Physics

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Radar Horizon Calculator.

Calculate geometric line-of-sight distance for two antenna heights.

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Results update as you type.

Horizon distance: 16,336.21476 m

Horizon distance

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Horizon distance: 16.336215 km

Horizon distance

0.000000km

Results update automatically as you type.

Use Cases

Radio and Radar Link Planning

Determine if two antennas are within line-of-sight for communication or radar systems, helping to plan tower heights and spacing.

Example: A radio tower at 50 m and another at 30 m with Earth's radius 6371000 m gives a distance of about 44.5 km.

Amateur Radio and HAM Operations

Estimate the maximum distance for VHF/UHF contacts where line-of-sight is critical, especially for mountain-to-mountain or hilltop operations.

Example: Two handheld radios at 1.5 m height each have a horizon distance of about 8.7 km.

Frequently Asked Questions

What does the Radar Horizon Calculator do?
It calculates the geometric line-of-sight distance between two antennas based on their heights above the Earth's surface and the Earth's radius. This is the distance at which the antennas are just visible to each other over the horizon, assuming no atmospheric refraction.
How is the line-of-sight distance calculated?
The calculator uses the formula: distance = sqrt(2 * Earth radius * antenna height 1) + sqrt(2 * Earth radius * antenna height 2). You input the heights of both antennas and the Earth's radius (default 6371000 m), and it computes the total distance.
Why is the Earth's radius included?
The Earth's curvature limits how far you can see. Including the Earth's radius accounts for the curvature, so the result is the straight-line distance along the surface (or the line-of-sight) between the two antennas, not just the sum of their heights.

Tips & Common Mistakes

Tips

  • Use the default Earth radius (6371000 m) for standard calculations, but adjust if you need a more accurate value for your location (e.g., polar vs. equatorial).
  • Ensure both antenna heights are entered in meters. If you have heights in feet, convert to meters (1 ft = 0.3048 m) before entering.
  • This calculator provides the geometric line-of-sight distance. In practice, atmospheric refraction can extend the radio horizon, so actual range may be slightly longer.
  • For a quick estimate, you can use the simplified formula: distance (km) ≈ 4.12 * (sqrt(height1_m) + sqrt(height2_m)).

Common Mistakes to Avoid

  • Forgetting to convert antenna heights to meters if they are given in feet or other units, leading to incorrect results.
  • Using the diameter of the Earth instead of the radius. The Earth's radius is about 6371 km, not 12742 km.

Last updated: August 13, 2026