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I'll respond to this onlyin part. If ohmic lossess in the waveguide walls aresmall, don't try to use the computed S parameters (which are computed only using the fields at the ports) to extract them, since numerical noise (due to the finite discretization) can wash out such small changes relative to unity. Instead, compute the thermal loss all along the path by integrating the surface losses on the walls. You can do this in either post-processing (Derived values -> integration -> surface integration) or by setting up a surface integration probe. Either way, integrate the quantity emw.Qsh over the lossy surfaces. Compare that to your input power (typically set to 1W, if you use the default value) and you'll immediately derive the conductive losses, and with greater confidence. I realize that this does not distinguish between losses in each mode present, if more than one mode is present. But I'll leave that question for others here who may wish to offer additional advice. Good luck.

p.s. You can also compute the losses in a straight rectangular waveguide, by mode, analytically. Refer to any decent EM theory / waveguide textbook. Many good ones have been written, and many still sit covered with dust on library shelves, going back to (roughly) the 1930s, although I mostly prefer the post-WW2 books. :-).

You would expect mode conversion only if there is some sort of disturbance in the waveguide, say a post or some other geometric change. Any TE20 that appears will probably change in amplitude if the mesh size, etc. is varied.

I agree that a reliable value for attenuation can't be calculated from Comsol S parameters, at least for highly conducting walls.

My favorite reference for these things is an old copy of Ramo Whinnery and Van Duzer obtained from a former colleague. For WWII era- the MIT Radiation Lab series can be found as a free download.

Thanks,

Do you think that bending the waveguide could also accentuate Ohmic losses, or just excite mode conversion?

Thanks,

Do you think that bending the waveguide could also accentuate Ohmic losses, or just excite mode conversion?

My intuition is that a bend will have additional ohmic loss (intuition easily checked by simulation). And that the bend will excite some higher modes.

By the way- higher order modes may be above cutoff. If so- they will be exicited by a disturbance but they decay exponentially with distance (evanescent modes) and are unlikely to be observed unless a port is quite close.

There exist published papers that address the subject of losses in waveguides with bends. You might want to do a search at IEEE Explore -- see ieeexplore.ieee.org/Xplore/home.jsp , for phrases such as "loss in waveguide bend".