Fresnel Zone Radius and 60% Clearance

Radio 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 (bulge = D^2/8k) and antenna height are added to the clearance budget separately. Per ITU-R P.526. A planning geometry; the path survey governs.

r_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.

The Fresnel zone radius and the 60%-of-first-zone line-of-sight clearance rule, per ITU-R P.526, by name; first-principles diffraction.

The 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.

Estimate. AHJ and licensed electrician govern. Verify against the NEC edition adopted in your jurisdiction.

Field names used by the API: frequency_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