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You should first define your Co-polarization and Cross-polarization. This depends on the antenna type. Co-polarization is the polarization level that matches the polarization you expect from your antenna. Cross polarization is any polarization orthogonal to it.
So, look at your antenna. If you expect it to produce "linear polarization in z-direction" for example, then adjust the farfield plot calculation to give you this component and the normal to it