VHF Interference

[martinwoolwich]

I was approaching Port Solent last weekend and had the VHF set to channel 80. I was about to contact them for a berth and suddenly got a load of French on the channel. Very clear lasted about 10 minutes. I'm pretty sure that it was the French coastguard giving weather information.

It seemed to me that other people were still talking on channel 80 whilst this was going on (in English).

Am I right in assuming that the French CG have a very powerful transmitter which is why I could hear them but they couldn't hear any one else?

Also how was it possible for two transmissions to be going on at the same time? I thought one transmission would "lock out" another

Third, I didn't think the French CG uses channel 80 at all, so what was that all about?

Sorry if the questions are a bit basic but this is my first season on the coast so I don't have a lot of experience with VHF other than a one day VHF course on the river Thames and getting my licence.

 

[Piers]

The answer is to be found in the Scuttlebutt forum, under the heading 'Whatever happened to VHF', Friday June 7th.

The phenomenon you encountered was 'ducting'.

Piers du Pré
MBM Cruising Club enthusiast
www.dupre.co.uk/fsPlaydeau.htm

 

[Twister_Ken]

Ducting, as Piers says below, and yes French CROSS stations do broadcast traffic and weather info on Ch80 regularly.

 

[Chris_Stannard]

I think a slightly fuller answer might help. Firstly the range of your VHF is not controlled by the power output but by the height of you aerial and the aerial of the other party. VHF at 5 watts to aircraft has a range of over 200 miles.

Ducting occurs in some weather conditions when there is layer of air near the surface, with a layer of different characterisitcs above it. It usually happens in high pressure weaTEHR systems or when the air is cold. You can sometimes see it because smoke from chimneys goes up and is then trapped and goes flat.

When this happens the VHF energy is trapped in the surface layer and you can get very extended ranges. I have heard Dover Coastguard whilst just off Cherbourg.
Incidentally the Met men will often refer to super refraction, rather than ducting, and some of us, who are long in the tooth, remember when it was called anomolous propagation, but I could not spell that, and looking at it I think I still cannot.

Chris Stannard

 

[martinwoolwich]

Thanks for that Chris, really helpful.

One final question if I may. How come people were able to transmit over the broadcast in the normal way? I thought this wasn't possible with VHF (or is it something to do with duplexing?)