Champagne Murphy
Well-Known Member
Er... well it says it won’t so I suppose not.
AIS aerial vs AIS/VHF splitter
- Thread starter Champagne Murphy
- Start date
Er... well it says it won’t so I suppose not.
MM5AHO
Well-Known Member
Summing up then...
* Stubby antennas have less rane (on transmit or on receive) that standard length VHF whips
* AIS is part of the marine band, so no special AIS antenna is required, and the standard size will do without detectable difference.
* Height above SL is the most critical factor in transmitting or receiving VHF at sea, the higher the better.
* to combine standard voice / DCS marine VHF and AIS to use the same antenna, a splitter is required.
* The splitter is different in a transmitting system than a receive only AIS. (a transmitting one is built to handle the transmitted power, but works well also for receive. The converse does not apply)
* VHF and AIS via splitter to same masthead antenna denies you the redundancy of a second antenna
I was recently sailing in company offshore where both yachts (same model) has transmitting AIS. Mine has masthead antenna (about 11m asl) and splitter, the other has a dedicated AIS antenna on the frame above the pushpit, perhaps 2m asl. We could "see" each other reliably at 2 miles, but lost signal at about 3-4 miles. This was in a swell of 2-3m, where whole boat visibility was zero at times. (they went below the horizon in the swell troughs). During that time, I could see the AIS of other boats at distances of 5-10miles (class B) and 20 miles (Class A). I also have a pushpit VHF antenna, but that is fed from a second voice VHF, so in effect I have the backup, though mst would not if using a splitter.
* Stubby antennas have less rane (on transmit or on receive) that standard length VHF whips
* AIS is part of the marine band, so no special AIS antenna is required, and the standard size will do without detectable difference.
* Height above SL is the most critical factor in transmitting or receiving VHF at sea, the higher the better.
* to combine standard voice / DCS marine VHF and AIS to use the same antenna, a splitter is required.
* The splitter is different in a transmitting system than a receive only AIS. (a transmitting one is built to handle the transmitted power, but works well also for receive. The converse does not apply)
* VHF and AIS via splitter to same masthead antenna denies you the redundancy of a second antenna
I was recently sailing in company offshore where both yachts (same model) has transmitting AIS. Mine has masthead antenna (about 11m asl) and splitter, the other has a dedicated AIS antenna on the frame above the pushpit, perhaps 2m asl. We could "see" each other reliably at 2 miles, but lost signal at about 3-4 miles. This was in a swell of 2-3m, where whole boat visibility was zero at times. (they went below the horizon in the swell troughs). During that time, I could see the AIS of other boats at distances of 5-10miles (class B) and 20 miles (Class A). I also have a pushpit VHF antenna, but that is fed from a second voice VHF, so in effect I have the backup, though mst would not if using a splitter.
BrianH
Well-Known Member
But these are yachts with 2W output - how far do you want them to present a target on your display, and for what ... collision avoidance? Because isn't that what we install our AIS transceivers and receivers for?
Personally, I am not interested in yacht targets, they are unlikely to kill me. In fact, I would be quite happy not to receive yacht signals beyond "3-4 miles" with the persistent alarms that they often spuriously trigger as they tack back and forth. However, my pushpit AIS antenna will consistently receive target reports from commercial shipping at 20nm and beyond.
As I have written in another, recent AIS thread, the only vessels that interest me are the ones that could prove lethal and they are the commercial, big ones. Because they are big their AIS antennas are invariably mounted high up. VHF propagation range depends, not just on your antenna height, but on the other vessel's antenna height too. Your antenna height is by far the minor component of the combination. I myself am content to let the potentially lethal big ships have all the cost and complexity of achieving antenna height.
,
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thalassa
Well-Known Member
Before I mounted a transponder, I had connected one of these. It worked well, albeit for only one season. Exchanged it for a new one next year, and it too gave up the ghost after six months. I suspect the circuit is susceptible to the VHF transmission power.
I now have a pushpit antenna from the marina skip. Works very well (about 10 miles) with the Emtrak transponder.
MM5AHO
Well-Known Member
The point was that a pushpit mounted antenna is very restricted as demonstrated between two yachts. Now scale that up to receiving the Class A from a big ship that could do real damage and a similar ration of distances would apply - ie restricted reception distance. I like to know early that a big ship is heading my way.
BrianH
Well-Known Member
Not sure how you can extrapolate your experience above to commercial vessels. Meanwhile, in the real world, I fitted an AIS receiver with a pushpit-mounted antenna in 2007 and since then have never had Class A targets only appearing at distances that could be worryingly close - say, under 3 nm - even when on starboard tack with the port quarter-mounted antenna depressed and the propagation pattern far from optimally horizontal.
Just as my CPA alarm range is triggered within 2nm and even with the fast ferries that infest Croatian waters with closing speeds of up to 40 knots, I would be quite satisfied to be seeing (and to be seen by) commercial vessels at only 5nm, although the usual range is well over twice that. Anything else is just clutter that needn't be processed.
Now, with an AIS transponder, I note large vessels in the middle of the Adriatic, when I am in their path, seem to change course slightly to avoid at about 2nm away, which implies to me that 5nm is the critical range for my safety.
Bobc
Well-Known Member
Before I mounted a transponder, I had connected one of these. It worked well, albeit for only one season. Exchanged it for a new one next year, and it too gave up the ghost after six months. I suspect the circuit is susceptible to the VHF transmission power.
I now have a pushpit antenna from the marina skip. Works very well (about 10 miles) with the Emtrak transponder.
My Glomex splitter also packed up after about a year, so just went back to having a separate AIS antenna on the pushpit which works fine. Will be fitting a radar post in the spring, so will re-locate the AIS antenna to the top of that, putting the tip about 5m above the waterline. We'll see what range that gives.
ghostlymoron
Well-Known Member
Does one really need to know what a ship 40 miles away is doing?
RichardS
N/A
If it's less effort to find out what a ship 40 miles away is doing than it is to find out what a ship 10 miles away is doing, why wouldn't you choose the 40 mile option?
Richard
pvb
Well-Known Member
If it's less effort to find out what a ship 40 miles away is doing than it is to find out what a ship 10 miles away is doing, why wouldn't you choose the 40 mile option?
Because most chartplotters can only display a fixed number of AIS targets, and the default position is to show those targets closest to the boat. Having the antenna in a position to receive targets 40 miles away doesn't mean they'll ever be displayed on the screen.
RichardS
N/A
PaulRainbow
Well-Known Member
But if it's less effort (to use a splitter than not), it's less effort.
Richard
Once you've fitted the equipment, how is either option more or less "effort" Richard ?
If fitting the equipment, how much extra "effort" is needed to fit a 2nd VHF antenna, as opposed to fitting a splitter ? I fit both and in most cases there isn't much to choose between the two.
rszemeti
Well-Known Member
Don't forget that a splitter, no matter how good, will always add some insertion loss, so it will degrade your VHF performance on transmit and receive.
Fitting a second antennae sounds great, but equally, you have to have somewhere to put it, and generally speaking putting it closer than 1 wavelength to another antennae in the same band is going to mess up your radiation pattern, and there is only so much room at the masthead ...
Fitting a second antennae sounds great, but equally, you have to have somewhere to put it, and generally speaking putting it closer than 1 wavelength to another antennae in the same band is going to mess up your radiation pattern, and there is only so much room at the masthead ...
RichardS
N/A
I'm talking about the effort in original fitting of the equipment rather than the effort in using it.
There's a world of difference on my boat. I already had the instrument locker open to attach the AIS unit to the inside with gorilla velcro. The VHF is also mounted in the same locker so all I had to do was unplug the radio antenna cable, plug in splitter, plug in AIS, connect +ve and -ve into connectors used for AIS power. About 5 minutes tops.
Mounting an antenna on the pushpit, or anywhere outside, and running the cables would be a major job requiring a deck gland or similar or running a cable up inside the hard bimini which would require some serious ceiling panel removal and drilling. I reckon a couple of hours minimum.
And the extra 1hr 55minutes work gives you ..... half the range. :ambivalence:
Richard
RichardS
N/A
PaulRainbow
Well-Known Member
For AIS you need an active splitter so no loss.
Richard
Impossible. You cannot transmit and receive with two devices, using the same antenna, without there being some losses. If you are using the radio, or the radio is even receiving other VHF transmissions, AIS data will be missed. Doesn't usually make a difference that you'd notice, but data can get lost.
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PaulRainbow
Well-Known Member
I'm talking about the effort in original fitting of the equipment rather than the effort in using it.
There's a world of difference on my boat. I already had the instrument locker open to attach the AIS unit to the inside with gorilla velcro. The VHF is also mounted in the same locker so all I had to do was unplug the radio antenna cable, plug in splitter, plug in AIS, connect +ve and -ve into connectors used for AIS power. About 5 minutes tops. <snip>
But i fit the splitters properly, so it takes quite a bit longer than 5 minutes.
Let's not forget the benefit of having a 2nd antenna, not at the masthead, when fitting pushpit mounted antennas.
My personal preference is for a 2nd antenna, but at the end of the day, both systems work.
savageseadog
Well-Known Member
RORC requires AIS antennas to be fitted at least 3m above sea level to compete in their offshore races.
3.29.12 a class A AIS Transponder which either:
Mo0,1,2 Mu1,2 3.29.13 an AIS Transponder which either:
MoMu0,1,2 3.29.13 a) shares the masthead VHF antenna via a low loss AIS antenna splitter; or
MoMu0,1,2 3.29.13 b) has a dedicated AIS antenna not less than 38 cm (15”) in length mounted with its base not less
than 3 m (10’) above the Waterline and co-axial feeder cable with not more than 40% power loss
(Loss Estimator)
3.29.12 a class A AIS Transponder which either:
Mo0,1,2 Mu1,2 3.29.13 an AIS Transponder which either:
MoMu0,1,2 3.29.13 a) shares the masthead VHF antenna via a low loss AIS antenna splitter; or
MoMu0,1,2 3.29.13 b) has a dedicated AIS antenna not less than 38 cm (15”) in length mounted with its base not less
than 3 m (10’) above the Waterline and co-axial feeder cable with not more than 40% power loss
(Loss Estimator)
RichardS
N/A
AIS includes digital error correction which is why the operating range is so much greater than analogue VHF. I don't think there is any chance that any data will be lost which relates to nearby targets. You might lose 1% of the data relating to target at more than 20 miles distant ..... whereas with the pushpit antenna you'll lose 100%.
Richard
PaulRainbow
Well-Known Member
AIS includes digital error correction which is why the operating range is so much greater than analogue VHF. I don't think there is any chance that any data will be lost which relates to nearby targets. You might lose 1% of the data relating to target at more than 20 miles distant ..... whereas with the pushpit antenna you'll lose 100%.
Richard
Wrong. You cannot digitally correct for data loss that was missed because the splitter was allowing use of the VHF. That transmission will have been missed, end of.