hfss eigenmode

Hello
I am using HFSS 10.0 to find out the resonant frequency of certain transmission line resonator in a multilayer substrate. kind of a stripline structure, i.e., the substrate's top-most and bottom-most surface are covered with good thin conductor as ground; the remaining 4 surfaces are just exposed to surrounding air.

I believe that the 'eigenmode' solution could help me do it.

So I draw the resonator and substrate and assigned the top and bottom surface to be silver. Then I draw a much bigger box (10 times the substrate's dimension in all x, y, z direction) to enclose the substrate. I assign the materials of the big box as 'air'

Then I simulate it with eigenmode solution. I got 5.2GHz as the 1st resonant freq

HOWEVER, when I reduce the big box's size to around 5 times of the substrate's xyz dimension, I got 10.2GHz!!

So what went wrong?

Please please help me!

Thanks

Adun

I thought what you got is the air box (cavity) resonance. For you transmission line resonance, you may try to use waveport or lumpedport to excite your structure and observe resonance.

Regards,