Antennas · VHF propagation · Installation
For marine VHF, a few extra metres of antenna height can matter more than a large increase in transmitter power. The reason is geometry: VHF coverage is governed mainly by the radio horizon.
VHF follows the horizon
Marine VHF normally travels in near line-of-sight paths. Raising either antenna extends the distance to the radio horizon. IMO’s planning criterion for VHF coverage expresses range as a function of the square roots of the two antenna heights: A = 2.5 × (√H + √h) nautical miles, with H and h in metres. For planning sea area A1, the ship’s antenna is assumed to be 4 m above sea level.1,2
Power cannot lift an antenna
More power can improve signal strength at the edge of coverage, but it cannot reliably transmit through the curvature of the Earth. The planning formula contains no term for transmitter power at all.1 A low antenna driven hard may still be outranged by a modest transmitter connected to a clear, elevated antenna.
Two boats, both with antennas 2 m above the water: 2.5 × (√2 + √2) ≈ 7 nm.
Move one antenna to the masthead, 10 m up: 2.5 × (√2 + √10) ≈ 11 nm.
Eight metres of height gives roughly 60 % more range. Doubling transmitter power does not move the horizon.
Calculated with the IMO planning formula; real range also depends on terrain, installation and conditions.
Placement includes the surroundings
Conductive structures, other antennas and high-power equipment can affect performance. IMO’s installation guidelines for shipborne AIS—which also uses a VHF antenna—set out the principles: omnidirectional vertical polarisation; an elevated position as free as possible, at least 2 m horizontally from conductive structures; at least 3 m from, and out of the beam of, high-power transmitters such as radar; and, when mounted near the main VHF radiotelephone antenna, at least 2 m vertical separation or at least 10 m horizontal separation.3
The same guidelines recommend double-screened coaxial cable equal to or better than RG214, kept as short as possible to minimise loss, and connectors that are waterproof by design.3 Manufacturers give similar figures for VHF/DSC radios; one installation manual, for example, limits RG214 cable runs to 25 m and total cable loss to 3 dB.4
The installation is a system
Antenna, feed cable, connectors, ground arrangements, power supply and radio all contribute. Height does not excuse damaged coax or corroded connectors. The useful comparison is never watts versus height alone, but the complete path from transmitter to antenna and onward to the receiving station.
References and further reading
- IMO Resolution A.801(19), Annex 3: provision of radio services for the GMDSS—definitions of sea areas and VHF range criterion. Summary: IHO, Definitions of Sea Areas A1, A2, A3 and A4 (PDF).
- Maritime and Coastguard Agency (2024). GMDSS VHF DSC procedures for small boat users. Updated 24 July 2024.
- IMO SN/Circ.227 (2002, as amended). Guidelines for the installation of a shipborne Automatic Identification System (AIS).
- Cobham SATCOM. SAILOR 7222 VHF DSC installation manual: VHF and DSC antenna installation.
- International Maritime Organization (2014). GMDSS.1/Circ.16: Master plan of shore-based facilities for the GMDSS (listed coast-station ranges).
