I have to simulate an antenna immersed in a dielectric.
what is the distrance for the dielectric box (radiation)? i noticed that the gain changes according with the distance...
You want to keep effective lambda / 4 for radiation boundaries and effective lambda / 10 for PMLs. However, if you want correct far field patterns, you will need to go to HFSS -> Boundaries -> Edit Global Material Properties and change the global material from air to whatever the immersed medium that the far field will be measured in.
Have Fun
Tallface
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Also, be aware that if your immersed medium has loss, it will matter