Fresnel zone

In radio transmission, the space between the transmitting and receiving antennas is known as the Fresnel zone. The Fresnel zone is a notional spheroid between the antennas.

The main portion of the energy is transmitted in the area of the Fresnel zone.

This zone should be free of obstructions (e.g. objects, houses, trees etc.). Metal obstacles such as control cabinets, machine parts, pipelines, iron beams etc. particularly hinder the development of the Fresnel zone!

Each hindrance of the Fresnel zone attenuates the transmission. If the Fresnel zone is half obscured, for example, the additional attenuation is 6 dB, i.e. the field strength is reduced to half of the free field value. Reception may then be disturbed or completely interrupted under certain circumstances.

If the Fresnel zone is free from obstructions, the propagating wave is only attenuated by the free space attenuation.