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HFSS - Air box (Radiation boundary) dimensions changes the results (that must not happening)


Apr 2, 2023
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Hey, I am designing a rectangular patch antenna and I am facing an issue that I don't know what should I do. I am trying to design an antenaa that will resonate inside of water box with some other substances that has dielectric constant of er=33. But I don't know how big the box must be and also why by changing the dimensions of the air box (radiation box) the results are also changing (the radiation box is always larger than λ/4 starting for the edges of the water box)?

The designers of HFSS Ansys say that this must not happen and that the dimensions of the radiation box must not affect the results (on condition that the dimensions are larger than λ/4).
Thank you in advance.
Welcome, kate_mylona.

I'm a bit confused as to what you are describing. Is the antenna embedded in water? Why is there an air box? Have you changed any of the default HFSS settings, such as background material?
Yes the antenna is inside of the water box.
The air box is a box that have been assigned as radiation boundary. (By unclicking the auto-region button (on the solution type) this vaccum box is automatically created). You can change then the offsets that this box has (practically the distance between the edge of the vacuum box to the design.

I am attaching some relevant questions that have been asked on the ANSYS Forum but haven't been totally answered:


Thank you in advance for your help
If the antenna is embedded in a water medium, why do you have an air/vacuum box?

The posts you link have nothing to do with water, and simply seem to be convergence issues with your simulation.
Hey @PlanarMetamaterials and thank you again for your help

You are right, I am not explain my problem correctly. Let me describe again what is the issue that I am facing.
  1. I create a vacuum box
  2. Right click --> Assign Boundary -->Radiation
  3. When I change the dimensions of this vacuum box the S11 parameter changes
Resonance frequency = 1 GHz

At the beginning I was using "Fast" as a Sweep Type and when the dimensions of the vacuum box was increasing I had totally different results. I changed that to "Discrete" (that step was after my initial question on Stack Exchange, when I found this comment on a similar question on Ansys Forum) but again I got a bit different values. I am presenting 2 different simulations:


My antenna.png

(*I wrote about the water box since the dielectric constant is different than vacuum and maybe the λ is different since:

λ'= λ/square_root(er of water box) *
Sorry if I confused you)
Try instead creating a box to draw a region which later assign as a boundry (Draw/Region).
You can use an absolute offset (distance in all directions from the structure) whatever you want.
I don't think anything is incorrect with your setup, this really just looks like a convergence issue. It appears you are looking for aggressive convergence values, which will result in fairly large simulations.

Please post your convergence settings and adaptive meshing results.

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