scattering parameters and electromagnetic reflection coefficient

[cherne-he]

Hi everyone,
I want to stimulate the reflection coefficient of the electromagnetic absorber. There are scattering parameters in HFSS or CST. Does the S11 represent electromagnetic coefficient?
Thank you!
Best regards,

 

[volker@muehlhaus]

I think you refer to your thread https://www.edaboard.com/threads/325239/

That setup looks totally wrong, because it tries model the absorber like a transmission line conductor, with a circuit port. These results mean nothing. To test your absorber, you need plane wave excitation.

 

[PlanarMetamaterials]

Hi cherne-he,

Yes, S11 is the reflection coefficient as seen by port 1.

A few comments on volker's link...

The setup with two wave ports should be just as accurate as one wave port with a radiation boundary on the far side. However, assigning radiation boundaries to the sides of the simulation may be introducing some error, as I've had issues with radiation boundaries being perpendicular to wave ports. If you can handle modelling the absorber as part of an infinite array, consider changing the tangential boundaries to perfect Es along one axis and perfect Hs along the other. Also, to model a plane wave, the wave port has to fill up the entire face of your bounding vacuum box, contrary to what was suggested in that thread (you can view the fields in both cases to find out why).

And to reply to volker:

From your response I would guess that you don't know what a HFSS wave port is. He's not using a lumped port. Why would you give this poor man such bad advice?

 

[volker@muehlhaus]

I apologize if I misinterpreted the setup.

 

[cherne-he]

Hi Volker,
Thank you very much for your warm reply. You are a kind-hearted man.

You are right. I refer to my thread posted 10 days ago. I want to test my electromagnetic absorber, and the magnitude of electromagnetic reflection coefficient represents its capability of electromagnetic wave mitigation. But I’m not sure if S11 is the parameter I want. The ratio of the electric field of the incident wave to the electric field of reflection wave is called electromagnetic reflection coefficient. Don’t you think the S11 represent electromagnetic reflection coefficient?
If I assign plane wave excitation, I cannot extract reflection coefficient. What should I do? Any suggestions?
Thank you in advance.
Cherne-he

I think you refer to your thread https://www.edaboard.com/threads/325239/

That setup looks totally wrong, because it tries model the absorber like a transmission line conductor, with a circuit port. These results mean nothing. To test your absorber, you need plane wave excitation.

 

[PlanarMetamaterials]

Firstly, I would like to apologize to volker: I came across as a bit more aggressive than I meant to, but yes, it does look like you misinterpreted the setup.

Secondly, volker was correct that you can use an incident plane wave, the problem is that analyzing the fields and extracting the reflection coefficient is much more complicated than just using a wave port. I can walk you through this process if you think you need to try it.

Thirdly, yes, the reflection coefficient is defined as the ratio of the incident to reflected electric field. The definition of S11 is similar: the ratio of transmitted to received electric fields at port 1. For simple cases like this, these will be the same value.

 

[PlanarMetamaterials]

My third point above was not quite correct (not sure where the edit button went). S11 will be equal to the reflection coefficient IF the absorber extends over the entire domain of the incident plane wave. If there is an air gap between the absorber and the transverse boundaries of the domain, then some power will "leak around" the absorber and S11 will not equal your absorber's reflection coefficient (which i believe is your case).

 

[jiripolivka]

Please note: testing absorbers does involve S11 measurement but it is a function of incidence angle. Absorber makers use the "inverted U" system to evaluate both factors.

 

[cherne-he]

Hi,
Thank you for your warm reply!
My model is simple. I just simulate the normal incidence angle. Do you have relevant experience to simulate S11. Could you spare time to help me to check it. My email address is chernehe@gmail.com. Thank you in advance!
Cherne he

Please note: testing absorbers does involve S11 measurement but it is a function of incidence angle. Absorber makers use the "inverted U" system to evaluate both factors.