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Looking at the plots, it seems that the second plots shows a radiating structure (alternating of minima and maxima in field) and the first plot shows the normal field decay of a non-radiating structure. So the structure is different.
I managed to find it some weeks ago. I post it just in case someone ever has the same problem.
What I wanted was to make the space of the simulation larger, to do so you have to go at the Modeling tab and there you will find Background option from which you can change the surrounding space in all direction (x,y,z). Of course changing the surrounding space will raise your Meshcells and that means longer simulation times.