4.1 Understanding VHF range in practice
Understanding VHF range in practice
VHF signals (Very High Frequency) travel in straight lines, which means they work best when there is a clear line of sight between the transmitting and receiving antennas. For this reason, antenna height and the type of equipment used are the most important factors affecting communication range.
Factors affecting VHF range
- Antenna height (the higher the antenna, the greater the communication range)
- Transmit power (25 watts for a fixed VHF radio, typically 5 watts for a handheld VHF radio)
- Conditions between the vessels (open sea, coastal waters, islands and waves can affect communication range)
- Terrain (mountains, cliffs and land formations can block radio signals)
Fixed vs Handheld VHF radio Function
Fixed VHF radio
- Transmission power Up to 25 watts
- External antenna, mounted high
- Range: 20–60 nautical miles
- Power supply: The vessels 12-volt electrical system
- Best suited for Longer range
Handheld VHF radio
- Transmission power Typically 5 watts
- Built-in antenna, mounted low
- Range: 1 - 5 nautical miles
- Power Supply: Dedicated battery
- Shorter range
💡 Remember
- A nautical mile (nm) is the standard unit of distance used at sea. One nautical mile equals 1,852 metres (1.852 km)
VHF range - Examples
- These figures are only examples and should not be considered exact. Several factors can affect VHF range, including obstacles, weather conditions and antenna height. Actual communication range may therefore be both shorter or longer than the examples shown here.
- Coast radio stations often have antennas located on high ground, sometimes up to 1,000 metres above sea level. This provides excellent coverage, even for small vessels with low- mounted antennas.
- Antenna height is the most important factor affecting VHF range. A small vessel with a low- mounted antenna will have limited coverage, but the communication range increases dramatically when communicating with large vessels or coast radio stations equipped with high-mounted antennas.