VHF black holes

What can you do to be prepared for VHF black holes?

We all know that mobile phone reception can be unreliable; the more remote your location, the less likely you are to have a good signal on your mobile phone simply because there may not be a local antenna within line of sight.

Although VHF reception should be available in GMDSS Sea Area A1 - this is the zone around the UK coast of the UK which extends about 30 miles off shore and within which continuous alerting by Digital Selective Calling (DSC) should be available - there may be areas where VHF reception is particularly poor or at times non-existent. Why is this and what can you do to be prepared for these black holes?

Little bend

VHF transmissions don’t bend to any great extent, so any obstruction between the transmitting antenna and the receiving antenna will tend to block the signal. The RYA VHF Handbook (G31) explains how VHF works and gives a table which estimates the distances you can expect VHF antennas to make contact over. According to this table a sailing yacht with an antenna height of 16m should be able to communicate with a coast station with an antenna height of 100m at a distance of up to 42 miles, but this assumes unobstructed line of sight between the two antennas.

Masthead antenna may not be high enough

VHF Antennas used by Coastal Stations such as the Coastguard Maritime Rescue Co-ordination Centres have, as far as possible, been situated to maximise the range over which they transmit and receive. If however, the yacht is anchored for example in a deep sided loch on the west coast of Scotland (where many of the known VHF black holes are located) an antenna even mounted at the mast head may simply not be high enough to clear obstructions for transmissions with a coastal station to have a chance.

Check your VHF installation

Although your location may be to blame, if you repeatedly experience problems, this could indicate that your VHF installation might be the problem.

It is worth checking that you have the right antenna and that the co-axial cable is sound. Also check any connections to ensure that these are sound and that water has not got in and caused corrosion. If these are all OK then it is worth checking that the press to talk or PTT switch on the handset is working properly. Try a radio check with a friend on a channel other than 16.

Hiding from bad weather

If you are looking for somewhere to hide from bad weather but you still want to be able to receive the Coastguard Maritime Safety Information (MSI) broadcasts, you may need to select your location carefully to ensure that the signal is as free from obstruction as possible. Whilst charts do not show the contours of the land, it might be the case that if the depth contours on the chart are very close together at the coast, the land will similarly rise steeply from the waterline. Pictures in pilot books may also help you to establish the nature of the land in the locations you are considering.

The key is not to assume that just because you are boating in sea area A1 the VHF will work 100% of the time. If you are tucked in under a cliff you may be able to communicate with boats at sea, but not via antennas positioned on the shore. Be aware of VHF black holes, the conditions in which they may occur and plan accordingly.

Stuart Carruthers RYA Cruising Manager

Images by Sarah Brown.