I agree with loucy that HFSS is not the best tool for this task. You should try using a planar EM simulator for this, e.g. Sonnet, Momentum (Agilent), Empower (Eagleware), IE3D (Zeland), Ensemble (Ansoft), etc. Depending on how much detail you want for the mode information, Sonnet Lite may suit your needs just fine and it's free, highly accurate and easy to use. You can download it from their website: sonnetusa.com The others all cost [plenty] and might not be as simple to use, depending on your background. There was a free version of Ensemble but since Ansoft has bundled it into their Designer product, it's no longer free. If you really want it, the old free student version should be available in the download section of this board. However, you're probably better off with Sonnet as it's much more capable than the free student version of Ensemble.