Radio Procedure

Priority of Radio Calls

All radio calls are subject to a priority rating as shown below, (note that routine radiotelephone calls have the lowest priority).

  • Distress
  • Urgency
  • Safety
  • All other communications, (i.e. routine radiotelephone calls)

Test Transmissions

Test calls may be made to check that the equipment is operating correctly. The duration of the call must not exceed 10 seconds and the station's identity must be included.

Example

Stornaway Coastguard, Stornaway Coastguard, This is Tobermory Lifeboat, Tobermory Lifeboat, Tobermory Lifeboat Radio Check, Over.

Typical responses would be: any combination of the two columns.

Signal Strength
and
Message Clarity
Loud
and
Clear
Good
and
Readable
Weak
and
Unreadable
Very Weak
and
Distorted
Fading
with
Interference

Interference Avoidance

The first rule in transmitting is to listen to the selected channel before transmitting. This is to ensure that the Channel is not already in use. If it is, you will have to wait for a break in the traffic before transmitting.

Capture effect

Even with this precaution it is still possible to cause unwitting interference and lost messages because of the way a VHF radio receiver works. All VHF radios make use of a phenomenon known as 'capture effect'.

Once tuned to a Channel they will disregard weak signals and only lock onto the strongest signal available, (the point in which they lock-onto a signal is controlled by the squelch control). Unlike MF radios, VHF radios only ever receive one transmission at a time and so appear to be 'interference free'.

If you are closer to the Coast Guard/shore station than another vessel you will block out their transmission. To help reduce this problem, routine transmissions should always be made at reduced power - minimum power for reliable communications is the rule. High power, (25 watts), should only be used for Distress, Safety and Urgency messages or when the Lifeboat is on service.



Casualty A is already transmitting to Lifeboat B, but Lifeboat C who is out of range of the casualty's signal transmits because it thinks the channel is clear.

When C starts to transmit, its stronger signal will drown out and over ride A's transmission Lifeboat B will then lose contact with the casualty