A radio link is determined by the electromagnetic wave propagation between the transmitting and receiving equipment. Such wave transmission depends on some physical phenomena. These include the diffraction of radio waves based on Huygens' principle, the widening of the wave beam of microwaves with greater distance from the transmitter and the possibility of interference with other frequencies.
The Fresnel zone can be considered as the connecting line between the external units of the transmitting and receiving equipment in microwave directional radio. This includes the area surrounding the link line that contributes to wave transmission. Some high- frequency waves follow a direct path from the transmitting equipment to the receiving equipment. Others, however, do not follow the direct link but follow a curved path. Such a curved wave propagation around a central axis is rotationally elliptical.
But there is not only one rotationally elliptical plane, but several, which are arranged around the direct axis of radiation and each is called a Fresnel zone. The size or diameter of the Fresnel zones depends on frequency and distance. As the frequency increases, the diameter of the Fresnel zones becomes smaller, and as the distance between the transmitting and receiving stations increases, it becomes larger.
The path that the waves travel on the rotational elliptical propagation planes is longer than the direct path, which results in a phase shift between the direct beam path and the indirect, curved one. If the phase shift between the direct and the indirect wave is one full wavelength, i.e. 360 °, the two waves add up at the receiving device.
If, during radio propagation, one of the rotationally elliptical Fresnel zones is impaired by topographical terrain, by trees or buildings, then the wave transmitted on this propagation plane will be attenuated. Such impairments should be avoided when setting up radio links and microwave links.