RF Module Mode Analysis Study for 3D Metal Cylinder Waveguide

Hello Forum,

Thanks in advance for reading and potentially responding with thoughts. I am trying to model a cylindrical metal waveguide, and in particular, I want to solve for the mode shapes. This is an analytically soluble problem, and so I am trying to reproduce the analytical result. While I know that this can be done with a 2D cross-sectional geometry, the structure I am actually interested in--a helix--can only be modeled in 3 dimensions.

When I run the Mode Analysis study, I get the error:

"The mass matrix is zero".

I have tried configuring the two port boundary conditions in many different ways, including making them a scattering boundary condition. In addition, I have played around with the parameters in the Mode Analysis study mode, all with no luck.

One other piece of useful information is that for this radius cylinder, the cutoff frequency for the lowest mode is about 39.9 GHz, and the cut off for the next modes are: [52.2, 83.1,83.5,118] all in GHz.

Can anybody offer me advice on how to get the mode shapes for a 3D structure, or any more insight into the source of the zero mass matrix error?

Thanks,
Charlie