Fresnel Zone and Microwave LOS Clearance Calculator
Is the path clear of obstructions, with 60% of the first Fresnel zone?
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Fresnel Zone and Microwave LOS Clearance Calculator by TheCalcTools
How the Fresnel Zone and Microwave LOS Clearance Calculator works
Enter the link length, frequency, antenna heights and the position and height of the obstacle.
The first Fresnel radius is 17.32 √(d₁ d₂ ÷ (f d)) metres, and earth bulge is d₁ d₂ ÷ (12.74 k).
At least 60% of F1 should be clear for free-space-like propagation.
The formula
Worked example
Using the default values shown in the calculator:
| Link length (km) | 12 |
|---|---|
| Frequency (GHz) | 18 |
| Antenna height at end A (m) | 40 |
| Antenna height at end B (m) | 35 |
| Obstacle distance from A (km) | 5 |
| Obstacle height (m) | 22 |
| Effective earth radius factor k | 1.33 |
- F1 = 17.32 √(5 × 7 ÷ (18 × 12)) = 7.0 m; earth bulge = 2.1 m.
- LOS height at the obstacle = 37.9 m; clearance = 13.9 m versus the 4.2 m needed. Clear: meets 60% of the first Fresnel zone.
Result:
| First Fresnel zone radius at the obstacle (m) | 7.0 |
|---|---|
| Earth bulge at the obstacle (m) | 2.1 |
| Line-of-sight height at the obstacle (m) | 37.9 |
| Clearance above obstacle + bulge (m) | 13.9 |
| Required (60% of F1) (m) | 4.2 |
| Result | Clear: meets 60% of the first Fresnel zone |
Frequently asked questions
Why 60%?
Clearing 60% of the first Fresnel zone gives close to free-space loss; less clearance adds diffraction loss.
What is k?
The effective earth radius factor for refraction. A standard atmosphere gives k = 4/3 (1.33); sub-refractive conditions have k < 1.
Sources and further reading
- Wikipedia: Fresnel zone
- ITU-R P.530: Propagation data and prediction methods for terrestrial line-of-sight systems
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This calculator gives estimates for information only. Check important results independently before you rely on them. See our disclaimer.