Isolation Improvement of Dual Feed Patch Antenna by Assimilating Metasurface Ground M. Habib Ullah 1, M. R. Ahsan 2, W. N. L. Mahadi 1, T. A. Latef 1, M. J. Uddin 3 1 Department of Electrical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, MALAYSIA; 2 Department of Electrical Electronic and System Engineering, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, MALAYSIA. 3 Networks Communications Engineering, Faculty of Science and Engineering, Queensland University of Technology, Queensland, AUSTRALIA Corresponding author: mhullah@yahoo.com ABSTRACT: A new octagonal ring shape metasurface structure (MSS) has been design and implemented at the back of a dual feed planar printed Ultra wide band (UWB) array antenna. The proposed MSS incorporated antenna has been design and fabricated on a high permittivity (Ɛr=15) ceramic filled biopolymer based sandwich structured dielectric substrate. A significant improvement of isolation (S21) between dual port array elements of the proposed antenna has been achieved by assimilating 6 9 octagonal ring shape metasurface structure with near zero refractive index in the ground plane. Measurement result shows that the proposed antenna with MSS loading has been achieved the reflection co-efficient (S11<-10 db) bandwidths of 1.3 GHz, 1.02 GHz, 0.75 GHz and 1.8 GHz. Stable radiation patterns with the gains of 3.42 dbi, 4.14 dbi, 5.46 dbi and 8.28 dbi have been measured at the resonant frequencies 3.1 GHz, 4.41 GHz, 6.2 GHz and 8.75 GHz. Keywords: Octagonal ring, metasurface structure, isolation improvement, dual feed, ultra wideband, microstrip patch antenna.
1. INTRODUCTION With the rapid development of wireless communication, the high performance modern communication systems with low cost and high data rate have been becoming a crucial requirement. The UWB technology is one of the potential candidates in the race of wireless communication system since the Federal Communications Commission (FCC) approved the commercial use of the bandwidth from 3.1 GHz to 10.6 GHz [1]. In contrast the UWB systems, some Narrow Band (NB) wireless systems have been licensed and used for a long time. Hence, it is necessary to design UWB antenna with notch band characteristic to mitigate the potential interference between NB and UWB systems. Recently, numbers of UWB antennas with notch bands are offered to reduce the potential interference [2] [4]. Dual/multiple feed microstrip patch antenna is one of the most cost effective solutions for designing compact multifunctional wireless communication systems [5]. For improving overall system performance, the isolation between the antenna elements and other system components needs to maintain properly to the highest value [6]. The low isolation may be introduced by the strong mutual coupling arises from surface waves, space waves and near field overlapping of the array elements. Typically, the mutual coupling can be reduced by increasing the distance between the antenna elements. However, this is impractical since it will increase the overall dimension and compactness cannot be ensured [7]. A possible approach to improve the isolation or to reduce the mutual coupling by using the techniques, such as slots and stubs in radiating element, cavity back, corrugations, split ring resonators, high impedance electromagnetic surfaces or electromagnetic band gap structures [7] [10]. The integration of periodic structures as metasurface is one of the effective methods to reduce the mutual coupling effects without increasing the antenna size. The periodic structures are considered systematic repetitive distribution of dielectric or conducting elements that can regulate the propagation of electromagnetic fields and waves at a particular frequency range. The metasurface structures (MSS) have been used for performance enhancement of antenna by several antenna researchers [6], [7]. By making the use of attractive features like simple planar structure, ease integration with antenna element and expedient property of zero refractive index, the MSS has become a competitive solution for isolation improvement for the antennas. A new octagonal ring shape 6 9 metasurface structure has been proposed to assimilate with the dual feed printed array antenna for isolation enhancement. The proposed antenna and the MSS have been designed and fabricated on a low loss (tanδ=0.0023) high permittivity (εr = 15) ceramic filled biopolymer sandwich structured dielectric substrate [8]. A significant improvement of isolation has been realized from measured performance results of the proposed MSS assimilated antenna. Measured maximum gains of 3.42 dbi, 4.14 dbi, 5.46 dbi and 8.28 dbi have achieved at four operating frequency bands with the resonant modes at 3.1, 4.41, 6.2 and 8.75 GHz respectively. 2. DESIGN OF METASURFACE AND ANTENNA STRUCTURE The design process of the antenna starts with radiating patch fed by 1 mm thick microstrip line at the bottom and connected with standard 50Ω SMA connector. Three closed rings which are shorted together for using as array element for the designed radiating patch structure. Two sets of 3 1 array elements placed at the top and bottom with two different excitation ports through microstrip line with 23.5 mm distant between them from center to center. Figure 1 shows design schematic of the (a) top radiating patch and (b) bottom MSS structure of the proposed antenna. The proposed MSS assimilated antenna of dimension 0.30λ 0.44λ 0.019λ (with respect to lower resonant frequency) has been designed and fabricated on a copper laminated 2 mm thick ceramic filled biopolymer sandwich structured high permittivity (εr = 15) dielectric substrate. High permittivity dielectric material has been used as substrate
in order to maintain the compactness of the antenna. Design approach of the radiating patch starts with a single element. Afterwards, two-element arrangement has also been analyzed and finally two sets of three element structure has been achieved the desired resonant frequencies. Furthermore, it is well known fact that array configuration offers higher gain [9] and multiband characteristics. Thus, two sets of 3 1 array element positioned in orthogonal direction to attain the desired properties of the antenna. A 28 42.5 mm 2 rectangular slot cut out from 32 46.5 mm 2 copper layer to form the rectangular ring ground plane. It is known that back radiation of the antenna increase due to defected ground plane and it has been dealt with the insertion of MSS. The proposed MSS unit cell constructed by cut out a circular slot from a 3 3 mm 2 octagon. The design layout of the MSS structure and unit cell of 0.02λ 0.02λ for the proposed MSS is shown in Figure 1 (b). The final optimized design of the MSS loaded dual port microstrip patch antenna is fabricated and presented in Figure 1 (c). Figure 1. Geometrical configuration of the proposed metasurface loaded dual feed antenna (a) Patch antenna (top) (b) Metasurface structure (bottom) and (c) Photo of the fabricated antenna The proposed 6 9 MSS has been inserted at the ground plane with a center to center gap of 4 mm is maintained around the unit cells. The outer cells are 6.5 mm and 7 mm away from the horizontal and vertical edge (wherever applicable) respectively. The size, shape and gap between cell matrix are well optimized to achieve the desired effective properties for the antenna. The proposed uniformly distributed periodic structures can be recognized as metasurface by identifying the effective parameters such as permittivity (Ɛeff), permeability (μeff), impedance (zeff) and refractive index (neff). The effective parameters of the proposed MSS element have been calculated and analyzed by using widely used effective parameter retrieval technique [10], [11]. The proposed design of MSS has found to hold the refractive index value near to zero for the functional bands. The refractive index approaches to zero since it is proportional to the square root of the product of Ɛeff and μeff values. The effect of the MSS configuration over the transmission coefficients/isolation characteristics (S21) are shown in Figure 2. It can be observed that lower S21 value at the operating bands have achieved from 6 9 MSS configuration. For the numerical analysis, the Ansys High Frequency Structural Simulator (HFSS), a 3D full-wave finite element method (FEM) method based electromagnetic software is employed [12].
Figure 2: Different MSS matrix and their effects on transmission co-efficient (S21) of the proposed antenna 3. RESULT ANALYSIS Performance characteristics of the proposed antenna prototype have been measured in a standard anechoic measurement chamber. Reflection co-efficient (S11) of the proposed antenna with and without MSS is shown in Figure 3. The measured operating bandwidths for S11<-10 db of the proposed antenna are 1.3 GHz (2.21 to 3.51 GHz), 1.02 GHz (4.02 to 5 GHz), 0.75 GHz (5.87 to 6.62 GHz) and 1.8 GHz (7.67 to 9.47 GHz) at the center frequencies of 3.1 GHz, 4.41 GHz, 6.2 GHz and 8.75 GHz respectively. The simulated and measured transmission co-efficient (S21) of the proposed antenna with and without MSS is presented in Figure 4. A significant reduction of mutual coupling between two port array antenna has been achieved by embedding the octagonal ring shape MSS in the ground plane. Figure 4: Transmission co-efficient (S21) of the proposed antenna with and without MSS. Measured radiation patterns of the proposed MSS loaded antenna at four resonant frequencies 3.1 GHz, 4.41 GHz, 6.2 GHz and 8.75 GHz have been presented in Figure 5. From the measured radiation pattern it can be observed that 3 db beamwidths are of 150 o (278-0-68) o, 65 o (328-0-33) o, 96 o (314-0- 50) o and 62 o (329-0-31) o for the proposed antenna system. Furthermore, lower cross polarization effects have also been observed compare to co-polarization of the proposed antenna as desired. Figure 5 Measured radiation patterns of the proposed antenna at a. 3.1 GHz b. 4.41 GHz c. 6.2 GHz and d. 8.75 GHz Figure 3. Reflection co-efficient (S11) of the proposed antenna with and without MSS. Figure 6 plots the simulated and measured gain of the proposed MSS assimilated dual feed antenna. Three-antenna measurement system has been used to measure the gain of
the antenna where two identical horn antennas as employed as reference [13]. Maximum gains of 3.42 dbi, 4.14 dbi, 5.46 dbi and 8.28 dbi have been measured from the proposed antenna correspondingly for four different operating bands. Figure 6. Comparison between the measured and simulated antenna gain plotted against the frequency 4. CONCLUSION A new 6 9 octagonal ring metasurface structure embedded dual port printed array antenna has been presented in this paper. A significant improvement of isolation (S21) between antenna elements has been perceived by incorporating the proposed metasurface structure with near zero refractive index in the ground plane. A good agreement between simulation and measurement results in entire operating bands has been realized. Symmetrical and steady radiation patterns with low cross polarization effect have also been achieved in four resonant frequencies. An optimized MSS has been configured by analyzing different sets of MSS elements in order to achieve better S21 of the proposed antenna. Furthermore, effective parameters and surface current distribution of the proposed MSS loaded antenna have also been analyzed. ACKNOWLEDGEMENT The authors would like to acknowledge and express sincere appreciation to the University of Malaya for assisting this study through the project FP012-2014A.
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