)Boundary conditions on EBG
106 D. Helena Margaret et al., 2017/Advances in Natural and Applied Sciences. 11(7) May2017, Pages: 104-110 Fig. 2:(b)EBG with appropriate boundary conditions For the Eigen mode analysis, the air box is terminated with a Perfectly Matched Layer (PML) which absorbs all the incoming radiation and prevents reflection. In addition, linked Periodic Boundary Conditions (PBCs) are applied in both the X and Y directions. Propagation along the X direction is considered for the Eigen mode analysis due to the symmetry of the structure. The phase shift transverse to the direction of the propagation is kept fixed while the phase shift along the direction of propagation is varied. The resulting bandgap is due to the contra-directional coupling of two modes. The edges (lower and upper edge) of the bandgap are determined by the leaky modes to cross the light line. To obtain the dispersion curve, the unit cell of modified mushroomebg is modeled by enclosed air box with height of 8 times larger than the substrate thickness and top PML boundary set up. The unit cell with the above set up is shown in Fig. 2(a) and Fig 2(b). The two dimensional dispersion diagrams are obtained for modified mushroom EBG structure. The dispersive curves for first two neighboring modes are found to predict the bandgap. The dispersion diagram is shown in the Fig.3.For this structure, the lower edge of the bandgap frequency occurs at 4.2 GHz and the upper edge occurs at 5.9 GHz with the bandgap bandwidth of 1.7 GHz. Fig.3:Dispersion diagram with band gap Parametric Analysis on Bandgap: The parametric analysis on the Bandgap is carried out by varying the parameters of modified mushroom EBG structure. The parameters considered are: Patch size (w) Radius of via (r) Gap between the patch and substrate (g/2) Slot width(b)
107 D. Helena Margaret et al., 2017/Advances in Natural and Applied Sciences. 11(7) May2017, Pages: 104-110 a) Effect of patch size on bandgap: The via radius, gap between patch and substrate, slot width are fixed and the patch width is varied. Here values of via radius r=0.2mm, gap g/2=0.25 mm and slot width b=0.5 mm. The patch width is directly proportional to the capacitance and the capacitance is inversely proportional to the bandwidth of EBG structure. It is observed that, as the patch size increases the frequency decreases. The decrease in lower and upper frequency ofbandgap with respect to increase in patch size is shown in the Fig.4. The bandwidth of the bandgap for variations in patch size is shown in Fig.5. As discussed above, bandwidth is decreased. b) Effect of via radius on bandgap: In this case, the patch size, gap between patch and substrate, slot width are kept constant and via radius is varied. The value of patch width w=7.5 mm, slot width b=0.5 mm and gap g/2=0.75 mm. The via radius is related to the inductance of the structure. From the analysis, it is shown that the frequency increases with increase in via radius. The variation of increasing in lower and upper frequency of bandgap with respect to increase in radius of via is shown in the Fig.6. The bandwidth of the bandgap for variations in via radius is shown in Fig.7. Fig.4:Patch Size Vs Bandgap frequency Fig.5:Patch Size Vs Bandwidth
108 D. Helena Margaret et al., 2017/Advances in Natural and Applied Sciences. 11(7) May2017, Pages: 104-110 Fig.6: Radius of via Vs Bandgap Frequency Fig.7: Radius of via Vs Bandgap c) Effect of gap(g/2) on bandgap: The via radius and patch width remain constant and the gap value is changed. The values are via radius r=0.2 mm and patch width=7.5 mm. The gap is directly proportional to the capacitance of the EBG structure and the capacitance is inversely proportional to the bandwidth of the bandgap. Here, it is observed that as the capacitance increases the bandwidth of the bandgap decreases and hence verified. Variation of decrease in lower and upper frequency of bandgap for different values of gap (g/2) is shown in the Fig.8 and the bandwidth of the bandgap with respect to variations in gap is shown in Fig.9. Fig.8:Gap(g/2) Vs bandgap Frequency Fig.9: Gap(g/2) Vs bandwidth
109 D. Helena Margaret et al., 2017/Advances in Natural and Applied Sciences. 11(7) May2017, Pages: 104-110 d) Effect of slot width(b) on bandgap: By changing the value of slot width in step size, it shows that the bandgap frequency is decreased due to the increasing of capacitance in the EBG equivalent LC circuit. The variation in the upper and lower cut off frequency and bandwidth of bandgap frequency is shown in the Fig 10 and 11 respectively. In this the values of EBG parameters are taken as patch size w=7.5 mm, gap (g/2) = 0.25 mm and radius of via r=0.2 mm. Fig. 10: slot width Vs bandgap Frequency Fig. 11: slot width Vs bandwidth Conclusion: A modified mushroom like EBG structure is proposed for WLAN antenna application with the band gap frequency from 4 to 6 GHz. The surface wave bandgap of EBG is analyzed by varying the EBG parameters patch size, periodicity and radius of via. The continuation of this work may be to optimize the proposed modified mushroom EBG parameters. REFERENCES 1. Yang,F.and Y.Rahmat-Samii, 2003. Microstrip antenna integrated with Electromagnetic Band Gap Structures: a low mutual coupling design for array application, IEEE Trans. Antennas and Propagate, 51(2): 2936-46. 2. 2007. Study of two bands characteristics of Mushroom like EBG structure by Long li, Qiang chen, Qiaowei yuan, Kunio Sawaya, changhong Ling ISAP. 3. 2012. Characterization of the Reflection And Dispersion Properties of `Mushroom'-Related Structures and Their Application to Antennas by shahzad Raza. 4. Lin Peng, Cheng-li Ruan, and Jiang Xiong, 2012. Compact EBG for Multi-Band Applications, IEEE Trans. Antennas and Propagation, 60: 9. 5. Farzad Mohajeri, Zeinab Danesh, 2013. Reduction of Mutual Coupling and Gain Improvement Using Step Electromagnetic Band-Gap (EBG) Structure, International Journal of Advanced Research in Electrical, Electronics and Instrumentation Engineering, 2: 9.
110 D. Helena Margaret et al., 2017/Advances in Natural and Applied Sciences. 11(7) May2017, Pages: 104-110 6. Cheng-Chi Yu, Meng-Hsiang Haung, Yao-Tien Chang, Luen-Kang Linand Tsung-Han Weng, 2009. A Novel Electromagnetic Bandgap (EBG) Structure for Electromagnetic Compatibility (EMC) Application, Electromagnetics Research Symposium. 7. Sievenpiper, D., L. Zhang, R.F.J Broas, N.G. Alexopolus and E.Yablonovich, 1999. "High-impedance electromagnetic surface with a forbidden frequency band," IEEE Trans. Microwave Theory Tech., 47: 2059-2074. 8. Yang, L., M.Y. Fan, F.L. Chen, J.Z. She and Z.H. Feng, 2005. A novel compact Electromagnetic Bandgap structure and its applications for microwave circuits," IEEE Trans. Microwave Theory Tech., 53(1): 183-190. 9. Martin Coulombe, Sadegh Farzaneh Koodiani, and Christophe Caloz, 2010."Compact Elongated Mushroom (EM) EBG structure for enhancement of patch Antenna Array, IEEE transactions on antennas and propagation, 58: 4. 10. Abhari,R.and G.V. Eleftheriades, 2003. Metallo-dielectric electromagnetic bandgap structures for suppression and isolation of the parallel-plate noise in high-speed circuits, IEEE Trans. Microw. Theory Tech., 51(6): 1629-1639.