SRC: The basis of VHF radio

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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 in the range.

Because of that, VHF radios integrates 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, will automatically “ring” another VHF radio. This is feasible because each VHF DSC Controller has allocated unique MMSI number that acts like telephone number. Only fixed version of VHF radios are available.


The VHF DSC radio sample (Simrad’s 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;
  • Power supply – often a 12 V or 24 V battery.

Siting the VHF radio

The VHF radio must be sited:

  • clear of weather 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 of any compasses, whether magnetic or electronic.

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. An emergency VHF antenna is recommended to be carried because accidents can happen that could lead to the loss of the VHF antenna.

Linking to navigation source

It is fully recommended to link the VHF radio to an external navigation source, for example GPS or chartplotter. This way a VHF radio can acquire the position and time. This is very important in distress situation as it is explained in details in next chapters.RD68 linked to navigation source

Linking to navigation source

Handheld VHF radio

A handheld VHF radio is usually used as a backup to a fixed one, for intership communications on large vessels and for use in life raft, because it is a completely self contained unit.

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

  1. Limited battery capacity. Use low power when transmitting whenever possible and speak clearly that you will be understood the first time because transmitting consumes battery power rapidly.
  2. Very limited range of VHF radio communications.
  • Antenna is a part of a handheld VHF radio itself, so it’s 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.

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 mantaining a continuous watch on VHF CH 70 (automatically).

The radio operators are required to keep also a radio telephone log in a diary of radio calls.

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