Could you tell me what the A is shown on the screen of my marine VHF radio?

Many marine VHF channels are duplex (TX and RX on two different frequencies). Duplex channels can only communicate with a shore base station that is configured with the TX and RX frequencies reversed. An example of this is channel 80, often used by Marinas. It allows the marina to talk to incoming vessels but prevents vessels from talking to each other and clogging up the airway. This is usually indicated on-screen with the small letters DUP.

Other Marine VHF channels are simplex (TX and RX on the same frequency). Simplex channels work back-to-back and will talk to any other marine radio on that channel. When a Duplex channel is reassigned/used as a simplex channel, it is programmed to TX and RX on only one of the two frequencies it would normally use. An example of this is channel M2. As a simplex channel M2 is one half of private duplex channel 37. As an example, when a duplex channel is programmed as a simplex channel it is often identified on the radio as 37A or 1037. In this case the suffix of A or the prefix of 10 is what identifies the channel as being simplex and not duplex.

15/12/2021