Problem with simulating a circular waveguide in HFSS

I am simulating a circular waveguide in HFSS and I am getting odd results for the s-parameters. The result from graphing gamma shows that the wave will not start propagating before 4GHz . Yet when I graph S11 it is less -50dB below 4GHz. Does anyone know what I am doing wrong.

Hi,
can you tell me, what is diameter of your circular waveguide? It must be wider than 43.96 mm.

It is wider that 43.93mm. I cannot remember the exact dimensions because I do not have the simulation in front of me right now. In the simulation the field patterns all looked right, the imaginary part of gamma looked right when it was plotted for multiple modes. However, when I looked at the s-parameters there were odd results.

Are you exciting the mode in the right way?
I mean does it need circularly symmetric mode like TE11 to excite?

I believe I am. I was using the same excitation as example for the conical horn from the HFSS v10 documentation. Which is define a circular waveport with the integration line going across the diameter.

At first glance your results might seem odd, but they are infact correct. Below the cutoff frequency the evanescent modes quickly decay and thus below 4GHz a very small amount of power makes it anywhere in the waveguide. Therefore the amount of power of these dying modes that get reflected back to the port will be very small as well hence a RL>50dB.