AIS Antenna

It might be adequate as a stopgap solution, but over time it would be frustrating. I'm sure that God invented masts for putting aerials on, so why not use them? Most of us have a radio aerial up there and a splitter is a fairly simple solution, and works well on my boat.
 
johnalison said:
It might be adequate as a stopgap solution, but over time it would be frustrating. I'm sure that God invented masts for putting aerials on, so why not use them? Most of us have a radio aerial up there and a splitter is a fairly simple solution, and works well on my boat.
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And will increase the range to 20-30 miles Class B and 40-50 miles Class A. ;)

Whether you think the extra range is useful is very much a personal decision and has been debated on here a zillion times. :)

Richard
 
Cotillion said:
What would be the range for an AIS antenna mounted on the push-pit?
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More than adequate. Most AIS displays have a finite limit on the number of targets they can display, and only display those nearest to you. So why would you want to possibly be able to receive AIS data from vessels 20 or 30 miles away?
 
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pvb said:
More than adequate. Most AIS displays have a finite limit on the number of targets they can display, and only display those nearest to you. So why would you want to possibly be abl;e to receive AIS data from vessels 20 or 30 miles away?
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The reason might depend on your area of sailing. South coast of UK it might mean overload of data for a masthead, and be sensible to cut it own with a lower antenna. But north west Scotland, sometimes seeing the few (2-3) boats within 15-20 miles is useful.
 
MM5AHO said:
The reason might depend on your area of sailing. South coast of UK it might mean overload of data for a masthead, and be sensible to cut it own with a lower antenna. But north west Scotland, sometimes seeing the few (2-3) boats within 15-20 miles is useful.
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How quickly do these boats approach you?
 
Some of the big commercial stuff does close to 20 Kts. Not all by any means but some do. 10NM warning would do for me?
 
If you’re worried the range from the pushpit is insufficient and you don’t fancy using a splitter, you could try a pole on the aft deck.
With my AIS aerial on a pole I regularly see ships over 15 miles away, which gives me plenty of warning.
 
Cotillion said:
What would be the range for an AIS antenna mounted on the push-pit?
Click to expand...
The range is your radio horizon plus that of the party you're communicating with. If your antenna is, say, 9 feet above sea level, your horizon is 4.2nm. If your correspondent has an antenna, say, 49' above sea level, his radio horizon is 9.8nm. Your combined range is 14nm. AIS signals from ships come from antennae higher than this so your range could be 20 miles or more. Range to a boat with an antenna at the same height as yours would be 8.4 nm, (Formula is 1.4 x root of height above sea level in feet. Answer is nm.)
 
pvb said:
More than adequate. Most AIS displays have a finite limit on the number of targets they can display, and only display those nearest to you. So why would you want to possibly be able to receive AIS data from vessels 20 or 30 miles away?
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You can't have too much range when it comes to safety. In any case, I might want to know where my 'buddies' are.
 
Mine is on a short mast made from half an extending sailboard boom. It is clamped to the push pit with 2 "U" bolts & if I want extra range when crossing the Dover straits & use the extend facility to raise the aerial up another 2 feet. Otherwise it stays down. I have been contacted by ships that I have not been able to see& range has been more than adequate. Being on the pushpit means I have an emergency VHF aerial because I have ensured that there is enough spare at the AIS control box to unplug & put into the VHF if needed
I consider that a much safer option that a splitter on the masthead aerial
 
Another variant if one doesn't fancy using a splitter but still want some more range, or have the antenna out of the way is to replace the backstay with a HMPE (dyneema) line and put the antenna along that, a couple of meters up the backstay.
Easy and cheap, building a dipole antenna out of a plain coax is described here: AIS Antennas

I use this solution, and get around 15nm reception for commercial traffic (class A). Leisure (class B) is around 6-8nm.
 
Another good reason for having a lower mount is you are less likely to be receiving as many class A signals that are totally beyond your visible horizon! Class A get priority for transmission over most types of class b transmitters. This may delay your own position transmission. I have seen my own boat transmission being delayed by at least a quarter of a mile behind my actual position when in amongst a large cluster of class a transmissions and I was doing over 10 knots!
Ais isn't a foolproof answer to all our collision avoidance problems but it will show how large class A have altered course for you out of visible range or before you can assess a collision course with a hand bearing compass! My only worry with a low mount antenna is a fast moving rib or mobo with class b that might give less warning of approach. I would hope they would be going considerably slower than normal in bad visibility probably at nearer displacement speed
 
I must admit that this thread has helped me out no end. I’ve just installed at AIT1500 and was debating about the little stubbie, now I think a 25mm pole on the pushpit would be most acceptable and avoid me purchasing that damn solar arch!
 
We have an antenna on the pushpit that we used for several years for the AIS, but with an active Class B transponder now I reckoned a splitter to the masthead antenna gives us better transmit range. Of course reception is better too, but we sail in a low AIS density area so Class A saturation as mentioned above is not a problem. The pushpit antenna is still available as an emergency spare - just a cable swap
 
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