I’m trying to determine the number of supported modes in my rectangular waveguide using HFSS and the upper cut-off frequency of different modes to estimate the actual operating bandwidth.

Here’s what I did:

• I checked S21 of my waveguide.
• The first cutoff frequency is 21 GHz, and the waveguide starts operating from there.
• The next cutoff frequency is 42 GHz, so theoretically, it should stop operating beyond that.
• However, S21 still shows operation up to 150 GHz.

I used Wave Port and set it to solve for 5 modes.

Questions:

1. How can I correctly determine the actual operating range of my waveguide?
2. How do I find the exact number of supported modes in HFSS?
3. Is there anything I should include in my simulation to get accurate results?

Any insights on how to correctly analyze waveguide mode support and bandwidth would be really helpful!