Strange behavior from rectangular waveguide

How to use openEMS. Discussion on examples, tutorials etc

Moderator: thorsten

Post Reply
PaulUK
Posts: 79
Joined: Wed 28 Oct 2015, 14:23

Strange behavior from rectangular waveguide

Post by PaulUK » Wed 18 Nov 2020, 14:24

I will post some images comparing a length of WG20 (WR42) simulated with both CST and openEMS. There are a number of discrepancies which I will list. I don't know if the issues are caused by incorrect use of openEMS or if it's a consequence of numerical noise or limitations of the software. I'm hoping that either Thorsten or someone else can shed some light on these issues.

List of issues:

1) When simulating the waveguide in CST FDTD solver (or CST F solver), the reflection S11 is below -90dB, openEMS gives values below -60dB but depending on the specified frequency range or z boundary PLM settings, it can vary. Is this a numerical noise limitation or something else? It could be an issue when designing waveguide filters or diplexers that require high isolation/rejection in the stop band.

2) When the openEMS waveguide model is changed from a normal waveguide structure using PEC with a finite wall thickness to just a waveguide defined only by the x and y boundaries being PEC, the S11 reflection now gets worse and is between -50 and -60 dB. The result should be identical to when the waveguide is defined by a rectangular channel of PEC. I don't know why there is a difference.

3) When taking the "realistic" rectangular waveguide model and replacing the PEC metal definition with a material definition with a given conductivity (kappa), the S11 is terrible regardless what conductivity I use that is close to any metal from brass, copper to silver. Also there is what seems like an oscillation on the S21 when you zoom into the plot. Is this a case of using the material definition function incorrectly?

I will post the two script files followed by some pictures, in this and the following thread.
Rect_Rect_WaveguideadapterWG20.m
Waveguide model with finite wall thickness.
(6.26 KiB) Downloaded 48 times
Rect_Rect_WaveguideadapterWG20_PEC.m
Waveguide model with PEC boundaries.
(6.19 KiB) Downloaded 48 times
CST Model
CST WG20 Model.png
CST model
CST WG20 Model.png (28.5 KiB) Viewed 670 times

PaulUK
Posts: 79
Joined: Wed 28 Oct 2015, 14:23

Re: Strange behavior from rectangular waveguide

Post by PaulUK » Wed 18 Nov 2020, 14:27

Continued images...

CST Results
CST WG20 SParameters.png
CST Results
CST WG20 SParameters.png (36.25 KiB) Viewed 669 times
openEMS Model
3DWG20.png
openEMS model
3DWG20.png (392.81 KiB) Viewed 669 times
openEMS Results for finite wall thickness PEC.
PML_12 S11 S21.png
openEMS Results for finite wall thickness PEC
PML_12 S11 S21.png (44.48 KiB) Viewed 669 times

PaulUK
Posts: 79
Joined: Wed 28 Oct 2015, 14:23

Re: Strange behavior from rectangular waveguide

Post by PaulUK » Wed 18 Nov 2020, 14:32

openEMS with PEC boundaries in x and y directions, vacuum inside.
PEC BC as WG Walls in x and y PML_8 S11_S21.png
openEMS with PEC boundaries in x and y directions
PEC BC as WG Walls in x and y PML_8 S11_S21.png (38.42 KiB) Viewed 669 times
openEMS results when finite thickness model PEC is replaced with a material with conductivity "kappa".
Silver PML_8 S11_S21.png
PEC metal replaced with a material and finite kappa
Silver PML_8 S11_S21.png (35.63 KiB) Viewed 669 times
End of post.

I hope that someone can shed some light on these peculiar observations. I hope that I have provided enough information, the scripts came from Gerald who is also a forum member. Thank you.

wettersat
Posts: 9
Joined: Fri 05 Jun 2020, 12:25
Contact:

Re: Strange behavior from rectangular waveguide

Post by wettersat » Sat 05 Dec 2020, 17:49

Hmmm. Looks like nobody has an idea or a tip.
Gerald, OE2IGL
http://wettersat.bplaced.net

Post Reply