When to use: Use for point-to-point microwave and Wi-Fi bridge links to determine how much physical clearance is needed above obstacles for a true line-of-sight path. The first Fresnel zone must be at least 60% clear of obstructions for the link to perform close to free-space. Obstructions inside the first zone cause signal diffraction loss. Includes earth-bulge correction for links over 5 km. Applies to rooftop-to-rooftop, tower-to-tower, and building-to-building links.
The Fresnel Zone Calculator computes the first and second Fresnel zone radii at the midpoint of a point-to-point radio link, determines the required 60% clearance distance, and estimates earth bulge for long paths — giving engineers the essential data to determine minimum antenna heights for microwave backhaul, Wi-Fi bridges, and public safety point-to-point links.
The nth Fresnel zone radius at the midpoint of a link is given by: R_n = 17.32 × √(n × D_km / (4 × f_GHz)) meters, where D_km is the total path length in kilometers and f_GHz is the frequency in GHz. This simplified midpoint formula assumes equal distances from each end (d1 = d2 = D/2). For obstructions not at the midpoint, the general formula is R_n = √(n × λ × d1 × d2 / (d1 + d2)).
The required 60% clearance is simply 0.6 × R1, the minimum vertical distance that any obstruction along the path must be below the line of sight. Earth bulge at the midpoint is calculated as d² / (8R) where d is the path length and R is the earth radius (6,371 km), giving bulge in meters. For accurate design, minimum antenna height = obstruction height + earth bulge + 60% × R1.
ITU-R P.530-17 is the primary international standard for point-to-point link design, specifying the 60% first Fresnel zone clearance criterion. FCC Part 101 licensing for fixed microwave services requires path profile analysis demonstrating adequate terrain clearance. For public safety communications systems, APCO and TIA recommend the 60% clearance rule as minimum, with 100% clearance for high-reliability links. NFPA 1221 does not directly address point-to-point microwave design but references TIA-TSB-88 for system performance benchmarks.
The first Fresnel zone radius scales as the square root of wavelength and path length, meaning lower frequencies require much larger clearances than higher frequencies. A 150 MHz link over 2 km has an R1 of approximately 47 m at the midpoint, while a 5.8 GHz link over the same path has an R1 of only 7.5 m. Trees present a unique challenge because crown height and shape change with season — always add 3–5 m of margin above measured tree height. For paths over 15 km, the K-factor (effective earth radius multiplier) varies with atmospheric conditions and can cause the effective earth bulge to double in superrefractive conditions, requiring additional antenna height.
Enter the frequency in MHz (e.g., 5800 for 5 GHz Wi-Fi, 800 for P25, 11000 for 11 GHz microwave). Enter the link distance and select the unit (km, m, or mi). Enter the TX and RX antenna heights above ground level. The calculator returns the first and second Fresnel zone radii, the required 60% clearance distance, earth bulge at the midpoint, and a recommended minimum antenna height. Use the Fresnel Zone Simulator for a full visual path profile with an adjustable obstruction position.
The first Fresnel zone is an ellipsoidal region around the line-of-sight path. Radio waves that travel through this zone arrive within half a wavelength of the direct path, adding constructively to the received signal. When at least 60% of this zone is unobstructed, the link achieves approximately free-space propagation. Clearing less than 60% allows terrain or structures to diffract the signal and reduce received power.
If the top of an obstruction exactly touches the line of sight (0% Fresnel clearance), the diffraction loss is approximately 6 dB compared to free space. If the obstruction rises above the LOS line, diffraction loss increases steeply — 10 dB for an obstruction 0.3×R1 above LOS, 15–20 dB for 0.7×R1 above LOS. These losses must be added to the link budget.
The second Fresnel zone radius defines a region where waves arrive between 0.5 and 1.0 wavelengths behind the direct path, which means they partially cancel the direct signal. Clearance between the obstacle and the second Fresnel zone boundary should also be verified in high-performance link designs, though the primary design criterion is always 60% of the first zone.
Enter 5800 MHz for 5 GHz Wi-Fi, set the distance in meters (or km), and enter the parapet or antenna height above the rooftop. The earth bulge is negligible for distances under 500 m. The required clearance shown is the minimum height above any obstacle at the midpoint. If there are buildings in between, verify clearance at the position of each building, not just the midpoint.
No — the Fresnel zone geometry is reciprocal. The same clearance requirement applies regardless of which end is transmitting and which is receiving. This is a consequence of the reciprocity principle in electromagnetic propagation (Lorentz reciprocity), which states that the channel response is identical in both directions at the same frequency.
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