Module 4 - Channels & Frequencies
Simplex vs duplex marine VHF channels
What you will learn
Use simplex and duplex arrangements to predict who can hear whom, then select a channel by assigned purpose rather than number alone.
On a simplex channel, stations use one shared frequency and take turns. A duplex coast-service allocation uses paired ship and coast frequencies, so hearing the coast station does not prove another ship can hear you.
Simplex voice operation places the participating stations on one shared frequency. Push-to-talk still means one station speaks while the others listen. This is why concise transmissions, Over and Out, and immediate PTT release matter.
A duplex allocation pairs the ship-transmit/coast-receive frequency with a coast-transmit/ship-receive frequency. A ship may hear the coast station but not another ship transmitting on the paired ship frequency. Duplex therefore does not create a normal ship-to-ship conversation or permission for two vessels to speak at once.
The arrangement and the service purpose are separate facts. A channel can be technically selectable yet unsuitable for the intended traffic. Use the channel assigned to the station and purpose in the current channel plan.
| Arrangement | What the ship station does | Operational implication |
|---|---|---|
| Simplex | Transmits and receives on one shared frequency | Stations take turns and can normally hear others on that shared route |
| Duplex coast service | Transmits and receives on paired ship/coast frequencies | The coast station may be heard when another ship on the pair is not |
| DSC on Channel 70 | Sends or receives digital signalling | Do not infer a voice route from the channel number |
Explain a one-sided coast-station exchange
You hear a coast station replying on a duplex channel but cannot hear the ship it is answering.
1. Recognise the pair
The coast and ship directions use different frequencies within the duplex allocation.
2. Reject the shortcut
Do not assume the channel is suitable for direct ship-to-ship traffic because the coast station is clear.
3. Use the assigned service
Follow the channel's current purpose and the coast station's instructions.
Sense check: Hearing only one side can be normal on a paired coast-service channel; it does not prove the other ship failed to transmit.
Common mistake or limitation
- Assuming duplex means two ships can speak and hear one another simultaneously.
- Treating dual watch as a type of duplex channel; dual watch is a receiver facility, not an allocation.
- Choosing a channel from its simplex or duplex label without checking its assigned purpose.
Recap
- Simplex uses one shared frequency and one speaker at a time.
- Duplex coast services use paired ship and coast frequencies.
- Technical arrangement does not replace the channel's assigned purpose.
Optional quick check
Section 8 of 9
Why might a yacht hear a coast station but not the ship speaking to it on a duplex allocation?
Sources and factual review
Reviewed 20 August 2026 - Compass Revision editorial review.
Connect channel type to channel purpose
The full module combines simplex, duplex, DSC, calling, inter-ship and local operational allocations.