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Old 03 August 2007, 15:40   #1
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Range of VHF

How far out from the coast would you still be able to communicate with the CG ? for example if your heading for the Scillies or Alderney would you have VHF coverage for the whole trip, assuming a standard Icom Fixed 25W Radio?

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Old 03 August 2007, 15:58   #2
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HM CG in the UK will be able to hear 25w transmissions from within 30m of the coastline.

This is known as area A1 in the GMDSS defined area of responsibility for the UK Coastguard Agency.
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Old 03 August 2007, 16:18   #3
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Atmospheric conditions can make a big difference. Two weeks ago I was delivering a boat from Gurnsey back to Portsmouth. I picked up, and spoke to Solent CG from just outside St Peters Port that was about 70 nms from the transmitter.

I have heard Aberdeen CG while on a rib in the river Itchen.
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Old 03 August 2007, 16:23   #4
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I hesitated to use the word "guarantee" before the 30nm - but that is their GMDSS requirement for A1.

And yes, when there is high pressure the blasted VHF signals go bouncing along a very great distance as young Solent Ranger has indicated.
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Old 03 August 2007, 17:01   #5
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You will probably hear HMCG from the french coast as their antenna's are 10's of meters high, don't think they will hear you though with 25w! On a similar thought, I was speaking to a radio ham, he suggested fitting a "linear amplifier" to the VHF to increase the power output.... does this work? Is it legal, what are the pros & con's?
If anyone knows about this it would be interesting to hear.
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Old 03 August 2007, 17:06   #6
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Yep a linear would certailnly work. However the maximum permitted Output power on Marine VHF is 25W (so highly illigal) And yes HMCG can hear you from a yacht in Cherbourg spoken to them a few times, however it does depend on conditions.
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Old 03 August 2007, 17:08   #7
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Ummmn - I think it also very much depends on the height of YOUR arial.... and its state of repair!
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Old 03 August 2007, 18:16   #8
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The distance that you can recieve/transmit on your VHF depends on two things.

1) The height of YOUR Aerial
2) The height of THEIR Aerial

so roughly:

Quote:
Here is the formula for calculating the range of an antenna:

Square Root of Height Above Water (in feet) times 1.42 equals Range in Miles

Example: Highest point of your boat is 6 feet above water. You attach your 3-foot antenna at that point. The antenna is now 9 feet above water. The square root of 9 (which is 3) times 1.42 equals 4.26 miles

So how can you receive that marine radio weather broadcast that is 75 miles away? You have to perform this calculation for the "other" antenna also and add the 2 distances together to get the maximum range between the antennas. The weather transmission may be from an antenna several hundred feet high and on top of a mountain giving it enough range to connect to your antenna's 5 mile range. You would be able to communicate with a boat that was 10 miles away from you if it had an identical set up as you. Each of your antennas could "reach out" 5 miles.
So if you were both using a handheld radio at 5ft above the ground, you'd be able to transmit about 3 miles

Anyone know the height of a HMCG antenna?
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Old 03 August 2007, 18:19   #9
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Its a bit late to work it out - but since they can receive from 30nm - its a long way up in the air!
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Old 03 August 2007, 20:42   #10
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Height makes all the difference.

Commercial airliners transmit on similar VHF frequencies and even their maximun transmit power is only 25w!!! I have often heard pilots talking to control coming into land in the Canary Islands and that is without strange conditions. That's a distance of 1800 miles - I suppose it helps when your aerial is 45,000ft above sea level.........
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