Fresnel Zone Radius Calculator

How it is calculated

The tool gives the radius of the first Fresnel zone at the middle of a radio link, where the zone is widest. With the link distance D in kilometres and the frequency f in gigahertz, r = 8.657 × √(D ÷ f) metres. The constant 8.657 is half of the usual 17.3 factor, because at the midpoint each end is D/2 away. The second result is the recommended clearance, 0.6 × r: the part of the zone that should be free of obstacles. Distance and frequency must both be above zero. The tool only covers the midpoint; the zone is narrower nearer the antennas, so these figures are the worst case along a flat path. They describe the free-space geometry only and do not include the Earth's curvature, which adds to the height you need to clear on long links.

Common mistakes

• Entering the frequency in MHz. The field wants GHz, so 5800 MHz must be typed as 5.8. Typing 5800 gives a radius of only 0.36 m for a 10 km link.
• Entering the distance in metres or miles. The input is kilometres, and a 500 m link is 0.5.
• Clearing only the straight line. An obstacle that does not touch the line of sight can still lie inside the Fresnel zone and weaken the signal; the full radius, or at least 60% of it, must be clear.
• Applying the midpoint figure to the whole path. It is the widest radius, so it is a safe worst case, but near an antenna the zone is narrower and the requirement is smaller.
• Forgetting antenna height and Earth bulge. The result is a radius only; the actual mast height must also cover the terrain and, on long links, the curvature of the Earth.