abhi_kolkata,
I have to disagree with costox. Without going into much details, I would suggest either IE3D (if the dielectrics are not too complicated) or CST microwave studio. CST is a time-domain solver, so you will get the entire bandwidth from one simulation run. On the other hand, CST (like HFSS) needs to mesh a big volume around your antenna in addition to the actual antenna, which can be time (and memory) consuming. IE3D on the other hand only meshes the antenna. IE3D is however (like HFSS) a frequency domain solver so you need to repeat the simulation for every frequency. In most case though, IE3D is (unlike HFSS) so fast it won't really matter. I guess CST and HFSS can both be used for similar problems (complicated 3D structures), but I would really only recommend HFSS over CST for narrow-band highly resonant antennas. This is clearly not the case here
I am fortunate enough to be able to choose tool for different jobs, being at a well-equipped compant. You should keep in mind that these programs costs a lot (>$10.000) and takes some time to get into. Therefore, if you do not already have all the different programs at your company, you should think carefully about future needs before doing the purchase.
Finally, if I'm allowed to be a bit subjective, I have used IE3D for over six years now and have done many many succesful designs with that program. On the other hand, with HFSS and some other programs (like Momentum), I have mainly experienced frustration. IE3D might not have the coolest or most good looking interface, but it usually gets the job done.
Best regards and good luck with your designs,
Peter