Hello,
I need to simulate a quad-ridged waveguide in CST. At first, I set settings to excite the first six modes in the waveguide and I see the S21 for the first mode is less than -18dB at all frequencies.
After that, I set the modes to one and I started the simulation again, but at this step, I see the S21 parameter started at -23dB and ended at -2 dB.
I need to know why CST shows me different S21 values in these two ways of calculation for the first mode.
Best regards,
I don't know what is happening with your simulation. Have you looked at the mode patterns that are giving you these results? They are in 2D/3D results and you can see the mode patterns at one frequency.
If the modes aren't what you expect you can enforce some modes using symmetry planes. I used a horizontal E wall and a vertical H wall to split the problem up into quarters, and the results were what I expected, except below cutoff, which was recently asked about here, and I don't understand the reason for that.
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