IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 11, 2012 1501 Mutual Coupling Reduction Between Planar Antennas by Using a Simple Microstrip U-Section Saeed Farsi, Student Member, IEEE, Hadi Aliakbarian, Member, IEEE, Dominique Schreurs, Fellow, IEEE, Bart Nauwelaers, Senior Member, IEEE, and Guy A. E. Vandenbosch, Senior Member, IEEE Abstract—Mutual coupling is an inevitable phenomenon in mul- tiantenna systems, usually reducing the system performance. Nu- merous works have focused on the reduction of this effect. The aim is maintaining the mutual coupling suppressing structure as simple as possible while having a high amount of mutual coupling reduc- tion. This letter presents a novel structure suppressing the mutual coupling between nearby patches. It is composed of only a simple U-shaped microstrip, which reduces the mutual coupling consider- ably. The structure has been constructed and tested. The measure- ment results prove the high efficiency of this configuration. Index Terms—Antenna arrays, mutual coupling reduction, par- asitic coupling, parasitic element. I. INTRODUCTION M UTUAL coupling (MC) has a direct impact on the per- formance of multiple-input–multiple-output (MIMO) and antenna array systems. This interaction between elements degrades the system performance in two ways, namely by increasing and by distorting the radiation pattern. This problem mainly arises when the antennas are close to each other. Dealing with the problem of MC has been a topic of interest since the early days of antenna array design. Several authors have proposed different methods to reduce the MC while keeping the antenna elements close to each other. Some of the most referred methods in literature for printed antennas are using defected ground structures (DGS) [1]–[3], electromag- netic band-gap (EBG) [4], [5] structures, and parasitic elements between the antennas [6]. Insertion of a slot was suggested in [1], with considerable MC reduction. Although it has the advantage of simplicity, it considerably changes the radiation pattern, especially at the rear side. In [2], a ground plane pattern was proposed that efficiently decreases the MC. However, this also significantly distorts the radiation pattern. Other DGS and EBG structures are used [3]–[5] to suppress the surface waves, but they are complex structures, and optimum designs are more difficult to achieve. A simple structure composed of two parasitic elements is presented in [6]. The difficulty with that scheme is that it needs an additional layer, along with Manuscript received September 28, 2012; accepted October 22, 2012. Date of publication December 19, 2012; date of current version January 03, 2013. This work was supported in part by FWO-Vlaanderen, KU Leuven within the GOA projects framework, and the Department of Economy, Science and Innovation (EWI) of the Flemish Government within the IS-HS II Project. The authors are with the Electronic Engineering Department, KU Leuven, Leuven 3001, Belgium (e-mail: saeed.farsi@esat.kuleuven.be). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LAWP.2012.2232274 TABLE I COMPARISON BETWEEN TECHNIQUES metallized holes for grounding. The radiation pattern is also changed. In [7], the antenna elements are physically linked via a narrow line. This technique also causes high MC reduction, but suffers from radiation pattern degradation, mainly due to the cross-polar component related to the current flowing over the long linking line. In general, even nowadays, establishing an efficient mutual coupling reduction technique while con- serving the radiation pattern is still highly challenging. Table I summarizes a qualitative comparison between some common techniques presented in the literature along with the method proposed in this letter. Note that for [6] in this table, although the used parasitic shapes are simple, the method has been denoted as nonsimple because it needs an additional layer and some via-holes. In this letter, a simple but highly effective structure is pre- sented to reduce MC between antenna elements. The structure is a U-shaped microstrip line section inserted between the cou- pled elements. The coupling reduction bandwidth fully covers the operating bandwidth of the antennas, and the structure is dedicated to linear polarization. It can be easily fabricated to- gether with the printed antennas without any extra cost. This research was mainly inspired by the unfolding research activi- ties in the field of MIMO technology, biomedical applications, and radar applications where, in many cases, a two-element an- tenna is used and an isolation as high as possible is needed be- tween transmit and receive antenna. This can be seen in the large number of publications using two-element structures, in- cluding [1]–[4], [6], and [7]. The idea can also be directly ap- plied to the MC reduction in arrays. The approach and design of a prototype are explained in Section II, and results are presented in Section III. Conclusions are drawn in Section IV. 1536-1225/$31.00 © 2012 IEEE