Thank you for all your answer.
I have been making some test. The measure in the anecoic chamber is in near fild and then, the conversion made to farfield, that means there is no multipath. IN the other hand, we have cheack the set-up and repeated it for other probes antennas where the obtained results is the same as the provided by the manufactured.
In addiction, we have use this horn antenna in order to feed a parabolic antenna. With this antenna I have made a "quick" measure checking that the maximum Power - minimum power measured at the antenna port rotating the horn antenna is 11-12 dB. i know this is not the best way to check XPD value, but this quick measure should give an difference near simulation (may be 25 - 30 dB) but the resoults are quite worse.
I don't know what I have to check. Is possible there is any parameter in the simulation that makes this a wrong design? I have cheqcked material, phase center, polarization port, mesh simulation, symmetry, boundary condition, accuracy... etc. I don't know what else. May be anyone could help me if i send him the cst or the 3D file.
Best regards