Fresnelzone

The Fresnel zone is a set of adjacent, ellipsoidal regions around the direct line of sight between a transmitter and a receiver. It is used to analyze how obstructions along or near the path affect the propagation of radio waves, particularly through diffraction. The boundaries between zones are defined such that the difference in travel time (or path length) to the two ends equals an integer multiple of the wavelength, with the first zone being closest to the direct path. Obstructions within the first Fresnel zone have the most influence on signal strength.

The radius of the nth Fresnel zone at a point along the line of sight can be approximated by r_n = sqrt( n * λ * D1 * D2 / D ), where λ is the wavelength, D1 is the distance from the transmitter to the obstruction, D2 is the distance from the obstruction to the receiver, and D = D1 + D2. This formula explains how zone size grows with wavelength and varies with geometry.

In practice, maintaining clearance of the Fresnel zone is important for reliable links. Objects that intrude into the first zone can cause diffraction losses that depend on the obstruction’s size, position, and the operating frequency. Higher frequencies (shorter wavelengths) produce smaller zones, making obstructions relatively more significant. The concept is widely used in planning point-to-point microwave links, broadcast reception, and other line-of-sight radio systems.

The term honors Augustin-Jean Fresnel, whose wave optics work laid the foundation for this concept in the early 19th century.