Radio Line-of-Sight Calculator
Enter antenna heights to compare pure geometric horizon distance with the common 4/3-Earth radio-horizon approximation.
By Aryan Khan, Founder & Editor · Updated October 2026
For example, two 30 m antennas enjoy a radio horizon of about 45.2 km between them — the air's refraction stretches their reach some 6 km past the 39.1 km that pure geometry allows.
How to use Radio Line-of-Sight Calculator
Radio waves bend a little. The plan prices both truths:
The Radio Line-of-Sight Calculator computes how far two antennas can see each other, comparing the pure geometric horizon with the 4/3-Earth radio-horizon model that accounts for atmospheric refraction. Enter both antenna heights and read each model's range — the first number every radio link plan stands on.
- 1Enter the first antenna height above local ground.
- 2Enter the second antenna height.
- 3Compare geometric and 4/3-Earth horizon estimates.
What people use Radio Line-of-Sight Calculator for
1. Point-to-point link feasibility
Before Fresnel clearance and path profiles, the horizon gate: if the 4/3 range does not reach, no antenna gain saves the plan. Clearance scoring lives on the Fresnel Clearance Percentage Calculator.
2. Repeater and coverage siting
A repeater's service edge is its horizon against the users' antenna heights — this page draws that circle honestly, twice (guaranteed and typical).
3. Ham radio VHF/UHF planning
Beyond-FM-simplex range questions are horizon questions. Two home stations at 10 m talk across about 26 km of radio horizon on flat land — terrain permitting, as always.
4. Explaining 'impossible' contacts
Tropospheric ducting can multiply ranges many-fold on special days — the calculator's standard model is the baseline those miracles are measured against.
Worked examples
A 100 m tower against a 10 m rooftop: geometric range 35.7 + 11.3 = 47.0 km; radio range 41.2 + 13.0 = 54.2 km. Refraction hands the planner 7 km — on standard days. Links engineered to need the gift on all days are engineered to fail on some.
Two handhelds at head height (1.8 m): radio horizons of about 5.5 km each, 11 km combined over perfect flatness. Real terrain, trees and buildings take their cut from there — which matches every hiker's experience of '5-mile' radios.
Antenna horizon distances
One antenna's reach to the horizon, both models — combine two for a link.
| Antenna height | Geometric | Radio (4/3 Earth) |
|---|---|---|
| 1.8 m (handheld) | 4.8 km | 5.5 km |
| 10 m (rooftop) | 11.3 km | 13.0 km |
| 30 m (tower) | 19.6 km | 22.6 km |
| 100 m (mast) | 35.7 km | 41.2 km |
| 500 m (mountain site) | 79.8 km | 92.2 km |
Antenna height means height above the surrounding terrain along the path — a 30 m mast in a deep valley starts its horizon from the valley's terms, not the sea's.
Estimate horizon-limited RF range from antenna height
Raising either antenna extends the geometric horizon because the line of sight reaches farther around Earth. The page shows both a pure geometric estimate and the widely used 4/3-Earth approximation, which represents average atmospheric refraction by increasing the effective Earth radius.
A horizon calculation is only one part of RF planning. Fresnel-zone clearance, terrain, buildings, frequency, antenna pattern, gain, transmitter power and local weather can determine whether a real link works. Treat the result as an early screening figure, not an engineering sign-off.
How to interpret the result
The most common errors come from using a correct result to answer the wrong geographic question. Keep these points in mind for Radio Line-of-Sight Calculator:
- Include your height inputs explicitly (eye height, mast height, deck level) when sharing visibility results: the same hill answers differently for a child, an adult and a 10 m antenna, and unstated heights make numbers unreproducible.
- Treat climate and hardiness outputs as 30-year character, not this year's weather: a zone tells you what winters usually allow, and microclimates — walls, slopes, urban heat — shift a real garden half a zone either way.
Limits & appropriate use
Terrain intelligence carries one dominant limit: the model sees bare earth at ~90 m spacing with metres of vertical error, and is blind to canopies, walls and wires. Every downstream answer inherits it — profiles show terrain truth with noisy amplitude, line-of-sight verdicts are pending trees, horizons assume open ground, and solar radiation averages assume an unshaded panel. The second limit is temporal: climate letters and hardiness zones are thirty-year character, not this year's behaviour, and seismicity is a mood, not a hazard model. Each result here is printed with its recipe precisely so these boundaries travel with the number.
The value proposition is screening-grade truth at zero cost: orientation for sites, gardens, antennas, shoots and curiosity, with the upgrade points clearly marked. When money or safety attaches to the answer — tower siting, flood insurance, structural shading, avalanche terrain — the escalation is professional: licensed survey, certified shade studies, regulatory flood determinations and geological hazard services. A good free tool makes that ladder visible instead of pretending to be the top of it.
- Construction and tower siting → licensed topographic survey.
- Flood decisions → regulatory determinations (FEMA/FIRM in the US).
- Solar finance → certified shade and production studies.
Data & methodology
Uses standard horizon approximations: 3.57√h km geometric and approximately 4.12√h km under a 4/3-Earth refraction model, summed for two antennas.
For source details, licences and model limitations, see Data Sources and Methodology.
Key terms worth knowing
- DEM
- Digital elevation model: a grid of terrain heights, e.g. Copernicus GLO-90.
- Refraction coefficient
- The standard 0.13 factor bending sightlines over the curvature.
Editorial guidance: MapBench explains the method and limitations so the tool can be used as a transparent reference, not as a black-box answer. For legal boundaries, safety-critical routing, surveying, engineering, regulated decisions or official statistics, verify the result with the responsible authority or a qualified professional.
Category: Earth Science. Results are intended for everyday analysis and planning unless a page explicitly states otherwise.
Method, assumptions & limits
- This tool’s method
- Uses standard horizon approximations: 3.57√h km geometric and approximately 4.12√h km under a 4/3-Earth refraction model, summed for two antennas.
- Before relying on the answer
- Check the location, height units and any model options shown by this calculator. A terrain lookup and a geometric Earth model answer different questions.
- What the result cannot establish
- Local terrain, atmospheric conditions and source resolution can matter more than numerical rounding. Do not use a planning result for safety-critical work.
Coverage: Worldwide. Search, basemap and external data requests may use third-party services; browser-based does not mean every operation is offline. Read the source and licence notes, calculation methodology and privacy policy. If a result looks inconsistent, report a correction with the inputs and mode used.
Frequently asked questions
Clear answers about the calculation, accuracy and appropriate use.
Does this guarantee an RF link?
No. Terrain, buildings, Fresnel-zone clearance, frequency, power, antenna gain and atmospheric conditions also matter.
What is the 4/3-Earth model?
A common engineering approximation that represents average atmospheric refraction by increasing effective Earth radius.
Is this suitable for engineering or legal decisions?
Treat the result as planning or educational context unless the page explicitly says otherwise. Engineering, hazard and legal decisions should use authoritative specialist data.
Why can terrain data differ between sources?
Elevation and earth datasets vary in resolution, collection date and processing method. Small local features may not be represented equally.
Can I verify the data source?
Yes. MapBench documents major providers and methodology on the Data Sources and Methodology pages.
Does antenna gain extend the horizon?
Gain focuses power — it extends the range at which a usable signal arrives once a path exists, but it does not bend the horizon. Beyond-horizon paths need refraction luck, diffraction loss budgets, or a relay.
What about diffraction over a ridge?
Signals do bend over knife edges at a loss — engineered paths sometimes use it deliberately. This page computes the clean-horizon models; diffraction design is the path engineer's next layer.
Lower frequencies reach farther?
HF signals refract off the ionosphere and ignore horizons entirely; VHF and up are the horizon-bound world this page serves. Frequency also fattens the Fresnel zone — the other half of link planning.
Height above sea level or above ground?
Above the terrain the path crosses — the horizon forms against the local surface. A mountain-top site's sea-level elevation is famous precisely because it is also its path advantage.
The 4/3 model fails when?
Under sub-refraction (unusual temperature/humidity gradients) the effective Earth can shrink below even the geometric case — coastal and desert paths know these moods. Conservative designs keep margin for them.
Two different heights?
Add their separate horizons — the table's rows are per-antenna contributions, which is why one tall site serves so many small ones.
Mobile and vehicle antennas?
A 1.5–2 m vehicle whip lives in the handheld row of the table: roughly 11 km of ideal combined range to another vehicle, before terrain edits it.
Related tools and resources
Visual version: the Line of Sight Helper; clearance: the Fresnel Zone Calculator; mid-path planet: the Earth Bulge Calculator.
More free tools
Sources, methodology & editorial trust
- ✓ Browser-first processing where supported
- ✓ External sources are documented
- ✓ No account required for public tools
- ✓ Review data sources
Calculations are documented with their assumptions, limitations and source context rather than presented as unexplained answers.
Reviewed and maintained by Aryan Khan, Founder & Editor of MapBench.
Part of our elevation & terrain hub
Radio Line-of-Sight Calculator sits in the elevation & terrain cluster, alongside 40 other tools that answer neighbouring questions with different methods or datasets.
You may also need
When this answer is only part of the job, these clusters cover the next step.
Read the guide behind this tool
How to Find Elevation on Google Maps — Check the elevation of any spot on Google Maps — desktop, mobile, and the precise method with exact numbers.