MF & HF-SSB Duplex Channels

We have already covered the recognized MF/HF Simplex Frequencies and their importance in ship to ship and distress communications. Here we will concentrate on the HF - Duplex Channels used in typical ship to shore communications.

In its simplest terms, a Duplex Channel is simply (2) different simplex frequencies paired together to form a channel. (1) frequency is used to transmit and the other is used to receive. As an example, If you select ITU Channel 401 on your HF-SSB to communicate on your radio, you will be transmitting on 4065.0 kHz and receiving on 4357.0 kHz.

It is important to note that Marine HF-SSB radios (much like the Marine VHF,) are not true duplex radios, but better described as "half-duplex." This means that the radio can only be in one of two states: either a transmitter or a receiver, it cannot be in both states simultaneously unless you have special equipment installed along with your radio. Most radios are toggled between these two states (transmit and receive) by the push to talk button. This however does not change the fact that the use of different paired frequencies are known as duplex channels.

Are there any advantages to the use of duplex channels? Surprisingly, very few. However, these advantages are major from the perspective of a coast station. They allow for "repeater" usage; which often extends the range of communications by allowing more transmit power to be used and also allows for the ability to locate the antenna in a more advantageous location. This being the typical reasoning behind the usage of duplex channels in ship to shore communications.

Frequency Bands

Like the simplex frequencies, the HF Marine SSB duplex frequencies are grouped into frequency bands. There are currently 9 bands that have been allocated for marine service:

We will ignore the 2 MHZ frequencies, due to their limited range, leaving us with 8 frequency bands that most duplex channels fall into.

HF Duplex Channel Pairs

In the dropdowns below, we have provided a complete list of all internationally accepted ITU standard frequency pairs. These frequency pairs have been broken down by their frequency bands and identified by their channel number.

In your typical marine HF-SSB radio a large number of these channels are pre-programed into the set by the manufacturer. They also have memory space available to allow the user to enter others.

   ITU 4 MHz Channels

   ITU 6 MHz Channels

   ITU 8 MHz Channels

   ITU 12 MHz Channels

   ITU 16 MHz Channels

   ITU 18 MHz Channels

   ITU 22 MHz Channels

   ITU 25 MHz Channels

Download the above HF Duplex Channel Tables 4 to 25 MHz