Discussion Forum

Hello,

I am new to Comsol. I am performing an eigenfrequency study on a rectangular microwave cavity, and I need it to resonate close to a specific frequency. I was able to find a geometry that was close enough, but when I was looking into how to run a frequency domain study, I saw that I need to set the "eigenfrequency search method around shift" to larger real part, which apparently gets rid of rigid body modes. I don't understand how it does this, since all it does is show eigenmodes that are larger than the initial frequency I gave it to look for. There are only 4 eigenmodes smaller than that frequency, aren't there usually 6 rigid body modes? Moreover, the frequency I am talking about is at 9.1918 + 0.00455i GHz, which seems too large to be a rigid body mode. Why else is it important to specify that the mode be larger than the starting frequency? Are the others unphysical? The mode which is important to my study is slightly smaller than the original frequency. If I change the geometry so that it has a larger real part, will this have some sort of physical significance when I actually build the cavity? Again, I don't understand why it matters if it is slightly smaller or larger. It shows up when I set it to smaller real part or closest in absolute value.

Sorry for the long post, I hope I've made it clear enough.