About marine VHF radio equipment – channels and it’s uses

Marine VHF radio refers to the radio frequency range between 156.0 and 174 MHz

The “VHF” signifies the very high frequency radio waves of the range.

In the official language of the International Telecommunication Union the band is called the VHF maritime mobile band.

In some countries additional channels are used, such as the L and F channels for leisure and fishing vessels in the Nordic countries (at 155.5–155.825 MHz).

Marine VHF radio equipment is installed on all large ships and most seagoing small craft. It is also used, with slightly different regulation, on rivers and lakes. It is used for a wide variety of purposes, including summoning rescue services and communicating with harbours, locks, bridges and marinas.

A marine VHF set is a combined transmitter and receiver and only operates on standard, international frequencies known as channels. Channel 16 (156.8 MHz) is the international calling and distress channel.

Transmission power ranges between 1 and 25 watts, giving a maximum range of up to about 60 nautical miles (111 km) between aerials mounted on tall ships and hills, and 5 nautical miles (9 km; 6 mi) between aerials mounted on small boats at sea level.

Frequency modulation (FM) is used, with vertical polarization, meaning that antennas have to be vertical in order to have good reception.

Modern-day marine VHF radios offer not only basic transmit and receive capabilities. Permanently mounted marine VHF radios on seagoing vessels are required to have certification of some level of “Digital Selective Calling” (DSC) capability, to allow a distress signal to be sent with a single button press.

Marine VHF mostly uses “simplex” transmission, 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.

A few channels are “duplex” transmission channels where communication can take place in both directions simultaneously when the equipment on both ends allow it (full duplex), otherwise “semi-duplex” is used.

Each duplex channel has two frequency assignments. Duplex channels can be used to place calls on the public telephone system for a fee via a marine operator. When full duplex is used, the call is similar to one using a mobile phone or landline.

When semi-duplex is used, voice is only carried one way at a time and the party on the boat must press the transmit button only when speaking. This facility is still available in some areas, though its use has largely died out with the advent of mobile and satellite phones.

Marine VHF radios can also receive weather radio broadcasts, where they are available.

Types of equipment
Sets can be fixed or portable. A fixed set generally has the advantages of a more reliable power source, higher transmit power, a larger and more effective aerial and a bigger display and buttons.

A portable set (often essentially a waterproof, VHF walkie-talkie in design) can be carried on a kayak, or to a ships lifeboat in an emergency, has its own power source and is waterproof.

A few portable VHFs are even approved to be used as emergency radios in environments requiring intrinsically safe equipment (e.g. gas tankers, oil rigs, etc.).

Marine radios can be “voice-only” or can include “Digital Selective Calling” (DSC).

Digital Selective Calling equipment, a part of the Global Maritime Distress Safety System (GMDSS).

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

DSC transmitters can automatically call a receiver equipped with Digital Selective Calling, using a telephone-type number known as a Maritime Mobile Service Identity (MMSI).
The DSC information is sent on the reserved Channel 70. When the receiver picks up the call, their active channel is automatically switched to the transmitter’s channel and normal voice communication can proceed.

A distress button, which automatically sends a digital distress signal identifying the calling vessel and the nature of the emergency

DSC can have a connection to a GPS receiver allowing the digital distress message to contain the distressed vessel’s position

The MMSI is used for seagoing vessels and consists of 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 example, here are MMSI prefixes of four station types:
Ship : 232, 233, 234 or 235 are the United Kingdom – e.g. a UK ship : 232003556

Coastal station : 00 – e.g. Solent Coastguard : 002320011

Group of stations : 0 – e.g. 023207823
Portable DSC equipment : for UK 2359 – e.g. 235900498

Here is an external link where you can find different countries MMSI Numbers http://www.vtexplorer.com/vessel-tracking-mmsi-mid-codes.html

For use on the inland waterways within continental Europe, a compulsory Automatic Transmitter Identification System (ATIS) transmission conveys the vessel’s identity after each voice transmission.

This is a ten-digit code that is either an encoded version of the ship’s alphanumeric call sign, or for vessels from outside the region, the ship MMSI prefixed with “9”. The requirement to use ATIS in Europe, and which VHF channels may be used, are strongly regulated, most recently by the Basel agreements.

Channels and frequencies
Simplex channels here are listed with the A and B frequencies the same. The frequencies, channels, and some of their purposes are governed by the ITU.

The original allocation of channels consisted of only channels 1 to 28 with 50 kHz spacing between channels, and the second frequency for duplex operation 4.6 MHz higher.

Improvements in radio technology later meant that the channel spacing could be reduced to 25 kHz with channels 60 to 88 interspersed between the original channels.

Channels 75 and 76 are omitted as their frequencies are either side of the calling and distress channel 16, acting as guard channels.

Other frequencies
The frequencies which would have been the second frequencies for simplex channels are not used for marine purposes and can be used for other purposes that vary by country. For example, 161.000 to 161.450 MHz are part of the allocation to the Association of American Railroads channels used by railways in the USA and Canada.

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