Antenna use case: out-of-band usage

Hi,

I would figure out what happens to the performance of an S-Band tuned antenna (let's say a QFH) if I use it in the X-Band frequency.
When I saw performance i refer to radiation pattern, axial ratio etc...

The question is vague. Here is an equally non-specific answer.

Antennas are generally high pass and using a S band antenna at X band will probably result in significant changes in the radiation characteristics.

Some exceptions exist, including Vivaldi's, LPD's and various spirals which often operate over octaves of bandwidth. L to Ku band operation is practical with surprising stability of radiation characteristics. Really broad band antennas were developed in the 60's, and that was before LSD.

Regards,

Azulukit