Mutual Coupling Reduction in Microstrip Patch Antenna Arrays by using Planar EBG Structure in X Band Seyed Mohammad Emamimeybodi * ,HomayoonOraizi ** * Department of Electrical Engineering, Amirkabir University of Technology,s.m.emami@aut.ac.ir ** Department of Electrical Engineering ,Iran University of Science and Technology,h_oraizi@iust.ac.ir Abstract: Periodic structures can reduce mutual coupling by constraining the surface waves. This concept is shown by electromagnetic band gap(EBG) based on selective surface frequency. In this paper we use two patches with spacing less than. Our goal is reducing coupling in microstrip arrays in the X band. The dimensions are selected for the antenna to have a good directivity. Furthermore, the substrate used in this antenna let it to have a good bandwidth. we first design a primary structure, then using the circuit models of planar EBG structures, improve the primary design. Reduction of mutual coupling in this structure is better than 19 db.[1,2] Keywords: Electromagnetic band gap, Circuit model, Mutual coupling, Periodic structure, microstrip antenna ,array 1. Introduction Reducing mutual couplings among antennas are important topics these days,because they have negative effects like pattern deterioration, reduction of gain and bandwidth.[5] Because of all these problems, researches are extended in reducing surface waves and mutual couplings. Basically, mutual coupling is created by free space radiation in array antennas. In such antennas, strong surface waves are stimulated on the E plane in TM mode. However , using high dielectric constant and thickness of substrates results in increasing stimulation of these waves. EBG structures have excellent capabilities in limiting these waves.[3,4].EBG structures mostly used in such applications have vias. Most structures use substrates with high dielectric constants. It is advisable that periodic structures and antennas,be placed on one surface of substrate. Using thick substrates will not allow using feeds with coaxial line. There exist many examples of EBG structures to reduce the mutual coupling, which have some common points. First, patch antenna on substrates with high dielectric constants become smaller. Second, low substrate thickness will cause narrow band antenna, which require a technique other than probe feeding. In addition, placing the Patch antenna and EBG on the same sheet has negative impact on impedance matching because of coupling. Also, materials with high dielectric constant will cause strong surface waves stimulation.[6]. The goal of this paper is reducing the mutual coupling in patch antennas in the X band. Feeding is done by coaxial probe which has high efficiency. EBG patches are completely flat which are superior to the mushrooms one. Initially, a basic antenna with EBG patches isdesigned and then it is improved by circuit model of EBG structure. The remainder of this paper is focused on the effects of size and number of EBG patches on reducing mutual coupling in micro strip arrays.