what is EBG structure?

Its full name is "electromagnetic band gap". It seems to be some patterning on the backside metal.

How does it work? Why it is called "band gap"?

Any explantion is much appreciated.

http://www.ursi.org/Proceedings/Proc...pers/p0236.pdf
http://www.umc.edu.dz/vf/images/emc/...rs/147_207.pdf
here are some links for its meaning.

Have a look at this presentation:
http://cc.ee.ntu.edu.tw/~jfkiang/ele.../ch12_2005.pdf