National Conference on Communication Technology Interventions for Rural and Social Development (NCCTIRD – 2012), February 16 & 17, 2012 34 ISBN 978-1-4675-1441-5 © 2012 Published by Bonfring Abstract --- An antenna system with reduced electromagnetic coupling exploiting the uniplanar compact photonic band gap (UC-PBG) structure to achieve higher isolation is presented in this paper to enhance the performance of MIMO antennas. Two closely-spaced monopole antennas operating in the frequency range 5-6 GHz is considered. It is shown that the use of UC-PBG is better than the existing methods in reducing the electromagnetic coupling. Other parameters of MIMO antennas like correlation co-efficient and diversity gain are also calculated. Keywords--- Electromagnetic Coupling, MIMO Antennas, UC-PBG I. INTRODUCTION HE use of multiple antennas has found various applications in the area of wireless communications. One such application has recently become very popular is the multiple-input multiple-output (MIMO) antenna system. These MIMO antennas have become center of research interest nowadays, because of rapidly increasing demand for high capacity communication services. The enormous capacity offered by MIMO systems is not realizable when the parallel channels are highly correlated [1]. In planar antennas surface waves that occur in the substrate layer reduces the total available power for radiation to space wave [2]. When multiple antennas are used in the system, like arrays, surface waves have a significant impact on the mutual coupling between array elements [3]. The inter-element spacing should be at least greater than 0.5λ0 where λ0 is the free space wavelength, at the center frequency, for an isolation of at least -20 dB. As with compact applications the separation should be as small as possible. Dr.B. Manimegalai, Assitant Professor, Department of Electronics and Communication Engineering, Thiagarajar College of Engineering, Madurai. D. Helena Margaret, Assistant Professor, Department of Electronics and Communication Engineering, Velammal College of Engineering & Technology, Madurai. E-mail: helenaraj@yahoo.com S.S. Keerthika, UG students, Department of Electronics and Communication Engineering, Velammal College of Engineering & Technology, Madurai. M.R. Subasree, UG students, Department of Electronics and Communication Engineering, Velammal College of Engineering & Technology, Madurai.E-mail: silverstar.suba@gmail.com S. Susithra, UG students, Department of Electronics and Communication Engineering, Velammal College of Engineering & Technology, Madurai. Various methods for achieving low mutual coupling between closely-packed antenna elements have been well studied. Ground plane structure with slits [4], wall with PBG structures loaded on it and slit ground plane [5], superstrates [6], de- correlation circuits[7], bridge line[8], parasitic structures[9] and split ring resonators [10] for reducing coupling between two closely packed antennas. Recently PBG structures are being used in antenna designs for reducing mutual coupling [11], [12]. The PBG structures are periodic structures that have the property to suppress the surface wave propagating in the substrate material [13]. Various 1D and 2D PBG structures have been used extensively in the last decade. This paper describes the use of Uniplanar Compact Photonic Band gap (UC-PBG) structures for the reduction of electromagnetic coupling between the antenna elements used in a MIMO system. Two closely spaced monopole antennas with UC-PBG ground plane operating in the frequency range 5- 6GHz is considered in the design. The greatly reduces the mutual coupling of signals between the antenna elements thus facilitating compact structures with closely spaced antenna elements. The performance and the mutual coupling are compared with and without the UC-PBG structures with the Method of Moment simulations. II. DESIGN OF UC-PBG AND BAND GAP CHARACTERIZATION The mushroom EBG was proposed by Sievenpiper et al. [14] to improve the performance of antennas by reducing the surface waves. They have periodic metal patches connected to the ground through vias. The concept of uniplanar PBG was then proposed in [15]. An important feature in the uniplanar EBG design is the removal of vertical vias which simplifies the design and fabrication process to a greater extent. Similar to the mushroom-like EBG surface, the operational mechanism of the uniplanar EBG surface can be explained by the lumped LC model. Instead of using vertical vias to provide an inductance L, a thin microstrip line on the same layer of the patches is used to connect them together. The capacitance C comes from the edge coupling between adjacent patches. Thus, this structure is named as a “uni-planar” PBG. Electromagnetic Coupling Reduction between Two Closely Spaced Planar Monopole Antennas for MIMO Applications using UC-PBG Structures Dr.B. Manimegalai, D. Helena Margaret, S.S. Keerthika, M.R. Subasree and S. Susithra T