Fresnel Zone? what is it

Can anybody explain what is fresnel zone and give me some url for the diagram of fresnel zone. Thank you. :)

The phenomenon explains why sometimes even line-of-sight itself is not sufficient... The wave reflected by some obstacles can be out-of-phase at RX. Fresnel provided a means to calculate how out of phase the bumps (deflections) between the transmission source (TX) and the receptor (RX) will be. Each Fresnel zone is an ellipsoidal

http://www.zytrax.com/tech/wireless/fresnel.htm

It is a concept of optics, you can get details about it in any physics book. I recommend Resinik Haliday Walker

But I see that the given url is related to wireless, is the optical concept and the wireless application the same ?

The optical concept is better viewed from Gaussian Beams theory.
However, GB approach is usable in RF too :)

eirp

The zones can be categorized into 4 zones:
1)near zone
2)intermediate zone
3)far zone
4)plane-wave zone

The first zone is the closest to the source. It is defined as the region where stored energy is greater than the radiating energy.

The second zone is the region between the 1) and 3).

The plane-wave zone is the region in the far zone where the radiation can be approximated by the plane wave.

In summary, the 4th zone is called Fraunhoffer zone and the combination of the remaining 3 are the Fresnel zone.

T Zul