This paper demonstrates how the error of a measured or simulated antenna radiation pattern can be decreased by calculating a spherical wave expansion (SWE) with an optimised truncation mode index. Four radiation pattern examples are examined for which the analytical expression of the electrical field is known. Upon adding random and -alignment errors to the exact electric fields, the SWEs are determined and compared with the corresponding ones for the exact fields. These comparisons show that the accuracy of the calculated SWE is much better than the accuracy of the original inaccurate field. Resting on the calculated examples, a method is created which can be used to determine the optimal truncation index from the modal power distribution of a SWE without knowing the exact field. Finally, the method developed is applied to measured antenna radiation patterns.