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?

Chris

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.

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.

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.

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.

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.

Ummmn - I think it also very much depends on the height of YOUR arial.... and its state of repair!

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:

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? :)

Its a bit late to work it out - but since they can receive from 30nm - its a long way up in the air!

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.........

There is a big difference between being able to receive and transmit. Your transmit capability is dependant on the height of your aerial, the height of the receiving aerial AND the Db of your aerial. ( effects transmit pattern)
High Db is good. Those stupid little 3 ft. whip aerials which are only 3Db are only good for strapping to a rubber duck in a pond. 8 ft, 6 Db. aerial is the best for a RIB in my opinion.
:eek:
Of course, Codders is going to disagree as usual and come up with some super tech. explanation of the subject.

An 8ft 6db arial would look a little silly on my 4.8 RIB, it would look like big a radio controlled toy boat!:D

There is a big difference between being able to receive and transmit. Your transmit capability is dependant on the height of your aerial, the height of the receiving aerial AND the Db of your aerial. ( effects transmit pattern)
High Db is good. Those stupid little 3 ft. whip aerials which are only 3Db are only good for strapping to a rubber duck in a pond. 8 ft, 6 Db. aerial is the best for a RIB in my opinion.
:eek:
Of course, Codders is going to disagree as usual and come up with some super tech. explanation of the subject.

Not really.........

There is a big difference between being able to receive and transmit. ....

...Of course, Codders is going to disagree as usual and come up with some super tech. explanation of the subject. No, it'll be me. I believe an antenna is equally good at receiving as it is at transmitting. However, there are other factors which come into play that may influence a signal and enable you to receive it.

VHF also is a "line of sight" frequency, that means that the signal drops expenentually if line of sight isn't acheived. I think I remember right that at a hieght of 6' from Sea level the horizon is only 3 miles away. so that means that two RIBs trying to contact each other with arials of no higher that 6' from Sea level will have trouble contacting each other of say more than about 10 miles apart. There are other factors such as signal reflection etc but that is too heavy for me.

The link below provides good info. on this subject. Of course the sytem chosen depends alot on type of use and personal preferences. Personaly I prefer stuff that works at max. possible performance.
http://www.westmarine.com/webapp/wcs/stores/servlet/westadvisor/10001/-1/10001/VHFAntennas.htm
:thumbs:

More good info. on this site
http://www.catfleet21.org/rigging/June.htm
T.

And so Codders does not get bored. Here is some real techy. stuff
http://continuouswave.com/whaler/reference/VHF.html
I am VHFed out for today. :yawn: :D

And so Codders does not get bored. Here is some real techy. stuff
http://continuouswave.com/whaler/reference/VHF.html
I am VHFed out for today. :yawn: :D

:D

And so Codders does not get bored. Here is some real techy. stuff
http://continuouswave.com/whaler/reference/VHF.html
I am VHFed out for today. :yawn: :D

To be honest I did get bored - started studying for my amateur radio exams but gave up on them - much more fun to mess around with directed energy radio waves - amazing what they can do to computers and other lectronics......

I thought the CG had some whizzo relay antenaas or something so they arent just recieving on their main "twig". Stornaway CG could hear us trying transmit on 1 watt (until we realised it had been switched to "LO") from the northern tip of Skye. I'll have a look on the chart and see how far that is in a bit, me teas on; Jerk chicken, rice n peas, calalloo fritters and dumplings - the Afro Caribean carnival was here yesterday.

Each MRCC (Coastguard Ops Room) controls a number of aerials along their stretch of coastline.

When a call comes in, they have to switch to the best aerial before transmitting their reply.

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.........

I don't know about 1800 miles but the principle is certainly sound - my 2 metre amateur sets (with expanded freq range) will pick up the incoming MOD flight when they call inbound at about 200 miles out and it is as clear as you could wish for. Calling from about 30,000 feet I think.

Each MRCC (Coastguard Ops Room) controls a number of aerials along their stretch of coastline.

When a call comes in, they have to switch to the best aerial before transmitting their reply.

Yep. In my day a light lit up on whichever aerial switch received a transmission. To answer we just pressed the lit up switch and used the foot treadle instead of a PTT button to speak. If several aerials received the call, we'd choose the aerial with the most general coverage (the 'best' aerial) first. The aerials were connected to the Ops Room by dedicated telephone lines, which quite frequently got chopped up by cows/sheep/deer/JCB's/ploughs - or just blown away in a passing hurricane.