MF/HF RT radio is often known as SSB radio. It is a transmitting-receiving system often referred to as a Transceiver (Tx/Rx), which allows the operator to either transmit or receive information by voice. MF/HF radios use SSB modulation for voice communication.

One of the greater disadvantages of an MF/HF RT radio is that it is not able to "address" a particular radio. A voice broadcasted over MF/HF RT radio can be heard by all other MF/HF radios within range.

Because of that, MF/HF radios integrate an MF/HF DSC Controller. Its function can be regarded as a cross between a normal telephone and a radio. The DSC functions via the DSC Controller or Modem, which simply sends a burst of digital code on the MF/HF DSC frequencies, will automatically “ring” another MF/HF radio. This is feasible because each MF/HF DSC Controller has been allocated a unique MMSI number that acts like a telephone number.


The main components of a MF/HF radio consist of:

* A receiver (Rx) with an earphone and/or loudspeaker;
* A transmitter (Tx) with a microphone – the microphone and the earphone are often combined into a single handset;
* A MF/HF DSC controller;
* A radio telex system (option);
* An antenna enabling both simplex and duplex operation;
* Power supply - often a 12 V or 24 V battery.

Medium Frequencies (MF) designated for marine communication range between 1605 kHz to 3800 kHz and are called "t" band.

BAND	DSC FREQUENCY	RT FREQUENCY
HF 2 MHz	2187,5 kHz	2182,0 kHz

High Frequencies (HF) designated for marine communication range between 4000 kHz to 27500 kHz and are called "u" band. This is divided into the following sub-bands: 4, 6, 8, 12, 16, 18/19, 22, 25/26 MHz. Only the first five are used for distress and safety communication.

BAND	DSC FREQUENCY	RT FREQUENCY
HF 4 MHz	4207,5 kHz	4125,0 kHz
HF 6 MHz	6312,0 kHz	6215,0 kHz
HF 8 MHz	8414,5 kHz	8291,0 kHz
HF 12 MHz	12577,0 kHz	12290,0 kHz
HF 16 MHz	16804,5 kHz	16420,0 kHz

Siting the MF/HF radio

The MF/HF radio must be sited:

• clear of weathering effects and direct sunlight,
• so that the engine noise and vibration or other background noise do not make it difficult for the operator to hear,
• in a convenient position that allows easy use and reading of its controls ,
• at least 1 m (3 ft 3 in) away from any compass, whether magnetic or electronic.

It is usually sited in the cabin of smaller vessels or on the bridge of larger vessels.

Fitting the MF/HF antenna

The MF/HF antenna does not require the highest possible location to get the greatest possible radio range, as the main factor influencing the range is 'day-time'. For night-time communication 4 & 6 MHz Bands are used. For day- time, 12 & 16 MHz are used.

Linking to navigation source

It is fully recommended to link the MF/HF radio to an external navigation source, for example GPS or chartplotter. In this way a MF/HF radio can acquire position and time. When the MF/HF radio is not connected to a GPS, the transceiver will give an alarm at regular intervals showing that the position is missing. This is very important in distress situation, as is explained in detail in the next chapters.

The SOLAS convention requirements

SOLAS vessels sailing outside Sea Area A1 must carry a MF/HF radio capable of transmitting DSC calls.
An MF/HF radio must be capable of mantaining a continuous, automatic watch on 2187.5 kHz (MF) when the vessel is in Sea Area A2.
When the vessel is in Sea Area A3 or A4 a continuous, automatic watch has to be maintained on 8414.5 kHz and on an additional HF frequency depending on the propagation condition i.e. 4 or 6 MHz night time and 12 or 16 MHz day time.
Usually, MF/HF DSC controllers are enabled to watch all DSC frequencies i.e. five from HF Band and one from MF Band.
The radio operators are also required to keep a radio telephone log as a diary of radio calls.