VHF Radio Horizon and Communication Range Calculator

Estimate the smooth-Earth marine VHF horizon between two antennas, compare standard-atmosphere and geometric limits, and check an optional planned separation. Inputs stay in this browser tab.

Planning estimate — not assured radio coverageDo not rely on this result for distress calling, voyage safety, or required communications coverage. Test the installed radios and antennas on the real route, maintain a watch on the appropriate channels, and carry the safety and backup equipment required for the voyage.

Antenna heights and path

Two radio stations

Examples only; use measured antenna-element heights.

Changing units converts both entered heights.

Radiating element above the local water or ground surface.

Use vessel, marina, coast-station, or receiving-antenna height.

Model and optional path check

Standard models typical refraction; geometric models none.

All three distance units also appear in the result.

Straight-line distance between the two stations.

Path checks are limited to 1,000 NM.

Privacy: calculations are performed locally. Input values are not sent, saved, or included in analytics events.

Radio-horizon estimate

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How to use the calculator

  1. Measure each antenna at its radiating element above the local water surface or the corresponding smooth-Earth surface.
  2. Enter both heights. Each antenna contributes its own distance to the combined horizon.
  3. Use the standard radio-horizon model for typical planning or the geometric model as a no-refraction baseline.
  4. Optionally enter the stations' straight-line separation to compare the planned path with the horizon estimate.
  5. Inspect and test the actual path and radio installation; being inside the modeled horizon does not guarantee a usable link.

What the result means

The combined figure is the sum of each antenna's modeled horizon over a smooth, unobstructed Earth. It is best treated as a curvature limit, not as a coverage prediction.

  • Inside the modeled horizon means Earth curvature alone does not rule out the path.
  • Outside the modeled horizon means the selected smooth-Earth assumption requires more antenna height.
  • Required height changes one station at a time while holding the other station's height fixed.
No range promise: islands, headlands, buildings, vessel structure, antenna faults, feed-line loss, interference, receiver limits, and weather can prevent communication inside the calculated horizon.

VHF radio-horizon formulas and assumptions

For antenna heights that are small relative to Earth, the distance to a smooth horizon is approximated from an effective Earth radius. The two antenna horizons are then added:

d = √(2 × k × R × h₁) + √(2 × k × R × h₂)

Here, R is mean Earth radius, h is antenna height above the modeled surface, and k is the effective-Earth factor. With standard refraction, k = 4/3:

d (NM) ≈ 1.23 × (√h₁(ft) + √h₂(ft))
d (km) ≈ 4.12 × (√h₁(m) + √h₂(m))

The geometric no-refraction option uses k = 1, about 1.06 NM per square root of height in feet. The model assumes a smooth spherical Earth and no intervening terrain or structures. It does not model diffraction, Fresnel-zone clearance, frequency-specific path loss, antenna gain and polarization, transmitter power, receiver sensitivity, cable loss, noise, interference, precipitation, sea state, or non-standard atmospheric refraction.

Normal refraction is only an approximation. Temperature and humidity gradients can shorten the effective horizon or create ducting that carries VHF unexpectedly far. A more distant contact does not invalidate the local line-of-sight estimate.

Frequently asked questions

How is marine VHF radio range calculated from antenna height?

Under the standard 4/3 effective-Earth assumption, add the two antenna horizons: approximately 1.23 × (√h₁ + √h₂) when heights are in feet and range is in nautical miles. This is a smooth-Earth estimate, not guaranteed communication range.

Why are both VHF antenna heights needed?

Each station can see to its own horizon. The theoretical line-of-sight limit is the sum of those distances, so increasing either antenna's height extends the modeled path.

Does more transmitter power increase the radio horizon?

No. More power can improve a weak received signal on an otherwise viable path, but it does not change the horizon set by antenna heights and Earth curvature. That is why wattage is not an input here.

What is the difference between radio horizon and geometric horizon?

The geometric option is a true-Earth, no-refraction baseline. The standard radio option uses a 4/3 effective-Earth radius to approximate typical tropospheric refraction, producing a horizon about 15.5% longer.

Can actual VHF range be longer or shorter?

Yes. Obstacles, low antenna efficiency, feed-line loss, interference, receiver performance, and low transmitter power can shorten usable range. Unusual atmospheric ducting can extend reception far beyond the normal radio horizon.

What antenna height should I enter?

Use the radiating part of the installed antenna above the nearby water surface. For an elevated shore station, use its antenna height above the corresponding Earth surface or mean sea level when the intervening path is over open water. Do not enter mast length alone unless it equals that total height.

References and safety limits

Safety limitThis calculator cannot confirm that a distress call, Digital Selective Calling alert, weather broadcast, Automatic Identification System transmission, or routine voice call will be received. Follow current regulations and equipment instructions, verify the installation, conduct appropriate radio checks, and use suitable backup distress communications.

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