Hi, rf-en:
Yes. It is what you need to do. If you think E-theta at phi = 0 is your co-pol, then E-phi at phi = 0 is your cross-pol.
Traditionally, designers will consider the stronger E-field pol as the co-pol and the weaker E-field pol as the cross-pol. Given an antenna with its properites well defined in a specific frequency range, we can tell what its col-pol and what its cross-pol are. However, from the viewpoint of generality, we can't define them automatically. Just a well-defined antenna, when you change its frequency to other than the regular frequency, its polarization will change. You may even have a case with E-theta and E-phi higher at different angles for the same cut. Apparently, we can't provide a general definition of co-pol and cross-pol unless it is for a some very simple antenna like a dipole because the polarization of a general antenna may change with many different parameters and situations. A designer has to decide on it. A designer should check the polarized field at a cut to see which one is higher and which one is lower (E-theta vs. E-phi, E-left vs. E-right, etc.) and decide which pol is the co-pol and which one is the cross-pol.
Regards,
Regards,