Re: Radiation boundary
Theoretically speaking, the simulated resonance frequency of the antenna must not depend on the airbox size but it does. The answer is that the simulation is affected by the amount of meshing tethraedra involved and thus by the volume size. Increasing the number of adaptive passes, reducing the convergence limit value, the solution tends to be more stable and closer to reality.
As I said in the last post the distance of the air box is also a value to be chosen properly. When we talk about numerical methods used in simulators like FDTD or FEM (the one used in HFSS) the separation between the simulated region (in a few words where u apply the mesh) and the outer region is of crucial importance. The boundary of the simulation domain is represented by a radiation boundary (which applies the Sommerfield Condition, Far Field Condition hence a of near2far field transformation). The 'mismatch' between the meshed region and the far field region represented by means of a radiation boundary implies numerical errors called 'numerical reflections', which are non-physical reflections at the domain edge due to such a discontinuity. It is proved that this reflections tend to reduce as the distance from the radiating/scattering element and the domain borders gets farther.
I hope now it's clear and u understood why larger distances implies a more stable solution.
I.