VHF radio is radio transmission in the 30-300 MHz frequency range, as a means of short-range, line-of-sight verbal communication. It is used in the sea and air transport industries, by emergency services such as the police, fire services, ambulances and by the military. See main article: VHF.

Marine VHF radio

Marine VHF radio in the frequency range 156 to 174 MHz, is mainly used for collision avoidance, summoning rescue services and communicating with harbours and marinas.

A marine VHF set is a combined transmitter and receiver of VHF transmissions on standard, international frequency bands known as "channels". Transmission power ranges between 1 and 25 Watts, giving a maximum range of 25 nautical miles between aerials mounted on tall ships and hills, and 3 nautical miles between aerials mounted on small boats at sea-level.

Marine VHF uses a number of "simplex" transmission channels, which have a frequency band each, where communication can only take place in one direction at a time. A transmit button on the set or microphone determines whether it is operating as a transmitter or a receiver. Channel 16, the calling channel, this most important of these. There are also a number of "duplex" transmissions channels where communication can take place in both directions simultaneously. Each duplex channel has two frequency assignments.

Sets can be portable or fixed. A portable set generally has the advantages of easily being carried to a lifeboat in an emergency, having its own power source and being easily water-proofed. A fixed set generally has the advantages of a more reliable power source and a larger and more effective aerial and bigger display and buttons.

Proper operating procedures include:

• using a set of international "calling" procedures such as the "Mayday" distress call and the "Pan Pan" urgency call
• using "pro-words" based on the English language such as Acknowledge, All after, All before, All stations, Confirm, Correct, Correction, In figures, In letters, Over, Out, Radio check, Read back, Received, Repeat, Say again, Spell, Standby, Station calling, This is, Wait, Word after, Word before, Wrong
• using the NATO phonetic alphabet: Alpha, Bravo, Charlie, Delta, Echo, Foxtrot, Golf, Hotel, India, Juliet, Kilo, Lima, Mike, November, Oscar, Papa, Romeo, Quebec, Sierra, Tango, Uniform, Victor, Whiskey, X-ray, Yankee, Zulu
• using a phonetic numbering system based on the English language: Wun,Too,Tree,Fow-er,Fife,Six,Sev-en,Ait,Nin-er,Zero,Decimal

There are two types of equipment: "voice-only" and "digital selective calling".

Voice-only equipment is the traditional type, which relies totally on the human voice for calling and communicating

Digital Selective Calling equipment, a part of GMDSS, provides all the functionality of voice-only equipment and, additionally, allows several other features:

• a transmitter can call a receiver automatically using Digital Selective Calling on Channel 70, using a telephone-type number known as a Maritime Mobile Service Identity or MMSI
• a distress button, which automatically sends a digital distress signal indentifying the calling vessel and the nature of the emergency
• a connection to a GPS receiver allowing the digital distress message to contain the distressed vessel's position

The MMSI is a nine digit number identifying a VHF set or group of sets. The left hand digits of MMSI indicate the country and type of station. For eamples, here are MMSI prefixes of 4 types of station:

• Ship : 23 is the United Kingdom – e.g. a UK ship : 232003556
• Coast : 00 – e.g. Solent Coastguard : 002320011
• Group of stations : 0 – e.g. 023207823
• Portable DSC equipment : for UK 2359 - e.g. 235900498