Millimeter waveguides with rounded inside?

[Plasmateur]

I'm looking to purchase a coax to waveguide adapter and noticed some of the adapters have rounded insides

found here: https://www.sagemillimeter.com/wr-42-waveguide-to-k-f-coax-adapter-right-angle/

while other have the square inside that I'm used to

found here: https://www.sagemillimeter.com/wr-28-waveguide-to-k-f-connector-adapter-right-angle-thermal-vacuum-safe/

Does anyone know what the major difference is between these two?

[ogden]

I'm looking to purchase a coax to waveguide adapter and noticed some of the adapters have rounded insides

found here: https://www.sagemillimeter.com/wr-42-waveguide-to-k-f-coax-adapter-right-angle/

while other have the square inside that I'm used to

found here: https://www.sagemillimeter.com/wr-28-waveguide-to-k-f-connector-adapter-right-angle-thermal-vacuum-safe/

Does anyone know what the major difference is between these two?

Flex waveguides are not strictly rectangular, yet usually their adapters are.  To minimize loss or cut the costs, some flex waveguide may omit stepped transition and have adapter shape that matches waveguide - oval. You do not want adapter shape mismatch without proper transition because it will lead to some (minimal) loss. So - avoid rounded/oval adapters because they are rare