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Old 16-01-2013, 02:56   #16
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Originally Posted by chrisjs View Post
At a combined closing speed of even 22kts (15 + 7) your 5 mile range equates to only 13 minutes before impact!! More (higher) is definitely better!!!!
You do need to avoid having your VHF and AIS antennas close (at the mast head) because they can interfere and the outgoing AIS signal may even damage your VHF. The idea of mounting at the spreader seems like the best compromise.
If one cannot wake up, stumble over the rum bottles, look at the display, find the autopilot and change course a few degrees in 13 minutes one has problems other than AIS range...
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Old 16-01-2013, 03:31   #17
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Mine(Furuno FA50) has a dedicated VHF antenna mounted at the masthead adjacent the main VHF antenna. To date it has worked perfectly returning contacts from at least 30 miles and have been advised that we are visible to other ships at similar ranges
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Old 16-01-2013, 03:57   #18
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Re: Where to Mount AIS Antenna?

Hi as i am just fitting my AIS trasnponder i decided to use a transponder splitter to use my mast head antenna. the extra cost of approx 130 EUR is more or less the same when buying a new Antenna and cable etc for a seperated AIS system.
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Old 16-01-2013, 05:45   #19
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Re: Where to Mount AIS Antenna?

I question the validity of the 5 mile statement. I assume it originates from the distance to horizon from a 3 feet elevation.
Remember, the other guy's antenna also has elevation (the big guy's a lot more than yours), which increases range. Additionally, there is a curvature in VHF propagation providing yet additional increase in range.

Next time you are out sailing in open waters, with a handheld VHF, meet a mate, going in the opposite direction, also with a handheld VHF - agree to call each other until you loose clear communication. Then call him on your mobile phone and get his position - Voila! You almost have your AIS range. Add a little distance, because digital data is more robust than speach.
Finally, remember. This is between two low antenna elevations.

Tom

Or, if both of you have AISes, use them. Sorry about that
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Old 16-01-2013, 06:12   #20
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Re: Where to Mount AIS Antenna?

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Originally Posted by LJH View Post
I will answer this with he just might.

Aircraft have an Air Traffic Control System and many collision avoidance systems, yet there are still many incidents that occur every year. You don't ask your pilot to turn his TCAS off when you board your flight.
The reason for my question is due to the fact that 90%+ of recreational vessels have no AIS at all. So if the ferry's ability to run at 45kts is predicated on all vessels having an AIS transponder, I'm curious how they pull that off.

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Why do so many people whose life is made safer by these systems resist putting similar systems to good or proper use in their boat?

Accidents usually occur as the result of a chain of events. Make yourself visible and identify your self early - increase your odds. It is about his decision making as much as it it yours. You quickly become one less contact for him to concern himself with and reduce his workload slightly. Safer for all!

If you are concerned about the number of contacts you have on your contact list, learn to use filters to reduce your target list to those that concern you.
If the distance of AIS transmissions are the bane of proper use, why is there a 2 watt Class B?
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Old 16-01-2013, 06:21   #21
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Re: Where to Mount AIS Antenna?

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If the distance of AIS transmissions are the bane of proper use, why is there a 2 watt Class B?
Because, without doing the math, line of sight range (AIS VHF 2 W) is 30 nm?
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Old 16-01-2013, 06:30   #22
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Re: Where to Mount AIS Antenna?

Good morning. I have the Vesper marine Ais. installed it in 2011 on a Radar Post about 12 ft above the water using a Metz antenna. On passage to Ireland in 2012 routenely got returns at 30 to 35 miles. plenty of time to avoid or be avoided by other traffic. Question though is Canadian 2 cents worth more than US 2 cents worth?
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Old 16-01-2013, 06:45   #23
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Re: Where to Mount AIS Antenna?

On a small boat there's really no significant benefit from picking up AIS signals at 30 miles versus at 15 miles. It is just more info to process. Any good display will have CPA/TCPA alarm set, so you will be informed of anything that comes within your requirements of safety. Class B are set at low power, 2 watts, to avoid cluttering and overwhelming the system with excess info. More info does not correlate with more safety - hence accident descriptions as "Radar assisted grounding". The 2 watt output does mean that the Tx antenna needs to be good, or the antenna, coax, connector losses can be very significant. Using an antenna splitter with the main VHF antenna usually works well, but it does cause some small loss of low level VHF signals.
I mounted my VHF antenna on the pushpit rail for a few reasons. 1. I wanted a spare VHF antenna in place that was not connected to the mast. 2. I didn't want to tie it together with the main VHF via a splitter. 3. It was much easier to run the coax and and mount the antenna.
The receive distance is a quite a bit less than my previous splitter/mast head system. Ships that show up at 15-20 miles on the low level antenna would have shown up at 20-30miles on the masthead. I don't have any specs on how far my 2watt is transmitting.
As noted above, by the time you buy an antenna and mount, you are at the same cost as a splitter.
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