I would like to know if it is necessary to define any boundary for subtrate under the Planar antenna, or just assign it as 3d object and dielectric material?
Thanks for your kind attention!
...if your planar antenna is single-sided microstrip and has a ground-plane, then you can assign a Perfect-E (perfect electric) boundary to the bottom of your 3D substrate object in order to model the ground-plane, instead of creating a thin PEC object for this ...
In addition for a microstrip feed operating in the Q-TEM mode , with a symmetric structure ( may look like a lot of conditions, but it happens in most microstrip fed p@tch antenna cases) , you can assign a Perf H symmetry plane which truncates the calculation domain to half.