Fresnel Zone
Calculate the Fresnel zone radius and clearance needed for a line-of-sight wireless link.
About Fresnel Zone
The Fresnel zone is an ellipsoid-shaped region around the direct line-of-sight path between two antennas; even without a solid obstruction blocking the direct path, an obstacle intruding into this zone can cause destructive signal interference through diffraction. The first Fresnel zone radius at any point along the path is radius = 17.3 × √(d1×d2 / (f×d_total)), with d1, d2 in km and f in GHz. As a rule of thumb, at least 60% of the first Fresnel zone should remain clear of obstructions for a reliable link.
Formula
First Fresnel zone radius
radius(m) = 17.3 × √(d1 × d2 / (f × (d1 + d2)))
- d_1, d_2
- Distances from each end to the point of interest, in km
- f
- Frequency in GHz
Worked example
With the default values below, Fresnel Zone returns First Fresnel Zone Radius of 11.36. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Total Link Distanceminimum 0.01 km
- 10 km
- Obstruction Positionaccepts 1–99 %
- 50 %
- Frequencyminimum 0.1 GHz
- 5.8 GHz
Results
- First Fresnel Zone Radius
- 11.36 m
- Recommended Min. Clearance (60%)
- 6.81 m
- Distance from Tx to Obstruction
- 5.000 km
- Distance from Obstruction to Rx
- 5.000 km
Step by step
- 1Values used
- 2First Fresnel zone radius
- 3First Fresnel Zone Radius= 11.36 m
- 4Recommended Min. Clearance (60%)= 6.81 m
- 5Distance from Tx to Obstruction= 5.000 km
- 6Distance from Obstruction to Rx= 5.000 km
Assumptions and limits
- The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax or fees or charges into account.
- Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
Why is the Fresnel zone widest at the midpoint of the link?
The radius formula's d1×d2 term is maximized when d1 equals d2 (the midpoint), since for a fixed sum (d1+d2), the product of two numbers is largest when they're equal — so obstructions near the middle of a link need the most vertical clearance.
Why does higher frequency mean a narrower Fresnel zone?
The radius formula has frequency in the denominator inside the square root, so as frequency increases, the required clearance radius shrinks — this is one reason higher-frequency microwave links can sometimes tolerate obstacles that would block a lower-frequency link.
Why is 60% clearance the common recommendation instead of 100%?
Signal energy contributing to constructive interference is concentrated mostly within the innermost part of the first Fresnel zone; keeping at least 60% of the zone's radius clear captures the great majority of that useful signal energy while allowing some practical tolerance for towers/terrain.
Does clearing the Fresnel zone guarantee a good link?
It's necessary but not sufficient — the link also needs unobstructed line-of-sight itself, adequate signal power (link budget), and freedom from interference on the same frequency. Fresnel zone clearance specifically addresses diffraction loss from near-path obstacles.