HFSS-air box size, wavelenth or something else?

[andy_larkeen]

Hello

I wonder how size of air box influences on results.
How you choose airbox demiension?
e.g. I have filter with center frequency 5Ghz, my air box height is 5mm.
Width and length is the same like substrate size, is it ok?
Substrate height is 0.5mm.

best regards
andy

 

[artiste]

Hi,

When defining the boundary conditions, it is important to surround the air box (or it can also be the vacuum box) by a radiating conditions that enable the wave to radiate infinitely far into space.

But Ansoft documentation does not precise how big should be the air box.
I know that a too small height for the air-box with a surrounding radiation boundary does not enable the wave to radiate correctly before touching the boundary condition.

You can also choose a minimum height according to the size of the waveport. There are rule for that which depands on the dimensions of the transmission line.
For instance, the minimum port height is 4 times the substrate thickness for a CPW line.
What is the type of transmission line you use ?

5 mm is not too big for me.
You can also plot the mesh. You'll see that the mesh is less dense in the air box.
You can also go step by step by increasing gradually the box dimension and looking on how the S-parameters vary, especially the insertion loss.

I Hope it will help you,
Regards,

 

[andy_larkeen]

Hi,

When defining the boundary conditions, it is important to surround the air box (or it can also be the vacuum box) by a radiating conditions that enable the wave to radiate infinitely far into space.

It dosen't matter what kind of layout you create?
Do you alwayes use radiating conditions for all box?
I make top of box: Finite Conductivite and rest is Perfect E to simulate metalic box.

What is the type of transmission line you use ?

SIW

5 mm is not too big for me.
You can also plot the mesh. You'll see that the mesh is less dense in the air box.

In my opinion in my solution mesh is quite big with compare to rest of layout.


But, with bigger airbox simulation is faster...... I'm not sure how to explain it.

Moreover results didn't change for radiation condition.

 

[ferdows]

hi
in hfss we dont have constant formula or method to design our antenna and other structure radiation box but u experience can help u in this way
if u radiation box is to small S parameter was fault like u gain in antenna my best suggestion is that u convert u made Sm3 file and insert in CST in cst u can find u structure write or not and found u structure radiation box
if u dont have CST i suggest chose lambda/4 to lambda/2 of structure lambda for radiation box
i thick 5mm is wrong and u result not reliable

 

[andy_larkeen]

So, you suggested that for 5Ghz I should use 30mm(lambda/2) height of airbox?

 

[ferdows]

hi
around 40 mm from top and 40mm from bottom

 

[andy_larkeen]

hi
around 40 mm from top and 40mm from bottom

For what from the bottom?
There is gruond plane.

We should specifed about what kind of circuit it is?

It is not antenna.

It is SIW.

 

[ferdows]

hi
not difference between antenna and other structure and if u have ground u can specify 10-20mm in bottom

 

[noor_awad]

to fedrows
if i simulate HFSS design with setup frequency (3-10)GHZ
could u tell me how to choose the center frequency ???
and i want to know if the airbox dimension depend on the lower frequency or the highest or the center???
please urgent