Enter Radio Link Details
Example Data Table
| Tx Height | Rx Height | Frequency | K Factor | Approx Radio Horizon | Use Case |
|---|---|---|---|---|---|
| 10 m | 10 m | 900 MHz | 1.333 | 26.1 km | Rural telemetry |
| 30 m | 20 m | 2400 MHz | 1.333 | 46.0 km | Wireless bridge |
| 60 m | 35 m | 5800 MHz | 1.333 | 66.0 km | Point-to-point link |
| 100 m | 80 m | 700 MHz | 1.333 | 99.5 km | Broadcast planning |
Formula Used
The radio horizon distance is estimated with:
d = 3.57 × (√(k × h1) + √(k × h2))
Here, d is distance in kilometers. The values h1 and h2 are antenna heights in meters. The value k is the effective earth radius factor.
The first Fresnel zone radius is estimated with:
F1 = 17.32 × √((d1 × d2) / (f × D))
Here, F1 is in meters. The frequency f is in MHz. Distances d1, d2, and D are in kilometers.
Free space path loss is estimated with:
FSPL = 32.44 + 20log10(D) + 20log10(f)
How to Use This Calculator
Enter the transmitter antenna height and receiver antenna height. Select the matching height unit. Add the operating frequency in MHz. You may enter a known path distance. Leave it blank when you only need the maximum horizon estimate.
Add an obstacle height when terrain, trees, rooftops, or towers sit near the midpoint. Select the distance unit for the result. Keep the earth radius factor at 1.333 for a common planning estimate. Change it when your propagation model requires another value.
Press the calculate button. The result appears above the form. Review the total horizon, Fresnel clearance, earth bulge, effective clearance, and link budget. Use the CSV and PDF buttons to save the result.
Radio Line of Sight Planning Guide
Why Line of Sight Matters
Radio line of sight is more than visual sight. A strong link also needs space around the direct signal path. Buildings, hills, trees, and earth curvature can weaken the signal. Higher antennas improve coverage because they extend the radio horizon. They also improve clearance over nearby obstacles.
Radio Horizon Basics
The calculator estimates the horizon from both antennas. It uses an effective earth radius factor. This factor accounts for normal atmospheric bending. A common value is 4/3. It allows radio waves to travel slightly farther than optical sight. The result is still an estimate. Real terrain and climate can change performance.
Fresnel Zone Clearance
The Fresnel zone is an oval space around the signal path. It should not be heavily blocked. Many planners use at least 60 percent clearance. Lower clearance may still work, but signal fading can increase. At higher frequencies, Fresnel zones are smaller. At longer distances, they become wider.
Path Loss and Fade Margin
Free space path loss shows how much signal weakens over distance. Longer paths create more loss. Higher frequencies also increase loss. Fade margin adds extra allowance for rain, alignment errors, cable loss, and changing conditions. A larger margin usually gives a more stable link.
Practical Planning Tips
Use accurate antenna heights. Measure from ground level near each antenna. Check maps and site surveys for terrain height. Add roof edges, trees, towers, and ridges as obstacles. Use the midpoint clearance estimate as a guide. For long links, test more points across the path. Final design should include terrain profiles and equipment specifications.
Frequently Asked Questions
1. What is radio line of sight?
Radio line of sight is the usable signal path between two antennas. It considers antenna height, earth curvature, and clearance around the path. A clear visual path helps, but Fresnel zone clearance is also important.
2. Why is the radio horizon longer than visual horizon?
Radio waves can bend slightly through the atmosphere. This effect is commonly modeled with an effective earth radius factor. The usual planning value is 4/3, which increases the estimated range.
3. What is the Fresnel zone?
The Fresnel zone is a rounded area around the direct radio path. Objects inside this zone can reflect or block signal energy. Good links often need at least 60 percent of the first zone clear.
4. Which height should I enter?
Enter the antenna height above local ground level. If the antenna sits on a building, include the building height and mast height. Use the same unit for both antenna values.
5. What frequency unit does this calculator use?
The calculator uses megahertz for frequency. Enter 2400 for a 2.4 GHz link. Enter 5800 for a 5.8 GHz link. Correct frequency improves Fresnel and path loss estimates.
6. What does blocked or marginal mean?
It means the estimated clearance is below the recommended 60 percent Fresnel clearance. The link may suffer fading, weak signal, or dropouts. Raising antennas may improve the result.
7. Does this replace a terrain profile tool?
No. This calculator gives a planning estimate. A detailed terrain profile is better for long paths or difficult sites. Use survey data for final engineering decisions.
8. Can I save the calculation?
Yes. After calculating, use the CSV button for spreadsheet data. Use the PDF button for a simple report. Both options help document link planning results.