📡️ Line of Sight Calculator

Before you design any point-to-point link you need to know whether the two ends can even see each other. The optical line of sight is the straight line between the antennas, but because the Earth curves, the practical radio horizon is shorter than the geometric one. Engineers use the 4/3-Earth radius model: radio waves refract slightly around the curvature, effectively stretching the horizon by a factor of √(4/3) ≈ 1.15. The familiar rule of thumb is that the horizon distance in kilometres equals 4.12 × √h(m).

Two antennas have line of sight when the straight segment between them clears every obstacle. The optical horizon to an antenna at height h is about 3.57×√h(km, h in metres); adding the 4/3 refraction factor gives the radio horizon at about 4.12×√h. Even with clear LOS you need Fresnel zone clearance — typically the first zone must be at least 60% clear at the midpoint — or diffraction around the obstacle will steal several dB.

For two antennas at heights h₁ and h₂ (metres), the total optical horizon distance is dₖ = 3.57·(√h₁ + √h₂) km. With the 4/3-Earth model the constant becomes 4.12. The calculator also works out the required Fresnel radius at the link midpoint, F₁ = ½·√(λd), and reports the clearance needed for 60% of F₁. Finally it checks whether your entered link distance is within the radio horizon.

The Fresnel radius at midpoint is the key derived metric: at 2.4 GHz over a 5 km link it is about 12.5 m, so you need roughly 7.5 m of clearance above any obstacle at the midpoint. Cross-check your planned link distance against the radio horizon using the Friis path loss calculator to complete the budget.

1. Enter the height of antenna 1 and antenna 2 in metres.
2. Enter the link distance in km.
3. Set the frequency (used only for the Fresnel zone calculation).
4. Click Calculate to see optical LOS, radio horizon and Fresnel clearance.
5. Compare the radio horizon with your distance to decide if the link is feasible.

What is the difference between optical and radio line of sight?

Optical LOS is the geometric straight line ignoring the atmosphere. Radio waves are slightly bent by refraction in the lower atmosphere, so the radio horizon extends about 15% further — hence the 4/3-Earth model. You can often hear a signal slightly beyond the optical horizon, but the path loss rises steeply.

Why 60% Fresnel zone clearance?

When 60% of the first Fresnel zone is clear, diffraction loss drops to about 0–1 dB, essentially negligible. Below that the loss climbs quickly: at 30% clearance you can lose 5–10 dB or more depending on the obstacle shape.

Does the Earth curvature really matter for short links?

For links under about 2–3 km, the Earth curvature is usually a few metres or less and rarely dominates. Above 5–10 km it becomes the deciding factor, which is why microwave towers are tall and why you plan hops rather than one giant link.

Can I extend the horizon by raising one antenna only?

Yes. Because the horizon distance adds the square roots of the heights, raising the lower antenna is usually the cheapest fix. Doubling the height adds about 41% more horizon distance — raising from 10 m to 20 m extends the horizon from 13.0 to 18.4 km.

What happens if an obstacle intrudes into the Fresnel zone?

The signal is attenuated by diffraction. A knife-edge obstacle at 60% clearance costs about 1–2 dB; a rounded hill or building can cost more. The calculator's clearance output tells you the minimum obstacle clearance you need to keep the loss acceptable.

Is line of sight enough for a reliable link?

No. LOS is necessary but not sufficient: you still need sufficient link margin (Friis equation), antenna alignment, and protection from interference. Use the LOS check first, then run the full path loss budget.

Use the 4/3-Earth model for planning, but remember that extreme weather (temperature inversions) can bend the path the other way. Trees grow — leave an extra 2–3 m of clearance over forested terrain. The 60% Fresnel rule applies at the worst obstacle, not just the midpoint; check along the whole path with a terrain profile. For short links (under 1 km) the Fresnel zone is tiny and antenna alignment matters more than clearance. Radio waves also need a few metres of clearance over rooftops to avoid the roof edge diffracting like a knife blade.