Revisit to Mutual Coupling Effects on Multi-Antenna Systems.

Due to space limitation, mutual coupling occurs almost invisibly in array antennas. In this paper, the mutual coupling effects on the two most popular categories of array antennas, i.e., phased arrays and multiple-input multiple-output (MIMO) antennas, are reviewed. Some misconceptions regarding the mutual coupling effects are uncovered. It is shown that the steering pattern of a phased array at an arbitrary scanning angle can be readily calculated once the embedded radiation patterns of the array elements (including the mutual coupling effect) are obtained. As antenna spacing decreases, absorption loss increases, yet the phase terms tend to add up constructively as antenna spacing reduces, which may overcompensate the absorption loss due to mutual coupling. Thus, the array efficiency may be increased by reducing the antenna spacing. A patch antenna array is used to illustrate this phenomenon. It is further shown that while mutual coupling tends to reduce the correlation of two-element arrays, it has a negligible effect on the overall correlations of larger arrays. Finally, various mutual coupling reduction techniques are briefly presented. Two feasible techniques for large planar arrays are used to illustrate the benefits of array decoupling.