Range of VHF reception

[Bodach na mara]

When reading the thread started by Claymore about Clyde Coastguard forecasts, I remembered that I wanted to tap into your collected wisdom about the range of coastguard station radios. I recall that the CG who taught a bit of the VHF certificate at our local club stated the range as approx 50 miles, with a yacht VHF transmission restricted to about 2 miles (low power) to 10 miles (on high power.) During the last two weeks, we have been receiving coastguard broadcasts in the Clyde from Belfast, Carrickfergus, Liverpool, Hollyhead and Forth. This despite the fact that up in Loch Ridden we could hardly get any Clyde transmissions and their Greenock aerial is not receivable much beyond Rothesay.

A related phenomenon is the varying reception on different channels. We clearly heard the opening calls on 16 about an incident in an area that we were heading for so switched to 67 to monitor the situation. Although we could hear the coastguard, we could no longer hear the vessel on the other end of the conversation.

Any ideas?

<hr width=100% size=1>Ken Johnson

 

[tony_brighton]

Yacht transmission is related to height of the aerial - I've had a chat with Solent CG from 5miles north of Cherbourg.

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[Aja]

Ken

Number of points here.

1 Loch Riddon and surrounding area above Colintraive and through to the boatyard in Tighnabruaich is a notorious blackspot for VHF reception

2 Recent weather (high pressure) will mean that you can get better reception but can also mean that you can also get more interference (skip)

3 Is your ariel on the mizzen? This will limit your 'line of sight' and will greatly reduce your receive and transmit facility.

4 When the CG switches to channel 67 he may also be using an ariel in the vicinity of the casualty.

Sitting in Arisaig recently I tested my emergency ariel on the pulpit. Could here all the forecast on my main ariel but it 'cut out' on the emergency ariel - and no - I didn't ask for a radio check to see if it worked! i also was able to pick up Stornoway/Clyde/Belfast from there. Not uncommon.

Hope this helps.

Donald

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[vyv_cox]

Several people have reported surprising reception range in the past few weeks, presumably due to the weather/high pressure. Anchored at Middelharnis on the Haringvliet we received broadcasts on Ch 16 from Yarmouth, Thames and Dover Coastguards, each about or a little more than 100 (sea) miles away.

In more normal circumstances I have managed to make myself heard from just outside Fishguard harbour to Holyhead Coastguard, about 50 miles. I could hear them clearly.

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[peterb]

My best was a conversation with Lerwick Coastguard from just outside Ostende.

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[Bergman]

In "normal" conditions range is limited to line of sight more or less. There can be varying degrees of difraction and refraction that affect this to some extent.

In certain atmospheric conditions usually hot and high pressure, a phenomenon called "ducting" can occur whereby range is greatly extended by signals effectively being trapped in a refractive "duct" in the atmosphere.

Those of us old enough to remember 405 line TV will remember most summers when warnings were transmitted about "atmospherics" affecting signal quality. Ducting was almost always the cause and some very strange things were received, in one case New York taxi cabs.

One other thing that may have affected you in that location is "knife edge diffraction" which occurs when a signal passes across a sharp ridge. The signal being "spread" and effectively re transmitted (at a much lower power level) from the ridge itself.

Power is not normally the limiting factor on range, the smallest change in power level that the receiver can detect is 3db or half power so that only at the edge of readability would low power be critical.

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[Sybarite]

I picked up Falmouth coastguard very clearly a few weeks ago when I was near Belle Ile in S Brittany.

John

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[BrendanS]

Off Salcombe last week, Alderney CG came through as though next door.

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[Chris_Robb]

At Easter we were at Ushant, and spoke to and received the weather forecast from Falmouth as clear as a bell - 100 miles away. All due to high pressure - apparently it happens quite regularly but you cannot rely on it!

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[Twister_Ken]

From just off Cherbourg last June got a full house.

Dover, Solent, Brixham CGs, Jersey and Aldernney radio, and more French Cros than youcould shake a baguette at. Plus ships in the Casquets TSS

<hr width=100% size=1><A target="_blank" HREF=http://www.writeforweb.com/twister1>Let's Twist Again</A>

 

[BrendanS]

No wonder CG are cutting back on doing radio checks. Can you imagine what that lot would be like to listen to on Ch16 if things carry on like this:

'Bahamas CG, Radio nut, Radio nut, Radio check.'
Radio nut, this is St Petersburg, Russia, any routine traffic to 67 and wait for response'

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[nordic_ranger]

Range and reception mostly due to atmosphirics. On the Clyde I think I am correct in saying that there is six Coastguard ariels because of the topography of the hills etc. They the Coastguard can always hear you, and you them, even if you can only hear one side of any other transmision. Most transmisions that you hear both sides of are generally at close quarters eg. Western Ferries ship to ship.
Loch Ridden and through the Kyles of Bute are rotorious black spots and sometimes it is necessary to do radio relay.

Jim

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[MainlySteam]

>>"I recall that the CG who taught a bit of the VHF certificate at our local club stated the range as approx 50 miles, with a yacht VHF transmission restricted to about 2 miles (low power) to 10 miles (on high power.)">>

Putting aside out of the ordinary propagation that is correct as a generalisation, but remembering that the yacht ranges are yacht to yacht. While power has a significant effect on range, and a yacht's transmitter is usually of lower power than those of organisations providing a safety watch (eg the coastguard in UK) that tends to be counteracted by the coast stations higher gain antennas- so, usually, if you can hear them, they can hear you. The coast stations advantage in height works both ways too of course.

Power has a significant effect on VHF range, despite the common statement that it is line of sight. The radio installation also has a significant effect on power actually radiated from the antenna at VHF frequencies. For example, when my own yacht was built we explored the difference between using RG58 (the small diameter coaxial cable) or RG8 (the bigger one) on the approx 20m run from radio to the antenna at the top of the mast (we did the check because the larger coax required another in mast conduit to be run and so was not just a simple case of running big coax). When I did the analysis, assuming a typical coast station elevation, the large cable gave around 55 mile theoretical range and the small around 30 mile, purely from the power loss in the cable.

The outcome is that one finds that there are large perfomance difference between various boats. A good installation on a yacht with a masthead antenna will consistently outperform the 10 miles on high power that you were quoted by the Coastguard. Over water paths, ship to ship and assuming a good installation on the other vessel, I find I consistently get at least 30 mile range (25 watt radio and antenna about 16 m above water).

John

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[MainlySteam]

>>"We clearly heard the opening calls on 16 about an incident in an area that we were heading for so switched to 67 to monitor the situation. Although we could hear the coastguard, we could no longer hear the vessel on the other end of the conversation."<<

Ken - I have had a think about what could have happened here. Usually this happens on duplex channels which are not being operated through a repeater (if a repeater is involved you hear both sides of the conversation), but Ch 67 is not, of course a duplex channel. Three possibilities spring to mind:

1. The vessel had a radio problem (unlikley if was ok on Ch 16)

2. In changing channels the vessel accidently swapped to low power (1W) (again unlikely I would have thought)

3. USA Channels 16 and 67 are the same as the international ones, but the USA channel 67 is restricted to 1 watt only (the radio does that automatically). So the vessel may have had the USA channels selected and not the international ones and so transmitted on high power on 16 but low power on 67. Furthermore, I have come across VHF's manufactured in USA where the power restrictions that are mandatory on some of their channels are also incorrectly implemented on some of the international channels - so it is possible that they had international channels selected but still low power on 67.

I would think that point 3 could be the most likely scenario, or something similar. For example, I understand that some European countries (have heard Holland mentioned in this) have (or had) strange power and channel use restrictions compared to the rest of the world. I stand to be corrected on that, but if so then maybe the radio was sourced in one of those countries when such restrictions applied within them.

If anyone has any other ideas as to the cause, I would be as interested as the original poster to hear them.

John



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[Bodach na mara]

Thanks for all the information. I had not realised that some VHF sets cut the power to certain channels.

Reading between the lines in a couple of posts, I get the impression that I am not the only one to get annoyed at the incessant stream of requests for radio checks and other clutter on channel 16. Perhaps the daftest is the growing habit by a number of vessels of ending their efforts to call someone with a message such as "Blabbermouth, Blabbermouth, this is Chatterbox, Chatterbox. Nothing heard. Out" For whom is this message intended?

<hr width=100% size=1>Ken Johnson

 

[Aja]

"Blabbermouth, Blabbermouth, this is Chatterbox, Chatterbox. Nothing heard. Out"

There is a good reason for ending a message as above. If the person being called receives the invitation to make contact and replies, there is no guarantee that the originator will still hear. He is then indicating that he has not HEARD any reply even though one has been made, and lets the intended vessel know that although they both wanted to make contact the originator cannot fulfil the conversation.

Good radio procedure.

Donald

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