ECC of MIMO antenna calculation in Matlab

Iam using s-parameters values to calculate the ECC values. I extracted S11, S21, S12 and S22 from the simulator and used in Matlab to find ECC. ECC values calculated from simulator(CST) and Matlab differs. Any suggestions to solve it

The rigorous definition of ECC involves a correlation of vector patterns of your antenna. The equation I think you are using in MATLAB is an approximation. That approximation is particularly bad for low efficiency antennas. I most suspect this is what is causing the discrepancy.

I agree with you ECC can be calculated using farfield method. However, ECC calculated using simulator and matlab uses S-parameter method.

The S-parameter method I have seen in literature is only good for high efficiency antennas. That means efficiency > 95% or so.