Analysis of the envelope correlation coefficient of MIMO antennas connected with suspended lines

Inserting a suspended line is a widely used technique to reduce the envelope correlation coefficients (ECC) of multiple-input multipleoutput (MIMO) antennas, but its ECC reduction mechanism has not been carefully investigated so far. In this paper, MIMO antenna pairs connected with different suspended lines were evaluated using a full-wave simulation and measurement results. We calculated the surface current density at the center of the suspended line inserted between the antenna elements and found that the derivative of the surface current density was closely related to the ECC values. Furthermore, parametric studies showed that the suspended line length determines the ECC reduction bandwidth of the MIMO antenna, and the antenna length controls the ECC minimum frequency point. These guidelines provide useful insights into designing a low ECC MIMO antenna employing a suspended line.