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. Modern-day marine VHF radios offer not only basic transmit vhf marine frequencies pdf receive capabilities. A transmit button on the set or microphone determines whether it is operating as a transmitter or a receiver.
Each duplex channel has two frequency assignments. 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. Sets can be fixed or portable.