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Number of cells per wavelength

Question asked by rinc on May 28, 2014
Latest reply on Jun 2, 2014 by volker_muehlhaus
Good morning everyone,
here i have a question about "Number of cells/wavelength" option in EM simulation.
I already read documents about the Mesh options in EM simulation. From documents, as you increase the number of cells/wavelength more, you get more accurate results. In this case, i assume to neglect the time of simulation, the main aim is the accuracy of simulation.
I made a test with this options, here it arises a question for me.
In my work, a 50 Ohm TL with RO4003c, conductor thickness = 17um, sub. thickness = 0.504 mm, at 2.45 GHz is 1.12mm ( this is a real dimension in LAB). From this information, i adjust the er of RO4003C as er=3.62. Then, i simulate this physical width by EM simulation.
1.case: 80 cells/wavelength: S11: -40dB
2.case: 180 celss/wavelength: S11: -55dB
in both cases, it is "matched", and 180 cells/wavelength give better results. At this step, you can conclude that: 80cells/wavelength is enough, because in reality for really good calibration, VNA only gives best result of -65dB (this is the best result we got in LAB), so we cannot distinguish the differences.
Now, i apply this method into my real circuit, the 50Ohm Coupler.
Figure1: coupler with 80cells (Bold lines is 80 cells, and thin line is 180cells)
Figure2: coupler with 180cells

As you can see, now the difference is dramatically visible. We care about the matching condition (S33), coupling (S31), and isolation(S41) of Coupling branch. There is no change in S31, but S33 and S41 change alot. For 180 cells, S33 is matched, but with 80 cells, in my opinion, it is not matched.
So, my question is, which values should we rely on?
I attached my workspace for you if you need some checks
Best Regards,
Rinc  

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