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I'm guessing that you may have unknowingly set up the software to allow it to mesh the problem differently in the two cases. Or you may have done something to cause the solver to use the previous solution as a starting value, and then your mesh was too coarse, resulting in inaccurate (but different) results. Take a closer look at your mesh and solver settings. And make sure your mesh is fine enough (wherever it matters) and your solver settings are clearly defined such there shouldn't be dependencies on previous answers. You also might wnat to post your model file for others to look at and offer you advice.

Thank you Robert,

I'h attached an example here. It gives -8.2502 dB for S11 at 2.4 GHz if i use range(2.3 [GHz],0.05 [GHz],2.5 [GHz]) phrase whereas it produces -8.3548 dB at again 2.4 GHz if I use 2.4 [GHz] phrase.

Thank you.

The issue is the mesh being different in the two cases. I executed your file on my workstation. Look at this taken from the message log:

Your 5-frequencies case: [Jun 12, 2019 9:54 AM] Used linear geometry shape in 8 mesh elements to avoid inverted curved elements. [Jun 12, 2019 9:54 AM] Used linear geometry shape in 8 mesh elements to avoid inverted curved elements. [Jun 12, 2019 9:54 AM] Number of degrees of freedom solved for: 205306. [Jun 12, 2019 9:55 AM] Solution time (Solution 1 (sol1)): 31 s.

Then, with just a single 2.4 GHz case: [Jun 12, 2019 9:56 AM] Used linear geometry shape in 18 mesh elements to avoid inverted curved elements. [Jun 12, 2019 9:56 AM] Used linear geometry shape in 18 mesh elements to avoid inverted curved elements. [Jun 12, 2019 9:56 AM] Number of degrees of freedom solved for: 205208. [Jun 12, 2019 9:56 AM] Solution time (Study 1): 11 s.

Note the slightly different #'s of degrees of freedom (and also the different count # of inverted elements). You have your mesh settings at the default "physics controlled." If you wish to ensure perfect consistency among your answers to the "same" conditions but in dfferent runs, set it to "user-controlled" and specify meshing details on various surfaces, volumes, etc., that correspond to ensuring adequate details where they are truly needed. A knowlegeable user can do a better job of specifying mesh densities than is done by the automated "physics controlled" rules.

I think I need to stick on "physics controlled" option for now since I am not a "knowlegeable user" yet :). But at least I understand that there is no problem in my model or comsol usage. Thanks again.

You're welcome.
Note: Please don't assume there are "no problems" in the model setup just because I took a quick look at it. I was simply looking for what might have given the variation in the results that you specifically asked about. I did not investigate whether your model otherwise makes sense or is otherwise properly configured to yield accurate results for the specific geometry, materials, computations, etc., involved. Good luck. :)