What Fresnel zone clearance percentage is required?

The required clearance depends on the effective Earth radius factor k (which changes with atmospheric refractivity gradient). Under standard conditions (k = 4/3), 60% of the first Fresnel zone clearance provides negligible diffraction loss (less than 0.5 dB). Most design practices require 100% F1 clearance under k = 4/3 to provide margin for adverse propagation conditions. For worst-case subrefraction (k = 2/3, common in continental climates), the clearance should be at least 60% of F1 even with the effective Earth bulge increased. Some operators require 100% clearance under k = 2/3 for high-reliability paths (99.999% availability). Path profiles should also account for tree growth (2 to 5 meters over 20 years), building construction, and seasonal variations.

How does Earth curvature affect clear zone calculations?

Over distances greater than a few kilometers, the Earth's curvature adds height to obstacles along the path. The Earth bulge at the midpoint of a path is approximately h = d^2 / (12.75 * k) meters, where d is the path length in kilometers and k is the effective Earth radius factor. For a 30 km path under standard atmosphere (k = 4/3), the Earth bulge at midpath is 52.9 meters. Under subrefraction (k = 2/3), it increases to 105.9 meters. This dramatically affects tower height requirements: to maintain 100% F1 clearance over a midpath hill, the combined tower heights must overcome the Earth bulge, the hill elevation, the Fresnel zone radius, and any additional margin. Path planning software (Pathloss, EDX, Google Earth with terrain data) automates these calculations using digital elevation models.

Propagation & Link Design

Clear Zone

Q: How is the first Fresnel zone radius calculated?

The first Fresnel zone radius at any point along a path is: F1 = 17.32 * sqrt(d1*d2 / (f*d)), where F1 is in meters, d1 and d2 are distances from the point to each end of the path in kilometers, d is the total path length in kilometers, and f is the frequency in GHz. The maximum radius occurs at midpath (d1 = d2 = d/2), giving F1_max = 8.66 * sqrt(d/f). For example, a 20 km path at 6 GHz has a maximum Fresnel zone radius of 15.8 meters. At 18 GHz, the same path has a radius of 9.1 meters. Higher frequencies require less physical clearance but are more sensitive to obstructions entering the smaller zone.

/kleer zohn/

The obstruction-free area around a line-of-sight microwave path required for reliable propagation, defined by Fresnel zone theory. The first Fresnel zone is an ellipsoidal volume where reflected signals arrive within λ/2 of the direct path. Adequate clearance of 60 to 100% of the first Fresnel zone radius ensures diffraction loss from terrain, buildings, and vegetation remains negligible. Clear zone analysis determines tower heights, path reliability, and is fundamental to microwave link planning.

Category: Propagation & Link Design

Target: ≥60% F1 (standard)

Standard k: 4/3

Understanding Clear Zone

Electromagnetic waves do not travel in infinitely thin rays but spread out as they propagate, occupying a volume described by the Fresnel zones. The first Fresnel zone is the ellipsoid where all reflected paths are within one half-wavelength of the direct path, contributing constructively to the received signal. If an obstruction penetrates the first Fresnel zone, it diffracts energy away from the receiver, causing signal loss. The diffraction loss is negligible when at least 60% of the first Fresnel zone is clear, approximately 6 dB when the obstruction just grazes the line of sight (0% clearance), and increases rapidly with deeper obstruction.

For microwave link engineers, clear zone analysis is the first step in path design. Using digital elevation model (DEM) data and path profile tools, the engineer plots terrain elevation, tree heights, and building heights along the path, then overlays the Fresnel zone ellipse to identify critical clearance points. Tower heights at each end are adjusted until adequate clearance is achieved under both standard (k = 4/3) and worst-case (k = 2/3 for subrefractive conditions) Earth radius factors. The Earth's curvature adds significant apparent height to midpath obstacles: for a 30 km path, the Earth bulge at midpoint is 53 m under standard atmosphere and 106 m under subrefraction. Higher frequencies require smaller physical Fresnel zones (F1 ∝ 1/√f), allowing lower towers but with less margin for atmospheric variation.

Fresnel Zone Calculations

First Fresnel Zone Radius:
F₁ = 17.32 · √(d₁·d₂ / (f·d)) [m]

Maximum F1 (at midpath):
F_1,max = 8.66 · √(d / f) [m]

Earth Bulge (at midpath):
h_bulge = d² / (12.75 · k) [m]

Where d = total path (km), d₁/d₂ = partial distances (km), f = frequency (GHz), k = effective Earth radius factor (standard: 4/3, subrefractive: 2/3).

Clearance Examples at Midpath

Path (km)	Frequency	F1 Max (m)	Earth Bulge k=4/3 (m)	Bulge k=2/3 (m)
10	6 GHz	11.2	5.9	11.8
20	6 GHz	15.8	23.5	47.1
30	6 GHz	19.4	52.9	105.9
20	18 GHz	9.1	23.5	47.1
20	38 GHz	6.3	23.5	47.1

Common Questions

Frequently Asked Questions

How is the first Fresnel zone radius calculated?

F₁ = 17.32 · √(d₁·d₂/(f·d)). For a 20 km path at 6 GHz, midpath F1 = 15.8 m. At 18 GHz, 9.1 m. Higher frequencies need less physical clearance but are more sensitive to obstructions entering the smaller zone.

What clearance percentage is required?

60% F1 under standard k=4/3 provides negligible diffraction loss (<0.5 dB). Most designs require 100% F1 at k=4/3 for margin. High-reliability paths require 60%+ F1 even under k=2/3 subrefraction. Account for tree growth (2 to 5 m over 20 years) and future construction.

How does Earth curvature affect calculations?

Midpath bulge = d²/(12.75·k). A 30 km path: 53 m at k=4/3, 106 m at k=2/3. Tower heights must overcome bulge + terrain + F1 radius + margin. Path planning software (Pathloss, EDX) automates these using DEMs.

Related Terms

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