A Radiospurs Propagation Tools Site
There are thre basic Waveguide types.
Circular Waveguide
The ultra low loss waveguide. If you can keep the TEM going round the bends.
Circular Waveguide
Double Ridge Waveguide
Put a dorsal spine down the rectangular guide and you get a Double Ridged Waveguide. Extra BW here.
Double Ridge Waveguide