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.