How much clear space a point-to-point link needs around its line of sight. Enter the distance and frequency to see the first Fresnel zone and every step of the calculation.
Radius of the 1st, 2nd and 3rd Fresnel zones. The 1st zone is the one that matters for planning.
Radio energy doesn't travel only along the straight line between two antennas: it spreads through an elongated, ellipse-shaped region around it. The first Fresnel zone is the part of that region where a signal bouncing off something arrives less than half a wavelength later than the direct signal. Obstacles inside it, such as trees, roofs, hills or the ground, reduce the signal even when the two ends can still see each other.
Keeping at least 60% of the first zone's radius free of obstacles is a common rule of thumb for point-to-point links: with that much clearance, the extra loss is small. Many designers aim for more where they can.
The zone is widest in the middle and shrinks to nothing at each antenna. It grows with the square root of the distance and shrinks with the square root of the frequency: twice the distance needs about 1.41 times the clearance, and a 5 GHz link needs less than a 2.4 GHz link over the same distance.
Over longer links the ground itself rises between the two ends. With the usual standard-atmosphere assumption (an effective Earth radius 4/3 of the real 6,371 km) that rise is d1 × d2 ÷ (2 × k × R). It's tiny below about 1 km and grows quickly after that. For the signal loss over the same distance, see the Free Space Path Loss tool.