Fresnel Zone Calculator

A Fresnel zone is an ellipsoidal region surrounding the direct line of sight between a radio transmitter and receiver. Radio energy does not travel only along an infinitely thin ray. Nearby paths can arrive with different phases, causing constructive or destructive interference at the receiver. The first Fresnel zone is the most important region for practical link design, and keeping much of it free from terrain, trees, buildings, and other obstacles helps preserve signal strength. This calculator finds the radius of a selected Fresnel zone at a particular point along the path. It uses the zone number, wavelength, distance from the transmitter to the obstacle, and distance from the obstacle to the receiver. Frequency entered in gigahertz is converted to hertz, and each path segment entered in kilometers is converted to meters. Wavelength equals the speed of light divided by frequency. The radius is the square root of the zone number times wavelength times both distances, divided by their sum. The zone is widest near the midpoint of a link and narrows toward either antenna. That is why the calculator asks for two distances rather than only total path length. On a symmetric two-kilometer link, an obstacle one kilometer from each endpoint lies at the widest point. Moving the same obstacle closer to one endpoint reduces the local Fresnel radius. Lower frequencies have longer wavelengths and require larger clear zones; higher frequencies produce smaller zones for the same geometry. A common planning guideline is to keep at least 60 percent of the first Fresnel-zone radius clear. The result includes that 60 percent value, but it is not a complete antenna-height recommendation. Earth curvature, atmospheric refraction, terrain elevation, vegetation growth, structure movement, and local regulations can require additional clearance. Distances should describe the same obstacle point and should be positive. For professional links, use surveyed path profiles and account for the effective Earth-radius model. This calculator is valuable for preliminary wireless bridge, microwave backhaul, point-to-point Wi-Fi, and amateur radio planning, but field verification remains essential.