Some hot forum about Microwave&RF learning？

This forum isn't hot enough? Search for existing discussions or ask what you want to know about.

From the site itself, if not also from it's forum, try the "microwaves101.com" site. It has an expansive encyclopedia reference section devoted to education of microwave concepts and devices with a emphasis on real practical usage, and how to relate design theory to actual devices.
What you get here is lots of expertise and discussion. The other site has web pages of information.