A duplex channel is like a two-lane road, in which traffic can flow freely in both directions.
Channel 80 is a typical duplex channel, intended for port operations. Coast stations transmit on 161.625MHz, and receive on 157.025MHz. This means that two coast stations can never communicate with each other on channel 80, but that is OK because there is no reason they should ever try to do so. Similarly, ship radio stations transmit on 157.025MHz and receive on 161.625MHz. Again, ship stations can’t communicate with each other on channel 80, but that is OK because they are not supposed to use it as an inter-ship channel.
Channel 80, however, is of particular significance to Small Craft, because in the UK, it is one of the three channels covered by the Coast Station (Marina) licence. It is, in effect, the “Small Craft Port Operations channel”. The snag is that most of the relatively economical radios sold for small craft use have only one antenna connection. You can’t receive through an antenna at the same time as you are transmitting through it, even on a duplex channel.
This doesn’t stop us from using duplex channels. The radio simply switches from receiving on one frequency to transmitting on the other, and vice versa. It all happens automatically when you press or release the PTT switch, whenever a duplex channel has been selected. The one snag with this so-called “semi-duplex” operation is that it sacrifices the very thing that duplex channels were set up to achieve: for most practical purposes, it is exactly like using a simplex channel.
Semi-duplex operation can be compared to a two-lane road, with road works. The fact that one lane is blocked means that two-way traffic can only be achieved by taking it in turns, just as in a simplex channel.
International and private channels
If you look carefully at the table of channel numbers, frequencies and approved usage on page 25, you’ll see that there is quite a wide range of frequencies (from 157.450MHz to 160.600 MHz) that seem to be missing. That is because these frequencies have been left out of the international allocation, and have been set aside as “private” channels for individual governments to allocate as they think fit.
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