Hi walaa,
the meshing will definetly have an effect on your design. Restricting the mesh in certain sensitive regions of your design will usually improve the accuracy of your results, so long as its done correctly. One way you can check how good your mesh is, is by plotting the convergence of S11 (using adaptive pass instead of setup1). You should notice the value of S11 converge after a sufficient number of passes.
As a side note I noticed you used a lumped port to excite the microstrip. You might want to try using a waveport but read the HFSS manual to figure out the appropriate size of your waveport. This will also require you to place one side of your airbox in the same plane as the waveport as waveports cannot be placed inside the radiation boundary.