It is known that a generic harmonic electric field which propagages in free space has an elliptical polarization (if you want to read the proof, look at this, slides 72 - 99).
My question is: what about the polarization of E and H fields in a waveguide or a transmission line? I have always read about elliptical, circular and linear polarization for electromagnetic waves in free space, but I have never heard/found something referred to guided waves. So, I'll say that waves in waveguides are simply linearly polarized. But:
I do not know if this is true. I have never read also the sentence "Waves in waveguides have linear polarization".
I do not understand why we cannot have elliptical polarization in waveguides. As you may see from the slides is the consequence of the harmonic behaviour of E and H fields, which is described by the wave equations. In theory, it is true also for waveguides...