Questions Concerning Radiation Boundary in HFSS

Hi

I have a question about the Radiation Boundary in HFSS itself. I am simulating a dipole with a lumped port feed and I notice changes in the resonant impedance/frequency when I change the size of the Radiation Boundary. According to the design requirements, I gathered that the minimum distance of the radiation boundary away from the radiating structure should be lambda/4.

I modeled my dipole for IR wavelengths, it's 5um in length (including the feed gap) and for the lambda/4 design I get a resonant frequency of 25.05THz. Whereas when I increase the radiation boundary to around twice more (in height, length and width) I get a resonant frequency of 24.7THz. Although the difference is small why is there a difference at all if both case abide by the lambda/4 rule? It feels like the first case has a sort of cavity mode associated with it or something.

Does anyone have a solution to determine the right medium dimensions for accurate resonant results?

Thanks in advance

The radiation boundary may not be appropriately meshed. Try applying a relatively fine seed mesh to it.

Thank you! I did apply an Element Length Based Mesh Refinement with a restriction to maximum length of elements = 0.5um and now for any size of Radiation Boundary, it gives you the same resonant frequency. Although for larger radiation boundary sizes, the processing time is way longer. Thanks once again!