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Knife-Edge Diffraction Calculator

Estimate the loss one sharp obstruction adds to a radio path, and how much of the first Fresnel zone it leaves clear.

Frequency in MHz (2400 means 2.4 GHz). Distances in kilometres: from the first antenna to the obstruction, then from the obstruction to the second antenna.

Metres above the straight line between the two antennas. Use a positive number when the obstruction rises into the path and a negative number when it sits below the line.

The loss follows the single knife-edge approximation of ITU-R P.526: the edge is reduced to the Fresnel–Kirchhoff parameter v from its height and the two leg lengths, and the diffraction loss is added to the free-space loss over the whole path. A negative height below the line gives a negative v, and once the edge sits 78 percent of the first zone or more below the line no loss is added. The page assumes a single sharp edge, flat ground, no atmosphere or terrain profile, and antennas that are small compared with the zone; it does not model rounded ridges, multiple edges, vegetation or ground reflection. Everything is worked out in your browser; nothing is sent or stored.