square to round waveguide transition

I want to make a square to round waveguide transition. I have plenty of info on rectangular to round and impedance calculations for the transition, but nothing for square.

Are the impedance calculations the same? I am not having much luck with this

Hi,
Maybe it helps: Online calculators help design sheet-metal transitions and cones
http://www.globalspec.com/reference/...ions-and-cones
http://www.docstoc.com/docs/22702283...GUIDE-ADAPTERS
K.