The frequencies used by VHF radios at sea are allocated by the International Telecommunications Union, (ITU), which is an international authority, established to ensure worldwide communications compatibility. These frequencies are known as the Maritime Mobile Band and were originally divided into 28 different Channels which were numbered from One through to Twenty-Eight. Each channel uses a different frequency; in fact some channels use two.
By 1972, the increased use of VHF radios was causing congestion. As no additional frequencies were available, extra channels were interleaved between the existing twenty eight to give a new total of 55 available channels. However, as the channel numbers 29 to 59 were already in use by other services, these extra channels had to be numbered from 60 to 88. This is why there is now an apparent gap in the sequence of channels used in the Maritime Mobile Band.
Many VHF sets have more than 56 channels available, but these extra channels are known as Private Maritime Channels and are not actually part of the Maritime Mobile Band.
SIMPLEX AND DUPLEX CHANNELS:
ALL THE CHANNELS ALLOCATED TO MARITIME MOBILE BAND VHF RADIOS CAN BE DIVIDED INTO EITHER SIMPLEX OR DUPLEX CHANNELS.
SIMPLEX CHANNELS ARE SIMPLY SINGLE FREQUENCY CHANNELS. A SIMPLEX CHANNEL ENABLES AN OPERATOR TO EITHER TRANSMIT OR RECEIVE ON THE SAME FREQUENCY, BUT THEY CAN NOT DO BOTH AT THE SAME TIME. THIS IS WHY THE PROWORD 'OVER' MUST BE USED AT THE END OF EACH TRANSMISSION, IT SIGNIFIES THAT THE OPERATOR HAS CEASED TRANSMITTING ON THAT CHANNEL AND IS NOW READY TO RECEIVE. ONLY ONE AERIAL IS REQUIRED TO USE A SIMPLEX CHANNEL, THE OPERATION OF THE PRESS TO TALK SWITCH (PTT) CHANGES THE AERIAL FROM TRANSMITTING TO RECEIVING AND VICE VERSA.
Duplex Channels use two frequencies, one to transmit, the other to receive. The result is a two way communication system, much like a telephone and these channels are used for all Public Correspondence. Public Correspondence channels were designed so that ships could connect to the shore telephone network via Coast Radio Stations, however all Coast Radio Stations in the U.K. have closed down. A fully Duplex Radio requires two aerials, one to transmit and the other to receive. These aerials must be widely separated otherwise much of the power generated by the transmitting-aerial would be received by the receiving-aerial, damaging the equipment. Because of this limitation, Duplex radios are restricted to large vessels and shore stations that can provide sufficient separation between both aerials.
In order to use Duplex Channels, the majority of Maritime Band VHF sets use a system known as Semi-Duplex. As they are limited to one aerial, they can still only transmit and receive on one frequency at a time, but the radio is designed to automatically switch between the two frequencies when the microphone switch is pressed and released. Because of this limitation, the proword, 'over' must still be used at the end of each transmission. Although the system is automatic and therefore appears identical in use to a Simplex Channel, operators must be aware of the nature and limitations of Semi-Duplex communications. Duplex Channels are for ship to shore communication only.
For any given Duplex Channel, all ships radio sets transmit on one frequency and all receive on a different frequency. It is therefore impossible to communicate with another vessel using a Duplex Channel, (one vessel would be transmitting on one frequency whilst the other would be listening on another). Equally, if an operator attempts to monitor a communication transmitted on a Duplex Channel using a Semi Duplex radio they will only ever hear the shore side of the conversation.