Use the free tool above, then read on for how it works, a worked example, and practical tips.
What this tool does
VHF and UHF signals travel roughly in straight lines, so your station’s reach is limited first by the horizon, not by power alone. This calculator turns antenna heights into optical and radio horizon distances, and combines two stations, so you can see whether a direct path is even possible before you worry about anything else.
How it works
The tool uses the standard horizon rules: the optical horizon in kilometres is about 3.57 times the square root of height in metres, while the radio horizon uses 4.13 times the square root of height, because normal refraction bends radio waves slightly around the Earth. For two stations it adds the radio horizon of each, since each station’s horizon reaches toward the other.
How to use it
- Enter your values in the boxes above.
- Press the calculate or play button.
- Read the result shown below the tool.
- Try a second set of values to compare options.
Worked example
A 10-metre antenna has an optical horizon near 11 kilometres and a radio horizon near 13 kilometres. Pair it with another 10-metre station and the two-station radio range is about 26 kilometres, which is why handheld-to-handheld range in flat country stays modest even with full power.
Practical tips
Height beats power for extending line-of-sight range, so a small climb or a taller mast often does more than doubling your transmitter output. Check the path for hills and buildings too, because the horizon figure assumes smooth, flat ground and any obstruction in between cuts the real range.
Common mistakes
A common mistake is treating the radio horizon as a hard wall, when in practice signals can scatter or duct beyond it on some days and fall short on others. Another is using height above sea level for a hilltop station without subtracting the local ground level, which makes the horizon look farther than it really is.
Questions and answers
Is this tool free? Yes. It runs in your browser, needs no account, and gives instant results.
Do I need to install anything? No. The tool works on any modern phone or computer browser.
How accurate are the results? The maths follows the standard formulas shown above. Real-world results vary with terrain, equipment, and conditions, so use the figures for planning.
Can I use it on my phone? Yes. The tool is designed to work on a phone screen, so you can use it at the operating desk, in the field, or at a club station without a computer.
Who uses this tool
Radio amateurs, students, and club members all use tools like this one, because they turn a formula from a book into an answer for your own station in seconds. Beginners use it to check homework and first setups, while experienced operators use it to sanity-check a plan before spending money on cable, masts, or antennas. Even a quick check can prevent a costly mistake, such as ordering the wrong cable length or aiming a beam in the wrong direction.
What to try next
- Run the tool with your real station values, not just the default numbers, so the answer means something for your setup.
- Change one input at a time and watch how the result moves, because that shows you which input matters most.
- Write the result in your station notebook next to the date, so you can compare later changes against a known baseline.
Safety note: this is an educational estimator only. Real transmissions need an amateur licence and must follow your local regulations, band plans, and power limits. Never transmit without the proper licence, and treat every result here as a planning guide, not professional or legal advice.
Versión en español: Calculadora de Horizonte de Radio – Alcance de tu Señal Más Allá del Horizonte