VHF ( Very High Frequency)

This definition specifically refers to the frequency spectrum but communic\tions equipment and broadcast receivers operating within this spectrum are referred to as VHFs.

The VHF spectrum is defined as the part of the electromagnetic spectrum which includes any radiation with a wavelength between 1 and 10 metres and a frequency between 30 and 300 megahertz. VHF signals are widely employed for television and radio transmissions.
 VHF electromagnetic waves, unlike longer electromagnetic waves, are not strongly reflected from the atmosphere; therefore, they do not bend readily around the Earth's curvature and cannot be transmitted beyond the horizon. Their range is further limited by their inability to pass through hills or large structures.  Accordingly, VHF systems are limited to use in short-range, line-of-sight communications,  including radio and television broadcasting, and in marine communication and navigation systems. They are  especially suited to such applications because their reception is not impaired by random  electromagnetic noise ("static") of longer wavelengths. Because of their limited transmission range, VHF signals of the same frequency can be used by transmitters several hundred miles apart without interfering with one another.

For Maritime purposes VHF equpments are used for short range radiotelephony and digital selective calling.
Maritime VHF equipments operate in the 156 - 174 MHz range. Maritime operating procedures and GMDSS requirements specify channels and that shall be used for specific purposes such as Distress calling, intership and Digital Seletive Calling (DSC).

The International Maritime Organisation (IMO), UK Maritime and Coastguard Agency (MCA) reiterate the International Telecommunications Union (ITU) regulations which state that:

(a) Channel 16 may only be used for distress, urgency and very brief safety communications and for calling to establish other communications which should then be concluded on a suitable working channel

(b) Channel 70 may only be used for Digital Selective Calling NOT oral communications

© On VHF channels allocated to port operations or ship movement services such as VTS, the only messages permitted are those related to operational handling, the movement and the safety of ships and to the safety of persons

(d) All signals must be preceded by an identification, for example the vessel's name or callsign

(e) The service of every VHF radiotelephone station must be controlled by an operator holding a certificate issued or recognised by the station's controlling adminisrtration. This is usually the country of registration, if the vessel is registered. Providing the station is so controlled, other persons besides the holder of the certificate may use the equipment.
 

Channel 13 is designated for use as a navigational safety communication channel (Bridge-to-Bridge), primarily for intership navigational safety communications. It may also be used for ship movement and port services.