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Old 02-03-2009, 10:25   #31
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Originally Posted by s/v Jedi View Post
- Your radio is supposed to have 50 nm range between you and shorestations and other ships. Sailboats achieve that by putting the antenna (3dB gain) on the masthead, motorboats use hi-gain stacked dipoles 8' long or more...These are the GMDSS requirements as implemented by coastguards/SAR services worldwide
Where did you get that information? That is just plain absurd! The GMDSS does not require your VHF to operate over a specific range other than to be able to communicate with the designated shore facility which is typically within 20nm for sea area A1. The shore facility is using a tower for it's antenna and the actual distance for that sea area is determined by the shore facility antenna height. There is no requirement for communication range to other ships via VHF within the sea area.

Eric
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Old 04-03-2009, 18:21   #32
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You are right that the VHF is for area A1. However, area A1 is allowed to stretch 50 nm from the nearest shore.

Find the ITU presentation here:

http://www.itu.int/ITU-R/conferences.../00-18_pp7.ppt

Intership normally assumes the range the same. You could miss a ship calling you from afar but on "intercept" course. I hear/see that regularly listening and watching AIS. AIS range is much more BTW, I see ships over 100 nm away with my antenna 60' high. This is similar to pactor: digital comms work when voice is not readable.

In Holland, the authorities used to survey VHF installations aboard before giving you your station-license. They tested with a coastguard station about 50 nm away. Always worked.

cheers,
Nick.
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Old 04-03-2009, 19:42   #33
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You are right that the VHF is for area A1. However, area A1 is allowed to stretch 50 nm from the nearest shore.
I repeat: "...typically within 20nm for sea area A1. The shore facility is using a tower for it's antenna and the actual distance for that sea area is determined by the shore facility antenna height"

Quote:
Intership normally assumes the range the same.
No way!

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AIS range is much more BTW, I see ships over 100 nm away with my antenna 60' high.
Don't think so unless it's through a repeater system. With your antenna at 60', the other antenna would have to be over 4000' high to get that kind of range direct.

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This is similar to pactor: digital comms work when voice is not readable.
VHF and HF are two different ballgames

Eric
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Old 04-03-2009, 19:54   #34
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I like my Radios with the old telephone type handset. Easier to hear when there is a lot of wind/background noise.
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Old 04-03-2009, 22:31   #35
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Eric,

I am sorry but the typical distance from within the A1 area to the nearest shore station is not the sufficient range: the maximum distance between a vessel in A1 and a shore station is what a vessel must be prepared to overcome and that is 50 nm, like defined by the ITU. This is also not a problem (as you state I think) because the shore stations have high antenna's, especially when they know they need to bridge 50 nm.

I started stating the 50 nm and you replied "absurd! A1 is just typical 20-30 nm". Both are correct except your "absurd!" part because the 50 nm is written by ITU as the allowed maximum distance and, as you state, 20-30 nm is typical, so we're both right ;-)

The former VHF station at Saba (Caribbean) had a service range of 500 nm. That's a total diameter of 1000 nm with their antenna in the center.

I hear ship-ship traffic where both ships are on my AIS and they are 30-50 nm apart often. Why do you think I am dreaming that up? I can also assure you that no repeaters are needed for that distance. We also have regular 40 or 50 nm ship-ship contacts, there's nothing special to that.

On AIS range: search this forum and/or the ssca forum. Many others confirm, it is common to see 100nm+ distances. Digital communications can overcome much worse SNR's than voice communications. This is much like morse vs voice. Both VHF and HF have the SNR issue even though they use different modulation techniques. You can make pactor links without hearing the modem on the other end. Voice would not work. I tried and accomplished that, as did many others.

The math formula you use is for reliable voice communications on vhf frequencies, not a strict limit. There is, in fact no limit when phenomena like ducting are considered where 1000's of nm are achieved by HAMs around the world during certain conditions. I managed a mere 300 nm once using 4W into a 21dB yagi. But I could do 300 nm without ducting at 250W power output. Power is a factor too.

But we're OT. This thread was about range of VHF radio's aboard our yachts. I stand by my statement that every installation should be able to do 50 nm for both ship-shore and ship-ship during average conditions. This is not hard, every decent installation can do it.

cheers,
Nick.
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Old 05-03-2009, 01:26   #36
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However, area A1 is allowed to stretch 50 nm from the nearest shore...

...the maximum distance between a vessel in A1 and a shore station is what a vessel must be prepared to overcome and that is 50 nm, like defined by the ITU...
First, the criteria are defined by the IMO, not by the ITU.

Second - Fairbank56 is correct. In actual fact the IMO Resolution setting out the criteria for determining Sea Areas requires for Sea Area A1 that the Area must be under continuous guard by at least one station, the path over the Sea Area must predominantly be over water and that part of the Sea Area covered by a particular station is a circle of radius determined by a given formula in which the only variable is the height of the shore station antenna. The mobile stations antenna height is required to be entered in the formula as being 4m above the water. The antenna gain and power of both stations is ignored and the result is essentially simple line of sight.

For example, if the shore station antenna is at 100m the maximum radius of the Sea Area is to be 30nm. Shore station at 50m, max radius is 23 nm.

So, one does not have to have a boat station that has the capability of working over 50nm at all. The path to be worked is a simple line of sight one and the outer limit of the Sea Area is completely determined by the height of the shore station antenna. A mobile station with a working radio can be expected to work anywhere within that declared area, whatever the extent of that area is, as long as its antenna is 4m high or more.

That is, if the IMO criteria established the outer limit of a Sea Area at 20nm and such a boat can work that 20nm path then it will just as capably work the path to the outer limit of other Sea Areas A1 which happened to be declared at 30nm, 40nm, 50nm or whatever.
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Old 05-03-2009, 04:20   #37
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GMDSS Sea Areas are defined by the responsible Government; and any distances given are mere approximations, and vary by Government:
Sea area A1 – This is an area within the radiotelephone coverage of at least one VHF coast station and in which continuous VHF digital selective calling (DSC) alerting is available. The area is as defined by the responsible contracting Government;

See:
http://www.itu.int/ITU-R/conferences...s/20-gmdss.doc
http://www.ccg-gcc.gc.ca/eng/CCG/SAR_Gmdss
www.uscgboating.org/safety/GMDSS_brochure.pdf
www.gmdss.com.au/concepts.htm
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Old 05-03-2009, 05:52   #38
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Wow....have we got far afield from the original question!

On a boat the effective range for 2-way communications will be determined by:

1. the height of the antenna;
2. the ERP from the antenna (affected by type and quality of coax, loss in the coax, full output of 25 watts from the transceiver, effective gain of the antenna);
3. proper adjustment of the receiver squelch; and
4. height and ERP of the antenna at the other station -- ship or shore.

This does not include the occasional boosts -- sometimes very great ones -- from ducting, scatter, etc. These are fun but cannot be relied upon for routine or emergency communications.

In practice, your mileage will vary. You may well be able to contact a Coast Guard station some 30-40 or even 50 miles away. You'll be hard-pressed to communicate with other sailboats more than about 10-15 miles away. On average. Ditto for communication with some shore stations, e.g., marinas with relatively low power and low antennas. You could communicate over distances as great as 100 miles or so with stations having very high antennas, or with aircraft.

But putting any number on the effective range of VHF on a boat, IMHO, is really extremely difficult. In USCG exams, questions on VHF range use 20 miles as the effective outer limit.

The bottom line is that you should do what you can to ensure that your installation on your boat is as good as you can reasonably make it. This means that (on a sailboat) you should:

1. use a mast top antenna;
2. use a 3db antenna because of it's wide VERTICAL radiation pattern (less affected by heeling than higher-gain models);
3. use the best coax for the job: RG-213 or 214 are good in many installations;
4. be sure the connectors are properly fitted to the coax, making watertight, secure contact and no short or open circuits;
5. ensure that there is adequate power getting to your VHF radio while transmitting (adequate wire size for the length of run, good fusing, clean connections, etc.); and
6. ensure that you know how to adjust and operate a VHF radio (squelch function, DSC if fitted, channel selection, communications protocol, etc.).

If you've done all these things, then don't worry any more about effective range. It is what it is :-)

Bill
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Old 05-03-2009, 05:57   #39
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See also an earlier discussion at:

VHF RADIO RANGE
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Old 05-03-2009, 06:27   #40
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I think Bill (btrayfors) pretty much nailed it.

Sure Bermuda Harbor Radio can be heard from a long ways off, but trying to raise them 50 miles out from the boats most of us have would be a waste of time. Which isn't to say that were I in trouble and 50 miles out, that I wouldn't be on the radio, but counting on it? Practically speaking, nah.

And that, I think, is the point of contention: what the rules say versus experience on smaller (i.e., not as big as a freighter) boats. So what say y'all agree to disagree, shake hands, and have a (virtual) beer. Either that or meet on the field of honor - cowpats at 10 paces...
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Old 05-03-2009, 06:38   #41
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Nick,

Your entire post is nonsense in my professional opinion. Anyway, my first reply was to dispute your claim that the GMDSS requires your VHF to have a range of 50nm between you and shorestations and other ships. It is simply not the case.

Eric
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Old 05-03-2009, 07:00   #42
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Does this mean it'll be cowpats at ten paces at dawn?

Honest, folks, this "I am right, you are out of your skull" stuff is getting old. How about just letting go?
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Old 05-03-2009, 15:01   #43
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GMDSS Sea Areas are defined by the responsible Government; and any distances given are mere approximations, and vary by Government:
Sea area A1 – This is an area within the radiotelephone coverage of at least one VHF coast station and in which continuous VHF digital selective calling (DSC) alerting is available. The area is as defined by the responsible contracting Government;

See:
http://www.itu.int/ITU-R/conferences...s/20-gmdss.doc
http://www.ccg-gcc.gc.ca/eng/CCG/SAR_Gmdss
www.uscgboating.org/safety/GMDSS_brochure.pdf
www.gmdss.com.au/concepts.htm
To add to the list the criteria set by the IMO for governments defining Sea Areas are in Resolution A801/19 - while old, I believe that is still the current Resolution. It may be on the internet somewhere.
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Old 05-03-2009, 15:34   #44
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WOW What a wealth of knowledge and good ideas. It sure sounds like you guys have done this once or twice before. My only caution would be with the antenna switch being one more thing to fail. As inexpensive as another antenna on the pushpit would be and the redundancy factor, I think I would go that route.

This would be overkill, but has anyone ever used 1/2 inch hardline up the mast from the radio?

Hal
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Old 05-03-2009, 15:59   #45
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A related query:

Does anyone know what the actual vertical beam width is for the various commercial higher gain (6,9, 12 dB, etc) VHF antennas ?
We are admonished to not use such equipment on our oh-so-tippy monohulls, while our brethren on motor yachts (many of which roll like pigs in bigger seas) seem happy with the high gain devices. Personally, I seldom want to use myVHF when I'm sailing at big heeling angles, and so wonder how big an issue it really is.

Cheers,

Jim and Ann s/vInsatiable II
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