Measurement of circularly polarized antenna

Hello,

I would just like to ask how to compute directivity (D) that is computed from two cuts of radiation patterns, for circularly polarized antenna. For normal antenna (linear polarization) you take vertical and horizontal plane (E and H). If you have antenna with dipole, the dipole is once horizontally and second time vertically positioned. From this two cuts you can compute directivity.

Which positions do you take if you have spiral? I cannot find anything useful on the internet.

Thank you very much for your answer.