###### Abstract  
In this paper, an effective technique for mutual coupling (MC) reduction between antenna elements of two multiple input multiple output (MIMO) microstrip patch antennas operating in the ultra-wide band (UWB) between 3.1 and 13.5 GHz is presented. The antenna array separation was kept at 44 mm for investigation, and the isolation was achieved through a modified electromagnetic band gap (MEBG) decoupling structure. The MEBG is embedded behind the radiating elements connected to the ground plane. HFSSv15 software was used to design and simulate the antenna. The effectiveness of the antenna and the MC reduction method was examined with and without the MEBG structure. The results revealed that the MC between the MIMO antenna elements was minimized when the MEBG structure was introduced. An MC of about \(-23\) dB was obtained over the entire UWB frequency spectrum. This is more than a 10 dB improvement over the reference antenna (without the MEBG structure). Without limiting the effectiveness of the antenna when the MEBG structure was introduced, the results of the envelope correlation coefficient (ECC) gave the antenna a satisfactory diversity performance. The MEBG UWB MIMO antenna has an ECC less than 0.09 with a wide bandwidth. In addition, the total gain and the Voltage Standing Ware Ratio (VSWR) results were analyzed, which show that the performance of the antenna was not degraded while reducing the MC effect between the MIMO antenna elements. electronic band gap; ultra-wide band; antenna; mutual coupling (MC); MIMO; VSWR; microstrip +
Footnote †: journal: Applied  
methods have been used to minimize MC among antenna arrays. Some of these include the embedding of EBG structures, defected ground structures (DGS), metal-surfaces, placing of metal strips between elements, and the use of orthogonal polarization and neutralization line techniques [16; 17; 18; 19]. Several studies in this domain have been reported in the literature. To achieve multiple resonance and excellent isolation between the radiating elements of two monopole UWB MIMO antennas, an F-shape stub was employed in the ground plane in [20] and achieved a MC \(<\)\(-\)20 dB. In this study, a low MC with S\({}_{12/21}<-\)20 dB was achieved. In [21], the effect of reducing the surface wave with an EBG structure and its use to minimize the MC was investigated. Applying a finite difference time domain (FDTD), its impact on an infinitesimal dipole source with and without the EBG structure was compared. While the FDTD method was established to examine the MC of the arrays of probe-fed patch antennas, EBG structures were also used to enhance antenna performance. The results indicate an MC of \(-\)24.6 dB. The studies in [22] are like those presented in [21], aside from the use of a uni-planar compact EBG (UC-EBG) inserted between antenna elements. A comparison between UC-EBG and without UC-EBG was made. Simulation results show a great reduction in the MC with a \(-\)19 dB difference. In [23], a comb-line structure that served as an EBG between the MIMO antennas to reduce the MC between the two antenna arrays was formed in the radiating element. A suppression of \(-\)24 dB covering the whole UWB bandwidth was achieved. In [24], a compact quad-band notched UWB-MIMO antenna with two symmetrically inserted L-shaped slots on each radiator and C-shaped stubs acting as a decoupling element was proposed. Within the UWB band of 3.1-10.6 GHz, a high isolation of less than \(-\)25 dB was obtained when the two elements were perpendicular to one another. The authors in [25] proposed a Pacman-structured MIMO antenna using two orthogonally positioned antenna elements operating between 2.9 GHz and 15 GHz. The simulation result shows that an isolation of less than \(-\)17 dB was achieved. Elsewhere [26], a miniaturized UWB MIMO antenna with a simple notched filter realized by a trident-shaped strip was utilized. The top layer consists of two 50 \(\Omega\) lines, while the bottom layer is a ground plane with two stepped slots that form a T-shape.