Why are Air Boxes Used in Bandpass Filters in HFSS?

[Paul678]

I've followed this tutorial, and it was simple enough to copy what was done:

But after I was finished, I realized that the air box was never turned into a Faraday cage. It remained
an air box, and even the outside diameters of the "coax", remained a vacuum, with only the center
conductors as PECs.

So how could this filter ever be used in the real world, with the resonators floating in space? Or is this
just meant to be a mathematical, electromagnetic exercise? All the bandpass filters I have seen in the
real world, had some sort of conductive filter cavities around them.

????

 

[volker@muehlhaus]

In some EM tools, you can set a "background material", which would be PEC here (perfect electric conductor) instead of air/vaccum.

 

[Paul678]

In some EM tools, you can set a "background material", which would be PEC here (perfect electric conductor) instead of air/vaccum.

Thanks for the response, Volker.

At what point in the video did you see them making the "background material" PEC?

I don't remember doing that.

 

[volker@muehlhaus]

I didn't watch the video, but from my EM experience it only makes sense with background material set to PEC. It's a common setting in 3D EM tools, so that you can choose if "nothing drawn" means air or metal.

--- Updated Jul 3, 2021 ---

It's a common setting in 3D EM tools, so that you can choose if "nothing drawn" means air or metal.

From this old thread, it seems that in HFSS you cannot choose background material, and PEC is always used. So in your case, anything outside the airbox is perfect conductor, without drawing that explicitely.

https://www.edaboard.com/threads/how-to-set-the-background-in-hfss.61004/

 

[Paul678]

From page 1.2-1 of this:

https://www.researchgate.net/profil...544@1505141883049/download/hfss_full_book.pdf

"By default Ansoft HFSS assumes that all structures are completely
encased in a conductive shield with no energy propagating through it. You
apply Wave Ports to the structure to indicate the area were the energy
enters and exits the conductive shield."

So it's not clear if the air box (and the coax vacuum cylinder) are just encased in
a PEC Faraday cage, or if everything outside the air box and coax cylinders are
PEC, but either way, the 3-D EM solution would be the same.

Thanks, Volker!