Hi
I am looking into using a circular waveguide for a dual polarized feed and have some thoughts of what diameter I should chose for the circular waveguide. An internet search gives tables for *standard* dimensions for circular waveguides. My frequency band is the commercial backhaul E-band, i.e. 71GHz to 86GHz and the table for circular waveguide dimensions gives some alternatives:
wg dimentions

Then I calculate the cut-off for the lowest modes, I see that several of these waveguides allow the TM01 mode in addition to the dominant TE11 mode. For example, cut-off for TM01 for a ⌀3.18mm waveguide is 72.22Ghz and it is specified 66GHz to 82GHz, i.e. the TM01 is present in the lower half of the frequency band (for some of these bands they have put the lower band limit just above the cut-off the TM01).
My question is why this mode is not considered in general? My simulation indicates that asymmetry in the waveguide (such as misalignment in a joint due to tolerance or the geometry of the OMT) gives a migration of energy from the TE01 mode to the TM11 mode that in turns give rise to cross polarization. I understand that taking this in consideration would reduce the bandwidth, but why is the specified frequency typically centered between the cut-off for the TE11 and the TE01 and with large marginal? I understand that you do not what to be close to the lower cut-off limit due to increased attenuation, but why not be closer to the upper cut-off limit?
In my case I would rather consider a diameter of 2.6mm. This would give me a cut-off for TE11 at 67.62GHz, a cut-off for the TE01: 140.7GHz and a cut-off for the TM01: 88.33GHz.
Thanks for answer
BR
Thomas