A Low Mutual Coupling MIMO Antenna Using Periodic Multi-Layered Electromagnetic Band Gap Structures

─ A multi-layered electromagnetic band gap (EBG) structure is proposed and incorporated into a MIMO antenna to reduce unexpected mutual coupling between antenna elements. The proposed multi-layered EBG (ML-EBG) structure is comprised of an improved EBG and three loading patches with a same distance. The proposed ML-EBG structure is designed at 2.55 GHz and it is utilized in a MIMO antenna array with an edge-to-edge distance of 0.13λ to reduce the mutual couplings. The simulated and measured results have been put forward to prove that the mutual coupling has been reduced by 30 dB between the antenna elements compared to the MIMO antenna without the ML-EBG structure. Index Terms ─ MIMO antenna, multi-layered electromagnetic band gap, mutual coupling reduction. I. INTRODUTION Mictrostrip antennas have been becoming an useful technology and a hot topic in wireless communication systems in the past decades [1-4] since they are small, light and low profile. Moreover, the microstrip antennas are easy to incorporate into a practical application to form a conformal antenna and they can be implemented to create multi-band antenna and circular polarization antennas [5-8]. In particular, the microstrip antennas are easy to integrate into a unified component with active devices and microwave circuits [9-10], rendering them suitable for various wireless communication applications. With the development of the wireless communication techniques, MIMO techniques have been attracting much more attention for next-generation applications [11-12]. MIMO antennas are to use in the future portable devices. Since the space of the portable device is limited, the configuration of the multiple antennas is difficult [13-14] to install in such a limited device. As a MIMO antenna aimed to use in wireless portable terminal, the distances between the antenna elements are narrow. Thus, the mutual coupling between the array elements is high [15-18]. After that, the mutual coupling reduction technology is urgent to be boosted. In recent years, a great number of techniques have been presented to reduce the mutual coupling between MIMO antenna elements, such as orthogonal structure [19], electromagnetic band gap (EBG) [20-26]. However, some of them have large array element distances, while others have high mutual coupling between array elements. Several amazing techniques have been reported to reduce the mutual coupling of a MIMO antenna. In [20], a double-layer EBG structure has been introduced into a MIMO antenna to achieve a low mutual coupling. In [21], a slit-patch EBG structure has been presented, which provides a significant mutual coupling reduction between a wide band UWB antennas. Then, miniaturized convoluted direction opening conversed slits have been proposed for reducing the mutual coupling between PIFA antenna elements. The size of each convoluted slit is about one-quarter wavelength and the convoluted direction opening conversed slits give strong effects on the mutual coupling between the PIFA antenna elements. In [2223], split-ring structured EBGs have been investigated for MIMO antenna coupling reduction. However, the antenna sizes are large. Then, a soft surface has been designed for antenna array decoupling [24]. The coupling has been reduced by about 9 dB with a large antenna size and three stripes. In [25], a multi-layers EBGs have been proposed with a coupling reduction of 24dB at the center frequency. Then, an improved EBG structure has been resented in [26] for a mutual coupling reduction of 26 dB. We can see that the coupling reductions of these antenna arrays are limited and some of them have large sizes. Additionally, a conformal antenna with a coupling reduction has been reduced by using slots. However, it is an un-planar antenna which is difficult to integrate into a portable terminal [27]. In this paper, a multi-layered electromagnetic band gap (ML-EBG) structure is proposed and it is integrated ACES JOURNAL, Vol. 33, No. 3, March 2018 1054-4887 © ACES Submitted On: June 23, 2017 Accepted On: November 29, 2017 305