Fresnel Zone Calculator

Q: What is the Fresnel zone?

The Fresnel zone is an elliptical region around the line-of-sight path between two antennas. Radio waves that pass through this zone can cause constructive or destructive interference. The first Fresnel zone (F1) is the most critical — obstructions within it cause signal degradation.

Q: How much Fresnel zone clearance do I need?

For reliable outdoor wireless links, at least 60% of the first Fresnel zone (F1) radius must be clear of obstructions. Ideally, 100% clearance is recommended. Trees, buildings, and terrain within the first Fresnel zone significantly reduce link quality.

Q: Why is Fresnel zone clearance larger at lower frequencies?

The Fresnel zone radius is inversely proportional to the square root of frequency. Lower frequencies (e.g. 900 MHz) have much larger Fresnel zones than higher frequencies (e.g. 5.8 GHz). This means a 900 MHz link needs more physical clearance above obstacles than a 5.8 GHz link at the same distance.

Calculate the first Fresnel zone radius and minimum recommended clearance for outdoor wireless links. Use this before installing point-to-point antennas to verify line-of-sight requirements.

Calculate Fresnel Zone Radius

Total Link Distance

Distance Unit

Frequency (GHz)

Obstruction at (% of total distance from TX)

Fresnel Zone Results

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F1 Radius at Obstruction

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Recommended Clearance (60%)

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Distance from TX

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Distance from RX

Formula

The first Fresnel zone radius at any point along the path is calculated as:

F1 radius (m) = 17.32 × √( d₁ × d₂ / (f × D) ) Where: d₁ = distance from TX to obstruction (km) d₂ = distance from obstruction to RX (km) D = total link distance (km) f = frequency (GHz)

The F1 radius is largest at the midpoint of the link and decreases toward each end.

Clearance Guidelines

100% F1 clearance: Ideal. Full line-of-sight with no obstructions in the Fresnel zone.
60% F1 clearance: Minimum recommended. Acceptable for most fixed outdoor links.
Below 60% F1: Expect signal degradation. Additional link margin or antenna height is needed.
Trees are particularly problematic — they sway in wind and absorb moisture, causing variable and unpredictable signal loss.
In hilly terrain, always check the Fresnel zone at each ridge, not just the midpoint.

Practical Example

A 5 km link at 5.8 GHz, checking the midpoint (2.5 km from each end):

F1 = 17.32 × √( 2.5 × 2.5 / (5.8 × 5) ) F1 = 17.32 × √( 6.25 / 29 ) F1 = 17.32 × 0.464 = 8.04 m 60% clearance = 4.82 m above the obstacle

This means any obstacle at the midpoint must be at least 4.82 m below the antenna line-of-sight path.

Frequently Asked Questions

What is the Fresnel zone?

The Fresnel zone is an ellipsoid around the direct line-of-sight between two antennas. Radio signals travel multiple paths, and signals reflecting off objects within the Fresnel zone can arrive out of phase with the direct signal, reducing link quality. Keeping the zone clear prevents this destructive interference.

How much Fresnel zone clearance do I need?

A minimum of 60% of the first Fresnel zone radius must be clear of all obstructions for adequate link performance. 100% clearance is preferred. If clearance is not achievable, you need to raise antenna heights, use a repeater, or accept reduced link margin.

Why is Fresnel zone clearance larger at lower frequencies?

The Fresnel zone radius is proportional to the square root of wavelength. Lower frequencies have longer wavelengths and therefore larger Fresnel zones. A 900 MHz link at 5 km has an F1 radius of around 40 m at the midpoint, while a 5.8 GHz link at the same distance has an F1 radius of only about 8 m.