What do Fresnel zone ellipsoids represent in antenna transmission?

[unseensoul]

I know fresnel zone is related to ellipsoids but what are these about? I was told they represent most of the energy transmitted by an antenna to another antenna. However, this makes no sense to me. Apparently, the width of the ellipsoid increases if the wavelength does so as well which makes it even more confusing. What do these ellipsoids actually represent? Why not another shape such as a rectangle?

 

[unseensoul]

Ok, I finally got it. This link http://www.zytrax.com/tech/wireless/fresnel.htm provides a very good explanation of it for those interested.