The wireless or WiFi signal travels in a shape like an elipse and the frenzel zone is the lower part of the elipse.
Fresnel Zone becomes more important if a factor in long-range point to point links of bridges and backhauls.
Fresnel Zone is an electromagnetic phenomenon, in which light waves or radio signals get diffracted or bent from solid objects near their path. The radio waves reflecting off the objects may arrive out of phase with the signals that traveled directly to the receiving antenna thus reducing the power of the received signal. Therefore you have to have more than visual line-of-sight: You need for the fresnel zone to be clear or the obstructions will block part of your throughput – you will have lower rate of through-put if there is an obstruction in this zone.
Below are examples of the correct and incorrect placement of the antennas/radios on the towers:Author: George Hardesty