A VHF RT radio 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. It can be fixed or handheld.

The fixed VHF RT radio sample (Simrad's RT64)

The handheld VHF RT radio sample (Simrad's HT50)

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

For this reason, VHF radios integrate a VHF DSC Controller. Its function can be regarded as a cross between a normal telephone and a VHF RT radio. The DSC functions via the DSC Controller or Modem, which simply sends a burst of digital code on the VHF CH 70, and will automatically “ring” another VHF radio. This is feasible as each VHF DSC Controller has been allocated a unique MMSI number that acts like a telephone number. Only fixed versions of VHF radios are available.

The VHF DSC radio sample (Simrad RD68)

The main components of a VHF 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 VHF DSC controller;
  
• An antenna enabling both simplex and duplex operation;
• A power supply - often a 12 V or 24 V battery.

Siting the VHF radio

The VHF radio must be sited:

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

It is usually sited in the cabin of a smaller vessel or on the bridge of a larger vessel. A waterproof extension speaker sited close to the steering position is recommended as it allows the person on watch to also monitor the VHF radio.

Fitting the VHF antenna

The VHF antenna should be fitted at the highest point of a vessel to get the greatest possible radio range, for example to the masthead of a sailing vessel. It is also recommended that an emergency VHF antenna is carried, as an accident might occur that could lead to the loss of the VHF antenna.

Linking to navigation source

It is strongly recommended to link the VHF radio to an external navigation source, for example a GPS or chartplotter. In this way a VHF radio can acquire position and time. This is very important in a distress situation, as is explained in detail in the following chapters.

Handheld VHF radio

For inter-ship communications on large vessels and for use in life rafts, a handheld VHF radio is usually used as a backup to a fixed one, because it is a completely self-contained unit.

Compared with a fixed VHF radio, any handheld VHF radio suffers from three disadvantages:

1. Limited battery capacity. Use low power when transmitting whenever possible and speak clearly so that you will be understood first time because transmitting rapidly consumes battery power.
2. A very limited range of VHF radio communications.
   
• Antenna is a part of a handheld VHF radio itself, so its height is very low. For example, just 1,5 m. in a life raft.
• To preserve battery power, high power in handhelds is only 5 watts compared to 25 watts on a fixed VHF radio.
3. They do not have a DSC controller.
   

The SOLAS convention requirements

SOLAS vessels must carry a VHF radio capable of transmitting DSC calls on VHF CH 70. A VHF radio must be capable of maintaining a continuous watch on VHF CH 70 (automatically).
The radio operators are also required to keep a radio-telephone log as a diary of radio calls.