MF & HF-SSB Simplex Frequencies

So What is a Simplex Frequency?

The simplest definition is:

A frequency that you both transmit and receive on.

Simplex frequencies whether MF, HF, or VHF are primarily used for Distress, Urgency, Safety, Calling, and Ship to Ship communications. Rarely, they may be also used for ship to shore communications.

The International Telecommunication Union (ITU,) the same organization that sets international regulations and treaties governing Marine VHF usage is also responsible for the Marine Medium and High Frequency spectrum. The intent is to provide the maritime radio communications user with a group of standard distress, safety, hailing, and working frequencies that are known to all mariners and can be used worldwide.

So why is it that Duplex Frequencies can't be used for ship to ship communications? Well, technically they can. The problem is that it would require a completely different equipment setup than today's common VHF or MF/HF Transceiver found aboard most vessels.

So, Why Not Duplex?

Let’s Formulate an Example to Show You Why:

Say you are cruising along in Penobscot Bay, ME around 1300 EDT and you decide to call a friend on HF-SSB using DSC Routine Calling.

You know he is about 200 NM south of Bermuda, putting him somewhere in the vicinity of 950 NM away.

A quick check of the propagation tables tell you that for the time of day and the distance involved the 12 MHz Band is probably your best bet.

So you send out a DSC routine traffic alert on 12577.0 kHz with a suggested duplex working channel of 1201.

You receive an acknowledgement to your alert from your friend, then switch frequencies to channel 1201. Your friend also goes over to 1201 and waits for your voice call.

You then begin calling your friend on the radio and Lo and Behold: Nothing Happens!

So What Went Wrong?

The (2) vessels were trying to communicate on a common duplex frequency pair (channel). You can choose any ITU channel that you wish, but for this example we chose channel 1201.

As you can see, both boats are transmitting on 12,230.0 kHz, and you are both listening on 13,077.0 kHz. Neither one of you are listening on 12,230.0 kHz the frequency everyone is transmitting on. Are we seeing the problem here? Neither party hears what the other is saying.

Public Coast Stations on the other hand are set up differently and that is why duplex channels work for them. When they dial up 1201 the frequencies are reversed. They are transmitting on 13,077.0 kHz and listening on 12230.0 kHz

This brings us back again to the use of simplex frequencies for ship to ship communications. Everyone transmits and receives on the same exact frequency, such as 12353.0 kHz. So everyone hears the traffic the other is sending.

Primary MF/HF-SSB Simplex Frequencies

Medium and High Frequency Simplex is provided for worldwide common use by ships of all categories and of all nations. By international agreement, it is used for communications with other ships or shore stations. Intership communications being the primary use for simplex frequencies.

Shared HF-SSB Simplex Frequencies

In addition to the primary simplex frequencies above, there are a large number of frequencies in the 4 MHz and 8 MHz bands that may be used on a shared basis with fixed services ashore.

Special Use Simplex Frequencies

In addition to the international frequencies listed above, individual nation states will often have other frequencies available for use for particular purposes or for use in particular geographic areas. An example would be in the United States there are additional 2 MHz simplex frequencies shown below:

Download all of the above MF/HF-SSB Simplex Frequency Tables

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