Air box and Radiation boundary condition

Hi,
I am in the process of designing a microwave filter using HFSS11. I would like to know whether how to determine the size of the air box in such a design? Should I cover my entire filter with the air box including the substrate or only the top surface where filter strip lines are placed?
In addition, I would like to know how to assign radiation boundary conditions for this system. Should I include for the whole box or only to the faces?
Can somebody help me regarding this?

Thanks a lot!

:D

Generally I surround the entire structure with an airbox that has at least a quarter wavelength from the edges. I generally use a third of a wavelength just to be safe. Then again, I am designing antennas.

For filters, you probably don't care too much about radiation. Nevertheless, it doesn't hurt to use a reasonable large airbox - say a quarter or third of a wavelength from the substrate. This will ensure that any spurious radiation is kept inside the box.

If you are using a waveport, make sure the port is assigned against the edge of the airbox. Otherwise, you will need a lumped port.

As for the radiation boundary, select the faces of the airbox and assign a radiation boundary that way. I would assign to each face.

Hi,
Thank you very much for the reply. ?Quarter wavelength? means is it the wavelength based on my simulation frequency range. In this case, I run the simulation from 1GHz to 40GHz. So should I just use "wavelength = 3*10^8/(10^9)" to get an approximate wavelength value?
Can you explain a bit about assigning ports as well? In my design, I have two ports and the desired filter starts about 50μm from the edge of the substrate. A 50Ω line is connected from the edge of the substrate to the filter start point. I created a wave port at the edge of the substrate and de-embedded 50μm. However, in this design my ports touch the faces of the air box.

Thanks a lot!

Hi,

If you are running simulation from 1GHz to 40 GHz so you need to take for airbox dimention calculation Lambda(maximum) / 3, that means 3e8 / 1e9. But the problem that you will get a very large mesh for very high frequencies and the simulation may run a long time or even won't work. Maybe you should make a few bands instead of one band and run a couple simulations.

Best Regards

Thank you!
:D

Is your filter shielded? You might not want to use a radiation boundary.

No it is not shielded. I increased the size of my air box but it did not change my simulation results. Do you know any reason for that?

Thanks!

If your results did not change, than probably the smaller airbox was enough.

Thanks!