Fresnel Zone Radius & Clearance
The first Fresnel zone is the ellipsoid around the direct line of sight that carries most of the RF energy between two antennas. Obstructions inside it cause diffraction loss, so a good link keeps at least 60% of the first zone clear. Enter the frequency and the distances from each endpoint to the point of interest (an obstacle or the mid-path) to get the zone radii and the clearance target.
Equations & Parameters ▸
\(r_n = \sqrt{\dfrac{n\,\lambda\,d_1 d_2}{d_1 + d_2}} \qquad \lambda = \dfrac{c}{f} \qquad \text{clearance} \ge 0.6\,r_1\)
| f | Link frequency (MHz). |
| d1, d2 | Distances (km) from each antenna to the point along the path being checked. The total path length is d₁ + d₂. |
| rn | Radius of the n-th Fresnel zone at that point (metres). r₁ is largest at mid-path. |
| 0.6 r₁ | Recommended minimum clearance; keeping this fraction unobstructed gives essentially free-space loss. |
References: J. D. Parsons, The Mobile Radio Propagation Channel, 2nd ed., Wiley, 2000. · ITU-R P.526, Propagation by diffraction.
Inputs
MHz
Link frequencykm
From antenna 1km
From antenna 2Results
Zone radii
1st zone, r₁—
2nd zone, r₂—
3rd zone, r₃—
Clearance
60% of r₁ (min.)—
Path length—
Diagram