Free marine-radio calculator

Estimate the VHF radio horizon between two aerials

Marine VHF is mainly line of sight. Enter both aerial heights to estimate their combined geometric radio horizon, then use the limitations to understand why the number is not a coverage promise.

Study VHF radio theory

Approximate radio horizon

Enter both aerial heights

Try a typical teaching example

Theoretical geometric estimate

About 10.5 nautical miles

Approximately 19.4 km between the two radio horizons.

First aerial horizon

2.7 M

Second aerial horizon

7.7 M

Why real range can be shorter—or occasionally different

  • Land, harbour structures, vessel superstructure and the operator's position can obstruct the path.
  • Feeder loss, aerial condition, battery voltage, installation, receiver sensitivity and interference matter.
  • Atmospheric propagation changes; transmitter power does not remove the radio horizon.
  • Never use this estimate to decide that distress coverage or a coast-station watch is available.

How the estimate is calculated

For each aerial, the approximate radio horizon in nautical miles is 2.23 × the square root of its height in metres. The calculator adds the two horizon distances. The coefficient represents a conventional effective-Earth-radius approximation for VHF/UHF propagation; it does not model a particular coastline, radio, aerial or atmosphere.

Range (M) ≈ 2.23 × (√height 1 + √height 2)

Why aerial height matters

Raising an unobstructed aerial extends its geometric horizon. A handheld used low in the cockpit therefore usually has less potential range than a sound fixed installation with a masthead aerial. Power helps signal reliability within the path, but it does not make VHF routinely follow the Earth's curvature.

Sources and factual review

Reviewed 12 August 2026. The model uses the ITU effective radio-horizon approximation. A withdrawn MCA safety notice is included only as an operational illustration of why lower portable aerials may not reach every shore station in a published area; it is not current regulatory guidance.