Animated cross-section of a point-to-point link. Shows 1st and 2nd Fresnel zone radii, earth bulge, terrain obstruction, and required antenna height for 60% 1st Fresnel clearance.
The Fresnel Zone & LOS Clearance Simulator draws an animated cross-section of a point-to-point radio link showing the first and second Fresnel zone boundaries, earth bulge, antenna heights, and terrain obstructions, making it straightforward to verify whether a proposed link achieves the required 60% first Fresnel zone clearance. It is used for microwave backhaul design, Wi-Fi bridge planning, and public safety point-to-point radio links.
The nth Fresnel zone radius at a point along the path is: R_n(d1, d2) = β(n Γ Ξ» Γ d1 Γ d2 / (d1 + d2)), where d1 and d2 are the distances from each end to the point in meters, and Ξ» is the wavelength in meters (Ξ» = 300 / f_MHz). At the midpoint where d1 = d2 = D/2, this simplifies to R1 = 0.5 Γ β(Ξ» Γ D). The clearance of an obstruction is the vertical distance between the LOS line and the top of the obstruction. The rule is: clearance must be β₯ 60% of R1 at the point of the obstruction for the link to behave approximately as free-space.
Earth bulge at the midpoint is approximated by h = d1 Γ d2 / (2 Γ Re), where Re is the effective earth radius (8,497 km for the standard 4/3 earth model). Earth bulge is significant for links over 5 km and must be added to the effective obstruction height. Links with less than 20% first Fresnel clearance incur significant knife-edge diffraction loss (6β20 dB).
ITU-R P.530 provides the authoritative guidance on Fresnel zone clearance for fixed point-to-point links, including the 60% clearance criterion. ITU-R P.834 covers tropospheric and diffraction effects for links below 3 GHz. For microwave backhaul links in the 6β11 GHz bands, the 4/3 effective earth radius model is standard per ITU-R P.453. FCC licensing for point-to-point microwave links (Part 101) requires a path profile analysis demonstrating adequate clearance above terrain and buildings. Public safety microwave links are also governed by TIA-TSB-88 propagation engineering guidelines.
The 60% clearance criterion applies for reliable LOS propagation under normal atmospheric conditions. For high-availability links requiring 99.999% uptime (40 dB fade margin), full clearance of the first Fresnel zone (100%) is recommended. Trees are particularly problematic because their effective height varies by season and wind β add 3β5 m to the measured tree height when calculating clearance. At frequencies above 10 GHz, rain attenuation becomes a dominant factor for links over 5 km and must be accounted for separately using the ITU-R rain model. For obstructed paths, knife-edge diffraction loss can be estimated using the Huygens-Fresnel diffraction method or the Roger Bullington simplified method.
Select the operating frequency from the dropdown. Use the Link Distance slider to set the total path length in km. Set Antenna A and Antenna B heights in meters above ground level (AGL). Set the obstruction height and its position along the path as a percentage from Site A. The diagram updates immediately to show whether the obstruction clears 60% of the first Fresnel zone. The clearance verdict at the top shows the actual clearance in meters and as a percentage of the first Fresnel radius. If clearance fails, increase both antenna heights until the verdict changes to ACHIEVED β the required minimum antenna height is shown in the Link Analysis table.
Theoretical free-space path loss is achieved when approximately 60% of the first Fresnel zone is unobstructed. This is because the contributions of the outer Fresnel zones partially cancel each other β clearing only the innermost 60% of the zone already captures most of the available signal energy. Full clearance (100%) provides only about 0.4 dB of additional signal over 60% clearance in ideal conditions.
Obstructions that penetrate into the first Fresnel zone cause diffraction loss. The amount of loss depends on how far the obstruction extends into the zone. An obstruction at exactly the LOS height (0% clearance) causes approximately 6 dB of diffraction loss. An obstruction above the LOS line by 0.6ΓR1 causes about 0 dB additional loss. Full blockage of the first Fresnel zone can add 15β25 dB of loss.
Lower frequencies have larger first Fresnel zones. At 150 MHz over a 5 km path, R1 at the midpoint is approximately 70 m. At 5.8 GHz over the same path, R1 is only 11 m. This is why VHF/UHF links are more tolerant of obstructions while microwave links in the GHz range are much more sensitive to terrain and building clearance.
Earth bulge is the apparent rise of the earth's surface above the straight LOS line due to the curvature of the earth. Using the standard 4/3 effective earth radius (K = 1.33), the bulge at the midpoint of a link is h = d1 Γ d2 / 17 km, where d1 and d2 are in km and h is in meters. For a 10 km link, midpoint bulge is approximately 5.9 m β a significant addition to effective obstruction height. Earth bulge is negligible for links under 2 km.
No β this simulator models outdoor point-to-point links with LOS geometry and earth curvature correction. In-building propagation involves multi-path reflections, diffuse scattering, and frequency-dependent penetration losses through walls and floors that are not captured by the Fresnel zone model. Use the RF Propagation Simulator or Antenna Coverage Radius calculator for in-building coverage analysis.
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