What is Fresnel Zone in Microwave Radio Link Communication.

Radio waves travel in a straight line, unless something refracts or reflects them. But the energy of radio waves is not “pencil thin.” They spread out the farther they get from the radiating source — like ripples from a rock thrown into a pond.
The area that the signal spreads out into is called the Fresnel zone (pronounced fra-nell). If there is an obstacle in the Fresnel zone, part of the radio signal will be diffracted or bent away from the straight-line path. The practical effect is that on a point-to-point radio link, this refraction will reduce the amount of RF energy reaching the receive antenna.
The thickness or radius of the Fresnel zone depends on the frequency of the signal — the higher the frequency,the smaller the Fresnel zone.