What are Fresnel zones and how much clearance is needed?
Fresnel zones are ellipsoidal regions around the direct path between two antennas in which secondary paths differ in length by multiples of λ/2. The radius of the first zone at a point d1 and d2 from the ends is r1 = √(λd1d2/(d1+d2)), or r1 = 17.32√(d1d2/(f·d)) meters with distances in km and f in GHz. Keeping at least about 60% of the first zone clear of obstacles makes diffraction loss negligible.[1][2]
How big is the first Fresnel zone?
At midpath of a 5 km link at 5.8 GHz: r1 = 17.32√(2.5·2.5/(5.8·5)) ≈ 8.0 m. At 2.4 GHz over 1 km, about 5.6 m. Large clearances explain why long links need tall masts even with clear optical line of sight.[1]
Why is 60% clearance used?
Knife-edge diffraction theory shows that when an obstacle tip is about 0.6 of the first-zone radius below the line of sight, the diffraction loss is close to 0 dB; grazing incidence (obstacle at the line of sight) costs about 6 dB.[1]
How does Earth curvature enter?
Path profiles include Earth bulge with an effective Earth radius factor k (4/3 for standard refraction), and links are planned for clearance under unfavorable k values as recommended by ITU-R.[1][2]
Frequently asked questions
Does foliage in the Fresnel zone matter?
Yes; trees cause diffraction and absorption, especially above 1 GHz.
Is the Fresnel zone a beam?
No; it describes propagation, independent of antenna beamwidth.
Do higher frequencies need less clearance?
Yes; the radius scales with √λ.
See also
References
Online sources accessed October 21, 2026; printed works cited by edition. See all sources.
- “Recommendation ITU-R P.526: Propagation by diffraction”. ITU-R. current edition.
- “Antennas and Propagation for Wireless Communication Systems, 2nd ed.”. S. R. Saunders and A. Aragón-Zavala, Wiley. 2007.