how to find cutoff frequencies of a simple waveguide?

I drew a simple waveguide in HFSS and i analyzed it by eigenmode solution method ! but the resonant frequencies which I see in data table for different modes doesn't match with theory ! what should i do?

use a waveport, look in HFSS help and getting started guides for help

Have Fun

tanx but I think you didnt understand my question ! firstly in eigenmode solution type we are not allowed to use excitations secondly i have drawn a rectangular waveguide with these dimensions 2.286 cm & 1.016 cm . I want to know exact cutoff frequencies first cutoff frequency is about 6.5 GHz and second one should be about 13 GHz but in eigenmode data i cant see these cutoff frequencies ! aren't resonance frequencies the same as cutoff frequencies? where is the problem?

I did understand your question and I encouraged you to look up a waveport in the HFSS help. I can only point the way...

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I'm no HFSS expert, but here is the view of someone who has used HFSS a little.

You don't want to be using the eignenmode solver since that is for finding resonances. You are not trying to find resonances.

You are trying to find cutoff frequencies. So draw a section of waveguide, put a waveport at each end, and find what frequencies propogate and what do not. That will show you the cutoff frequenciies. When setting up the waveport, you I believe specify the number of modes it will look for. Obviously push that to more than one.

I'm sure one of the HFSS experts will correct me if I'm wrong, but I think your basic problem is trying to use the eignenmode solver, which is for something different to what you actaully want.

Dave

Drkirby is overly humble :) I would do exactly as he says for the mode information.

Have Fun.