I am trying to simulate an NFC coil in HFSS. I have simulated it with air box dimensions as per the free space wavelength of 22m. As expected the simulation time is huge and also, I am not getting any response. However, if I reduce the size of the air box to the expected radiation range of the antenna (15cm in my case), I am getting a clear response. However, I am unsure if it is the correct approach.
According to HFSS guidelines, the radiation boundary should be at least lambda/4 (at the operation frequency) in the directions where energy radiates. A 22m box is quite large actually so I am not surprised you didn't get any results. If your free-space wavelength is 22m, then 22/4 = 5.5m away from the coil in the directions where energy is radiating should be sufficient.
With that being said if the radiation range is only 15cm and the radiated energy is negligible beyond that, then perhaps 15 cm is enough. You can incrementally increase the size beyond 15cm and observe if there are any changes in the response. If you have a theoretical response to compare HFSS with that could provide some guidance.
Phalguni Mathur In HFSS there should be a create region button in the draw tab. HFSS will generate a box that contains your model geometry. This has always been sufficient for me. Alternatively you could plot fields and make sure they don't have sufficient magnitude where your box terminates.
------>The radiation box should always have at least a quarter wavelength in the embedding medium, i.e. AIR, between the closest radiating edge and the closest edge of the box, at the lowest frequency.
------>A larger radiation box shouldn't be an issue, it might just take longer to simulate.
You can assign the radiation boundaries from at least lambda0/4 distance. Although you can calculate the distance using the standard near field and far field radiation distances with respect to your radiator size and frequency.
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