Simplex is the communication method where both transmitter and receiver are operating on a single (or the same) frequency, for example VHF CH 12 and VHF CH 16. On simplex channels it isn't possible to transmit and receive simultaneously. Simplex channels are mainly used for Distress, Urgency, Safety and Routine Calling purposes, port, pilotage, harbour and inter-ship operations. Because a single frequency is used, there is no privacy as we can be heard on other VHF radios.




Place the mouse on the vessel or the coast station and check who hears the broadcast through VHF radios on the chosen object.

Duplex channels are normally only used for communication between vessels and CRS and for port operations and ship movement. With duplex channels our transmission can only be heard by the CRS. However, all ship stations listening to the same channel can hear the CRS transmission. For example, VHF CH 23 is a commonly used duplex channel for public correspondence. CRS transmit on 161,75 MHz and receive on 157,15 MHz. This means that two coast stations cannot communicate with each other on VHF CH 23. That is OK because there is no reason why they should ever wish to do so. Similarly ship radio stations transmit on 157,15 MHz and receive on 161,75 MHz. Again, ship stations cannot communicate with each other on VHF CH 23. This is OK as they are not supposed to use it as an inter-ship channel. On duplex channels it is possible to transmit and receive simultaneously like over normal telephone. But this requires a duplex radio and two antennas or a special duplex filter that is mounted only on larger vessels.




Place the mouse on the vessel or the coast station and check who hears the broadcast through VHF radios on the chosen object.

The reader can wonder why the channels are not all just simplex, and the answer is only to get more privacy with the personal and commercial comunications and get also a link like the normal telephone.
Last modified: Saturday, 25 April 2020, 7:50 PM