Plane of Incidence for the wave transmitted with Horn Antenna.

Q1: Suppose Horn Antenna is used to transmit wave toward a target equipment under test (EUT), then what is the plane of incidence. Since with reference to the Incidence plane, TE and TM is defined. Plane of Incidence is a plane perpendicular to the EUT surface which also contain the incidence wave.

Q2: Is any formula available to calculate the S-Parameter for arbitrary polarization if S-Parameter results for TE/TM case is known. Since field can be decomposed into orthogonal components.