Calculate the Fresnel zone radius and the 60% clearance radius for a radio or microwave link from frequency, path length, and obstruction position.
Quick Facts
Formula
r_n = 17.32 × √(n·d1·d2 / (f·D))
r_n in meters; d1, d2, D in km; f in GHz; n = zone number.
Clearance rule
Keep 60% of Zone 1 clear
Below that, diffraction losses grow even with a clear visual sightline.
Results
Calculated
Fresnel zone radius
—
At the obstruction point (Zone n)
60% clearance radius
—
Minimum recommended clear radius
Distance to obstruction
—
From the transmitter (d1)
Distance to receiver
—
From the obstruction (d2)
Add this calculator to your website
How to use this calculator
Enter the link frequency in gigahertz, the total path length between the two antennas in kilometers, how far along that path (as a percent from the transmitter) you want to check for clearance, and which Fresnel zone to calculate — almost always zone 1. Click Calculate to get the zone radius at that point, the 60% clearance radius, and the distances to each end of the link. Click Clear to reset all fields and start a new calculation.
Understanding the inputs
Frequency and path length come from your link budget or site survey. The obstruction position is where along the path a hill, tree line, building, or tower sits — expressed as a percentage of the total distance from the transmitter, so 50% is the midpoint, where the Fresnel zone is widest. The zone number n is almost always 1, since keeping the first Fresnel zone clear is the standard design target; higher zones are rarely used in practice.
Interpreting the results
The Fresnel zone radius is the full radius of the ellipse at that point along the path. The 60% clearance radius is the widely used minimum: as long as the ground or obstruction stays outside that smaller radius, the link should perform close to free-space conditions. The distances to the obstruction (d1) and to the receiver (d2) let you cross-check that the split matches your site geometry.
Frequently Asked Questions
What is a Fresnel zone?
A Fresnel zone is one of a series of concentric elliptical regions around the direct line-of-sight path between a transmitter and a receiver. Radio waves can arrive at the receiver by slightly different paths through these zones, and if an obstruction blocks too much of the first zone, the signal can partially cancel itself and lose strength, even when there is a clear visual line of sight.
What is the Fresnel zone formula?
The radius of the nth Fresnel zone at a point along the path is r_n = 17.32 × √(n · d1 · d2 / (f · D)), where r_n is in meters, d1 and d2 are the distances from that point to each end of the link in kilometers (d1 + d2 = D), f is the frequency in gigahertz, and n is the zone number (n = 1 for the primary zone). The radius is largest at the midpoint of the path and shrinks toward zero at each antenna.
Why is 60% clearance the target?
Engineering guidance for point-to-point radio links calls for keeping at least 60% of the first Fresnel zone free of obstructions such as terrain, trees, or buildings. Below that threshold, diffraction losses grow quickly and the link can degrade even though a straight visual sightline still exists between the antennas.
How does frequency affect the Fresnel zone size?
Fresnel zone radius is inversely proportional to the square root of frequency, so lower-frequency links need much more clearance than higher-frequency ones over the same distance. A 900 MHz link requires a noticeably wider clear zone than a 24 GHz link spanning the same path.