Hi Meghjit,
The Modal solution option is preferred for multimode or higher mode analysis, whereas the Terminal solution option is used for single mode analysis. Note that each of these solution options can be used for your case for a TEM mode excitation from the coaxial feed in your patch antenna.
More information about these two options can be found in "HFSS Help" and "An Introduction to HFSS" documents accessible from the Help menu.
Best regards,
Praneeth.
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