Fresnel Zone Radius and 60% Clearance
The line-of-sight clearance a point-to-point wireless link needs: the Fresnel zone radius and the 60% rule.
Example
You enter
- Frequency (GHz) 2.4
- Distance end 1 to obstruction (km) 2.5
- Distance end 2 to obstruction (km) 2.5
- Zone number (1 = first) 1
You get
- First zone radius m 12.5
- 60% clearance needed (first zone) 7.5 m
Checked against published worked examples from: Fresnel zone (higher-zone cross-check).
Details, formula, and sources
ScopeRadio energy travels through an ellipsoidal zone around the sightline, so an obstruction technically below the line still robs signal by diffraction. The first-zone radius at the obstruction is r1 = 17.32 sqrt(d1 d2 / (f_GHz D)), with d1 and d2 the distances (km) from each end to the obstruction and D their sum, largest at midspan: a 2.4 GHz 5 km link has a 12.5 m first-zone radius at its midpoint. The rule of thumb keeps at least 60% of the FIRST Fresnel zone clear of terrain, buildings, and trees (7.5 m here); less than that and the link loses margin even with a nominally clear sightline. Higher zones are reported for reference. Earth curvature (midpath bulge = D^2/(8kR), R the earth radius) and antenna height are added to the clearance budget separately. Per ITU-R P.526. A planning geometry; the path survey governs.
Formula or tabler_n(m) = 17.32 sqrt(n d1 d2 / (f_GHz D)), d1/d2/D in km, D = d1 + d2 (17.32 = sqrt(300) from r_n = sqrt(n lambda d1 d2 / D)); recommended clearance = 0.6 x first-zone radius.
Source and editionThe Fresnel zone radius and the 60%-of-first-zone line-of-sight clearance rule, per ITU-R P.526, by name; first-principles diffraction.
Free accessThe Fresnel-zone geometry is public first-principles physics and ITU-R P.526 is a free ITU download; the frequency and obstruction geometry are the user's link inputs.
What governsEstimate. AHJ and licensed electrician govern. Verify against the NEC edition adopted in your jurisdiction.
API field namesfrequency_ghz, d1_km, d2_km, zone_number, first_zone_radius_m, clearance_60pct_m
- Fresnel radius r_n = 17.32 sqrt(n d1 d2 / (f_GHz D)), km and GHzITU-R P.526 / Fresnel geometry
- 60% rule keep >= 60% of the first (n=1) zone clear; earth curvature and antenna height are separateRF path-survey practice