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CST MWS Resonant Structure

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Dec 16, 2005
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calculate q in cst mws


I work with CST MWS and I simulate (transient solver) a high resonant structure (a split ring) in the GHz region, where I don't know the exact resonant frequency. I placed a magnet field probe in the middle of the ring.

The structure will be excited with a plane wave. For the discretisation I set the values for “Lines per wavelength” and “Lower mesh limit” to 25. The result is a resonant frequency of 1.72 GHZ, but if I increase the discretisation values I achieve a higher resonant frequency, e.g. 1.76 GHz for 35 and so on.
For higher values of Discretisation I got higher resonant frequencies, which doesn’t converge against a certain value.

How can I achieve the real resonant frequency from my simulations?

if you attach your project file, maybe I can help

Hello irfan1,

I glued my model on this message.

An additional question is: Can I also get the "real" Q-factor, for instance from the H-probe, because there is no S11 and S21 due to the plane wave excitation?
I know that the Q-factor is around 20-30. If I try to get it from the H-probe I got 48.

Thanks for help!

Even if the excitation is a plane wave, you may still obtain the reflection. There is a discussion on the board about that. So, it may be possible to obtain the Q-factors.

Added after 5 hours 47 minutes:

I checked your structure. A couple of remarks about the structure:
This is a highly resonant structure. So to get the resonant frequency correctly, you need to care for two things. First, you need to let the energy to drop to -80 dB. Second, you need to determine the high field locations in your structure. Because, these locations contribute to the response of the structure the most. For the case of split ring, it is important to accurately describe the split. For a fine simulation, to my experience, you need at least 4 mesh cells inside the split. One more final remark, Instead of using the H-probe, I would suggest locating an E-probe some distant away from the structure to determine the transmission. The resonant frequency will appear as a dip in the transmission. And it will be quite sharper.


Thank your for the remarks.

Unfortunately my experiences with MWS are if you try to answer one question you will find two (and more) new questions.

Question 1
How can I consider the split with 4 meshcells? –
I see to possibilities:

a) I can increase “Lines per wavelength” and “Lower mesh limit” up to 100 than I have 4 meshcells in the split, but I also have very long calculation time (especially when the energy dropes to -80 dB) and I have more than 1 million meshcells.

b)Another opportunity is to use subgrids [Mesh properties -> “Use local volume refinement factor” set to 8 and tick the box “Use subgrids for voume refinments”] it will decrease the number of meshcells to 17000.

Is b) the right way?

Question 2
a) What is the advantage to use the E-Probe instead of the H-Probe?

b) How can calculate reflection and transmission?

I tried to obtain the Q-factor for a plane wave excitation, like it was discussed in another thread. I placed a probe in front (H_i) and behind (H_o) my structure along the k vector. After the simulation I can calculate the S-parameter (with Matlab). H_i and H_o are complex

Reflection: S11 = (H_o ./ H_i).^2
S11_abs = abs(S11)
Transmission: S12_abs = 1- S11_abs

This is my first idea but I'm not sure if it is right

one way to increase the mesh just between the splits is to insert an block (air block) between the split. And you can locally define the mesh properties of the air block. This way you can obtain 4-mesh cells in the split area.

The formula you wrote may not be applied to open domain problems. But you can obtain the reflection. Insert an E-field probe infront of the structure. Run the simulation. The result of the probe will be reflected field+incident field. Lets call this Etot. Then run a free space simulation (You can do this by setting the all the objects as vacuum). Lets call the result of the probe for free space simulation as Efree. Then the reflected signal will be Eref=Etot-Efree. Then the S11 will be Eref/Efree.


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