E-theta and E-phi in microstrip antenna

what is the significance of E-theta and E-phi in microstrip antenna and how the cross polarization level is measured from these two values.................

Theta and Phi together will represent the variation of E in free space.

Cross Polarization for Linear or Circular polarization ?

If one antenna wishes for linear polarization?

I don't know if the ludwig's definition can be used here?
some rotating,shifting of the axis makes the answer different@@

This question makes me puzzled for a long time.