Hi

Are there any graph that shows the relation between the Gain and the operated frequency ?

Because I noticed that changing frequency can also effects the Signal level.

For a fixed aperture, the gain is proportional to the frequency squared.
However, you are probably referring to the characteristic of one particular antenna, rather than such a generic statement.
In that case you need to refer to the diagrams in that antenna's specification. It may be that part of the antenna
operates as a tuned element, and there is some band over which it operates well (and less well at the edges of the band).

For a fixed aperture, the gain is proportional to the frequency squared.
However, you are probably referring to the characteristic of one particular antenna, rather than such a generic statement.
In that case you need to refer to the diagrams in that antenna's specification. It may be that part of the antenna
operates as a tuned element, and there is some band over which it operates well (and less well at the edges of the band).

Thanks for your help

What is the optimum frequency to work with for MikroTik 5GHz range RB911G-5HPnD or SXT Lite 5 ?
Any Idea where the maximum gain could be? IS it at the 5.8GHz frequency ?

I think there are no exact specifications about that. Experiment.
Besides, gain is not the only thing that matters.

Actually SXT does have such graph:
http://i.mt.lv/routerboard/files/sxt-patterns.pdf