AIS Antenna Height

[Tex]

I have fitted an AIS transponder antenna to stern rail and reception is very good, 20M+, I am however concerned about transmissions.
The unit is transmitting as I am being picked up by other vessels in the marina, however I do not show on Marine Traffic and they do.
I changed over to try with the VHF mast head antenna and immediately appeared on Marine Traffic.
Should I now consider a splitter instead?

 

[chubby]

The experts will tell you that splitters cause some loss of VHF power, I put two aerials on my mast head, one standard VHF and one AIS which is optimised for the particular frequency AIS uses but would still be a spare general aerial, put the brackets pointing 180 degrees away from each other to separate the aerials and run the cables down together, seems to work well1

 

[Twister_Ken]

Without knowing where the Marine Traffic RX aerial is (distance & height) in relation to your parking space, it's impossible to say whether you are getting sufficient range from your pulpit mounting. At what range do you want to be seen by shipping (assuming it is maintaining an AIS watch)?

The situation might also be different at sea, where you are not surrounded by metal masts, other interruptions to line-of-sight, and sources of interference.

 

[nigelmercier]

... I do not show on Marine Traffic ...

That isn't a problem, you may be many miles from the nearest ground station. You will get plenty of range at sea, where it actually matters. Marine Traffic is a bit of fun, nothing more.

Don't even consider a splitter, they should be banned in crucial safety gear. Not only are you adding a loss to the signal, you are also adding at least four more points of failure.

Another advantage of a stern mounted AIS antenna is that it forms the perfect emergency VHF antenna.

 

[William_H]

That isn't a problem, you may be many miles from the nearest ground station. You will get plenty of range at sea, where it actually matters. Marine Traffic is a bit of fun, nothing more.

Don't even consider a splitter, they should be banned in crucial safety gear. Not only are you adding a loss to the signal, you are also adding at least four more points of failure.

Another advantage of a stern mounted AIS antenna is that it forms the perfect emergency VHF antenna.

+1 However you may find it easy to raise the stern antenna about 1 metre above the rail. This should improve range a lot. good luck olewill

 

[onesea]

To me asking the height of an AIS antenna is like asking which size of anchor to use.

In a perfect world you want it as high as possible, a second Ariel at the top of the mast would be better. Or there is hot topic of the ariel splitter.

The thing to watch for is not picking up big ships at 20 miles in open water, its how far you can see smaller vessels yachts, fishing boats etc If entering or leaving a walled harbour (Dover?) you AIS range will be limited till clear. As others have said sounds like this effect is stopping your signal getting to the receiver for Marine Traffic.

Parents had a motor sailor with pushpit mounted VHF ariel, they soon moved it to the top of the mast because every time they called for lock in they had to turn the boat round.

Its all a compromise whats acceptable to one may not be to another.

 

[Ian_Rob]

Should we not be worried about emissions? I have recently been looking at the Garmin AIS 600 and noted that the recommended MPE distance for the ariel is 1.5m. Difficult to achieve with any pushpin installation. I assume other transponders would have similar safety issues?

 

[savageseadog]

Anyone racing at ORC CAT2 (Fastnet/ round Ireland etc) the new regulations state a masthead antenna for the transponder. We have a full size whip for ours on the back - it's neatly out the way. But now facing the fact we need a masthead one!

 

[jwilson]

Should we not be worried about emissions? I have recently been looking at the Garmin AIS 600 and noted that the recommended MPE distance for the ariel is 1.5m. Difficult to achieve with any pushpin installation. I assume other transponders would have similar safety issues?

Emissions: I was contemplating fitting an fixed waterproof AIS-equipped VHF set in the cockpit with a whip aerial on the pushpit, to feed AIS data to an existing at-the-helm plotter. ( I already have a fixed VHF at the chart table with a masthead aerial ). Most VHF sets say 1.5 metres MPE distance, which is not possible with a pushpit aerial. On the other hand I drove around in a RIB a lot when teaching dinghy sailing with a VHF aerial on the steering console within a couple of feet....

Should I worry about a pushpit aerial?

 

[Joker]

I have recently been looking at the Garmin AIS 600 and noted that the recommended MPE distance for the ariel is 1.5m. Difficult to achieve with any pushpin installation.

The broadcast power is about the same as a hand held VHF. You might be struggling to use a hand-held at a safe distance of 1.5m.

 

[Salty John]

Just to give some idea of range: On a small boat a pushpit mounted antenna might be 10' above sea level and will be presenting it's transmission up to 4 miles distance. If there is a similarly located receiving antenna on another small boat they will communicate at 8 miles. If the receiving antenna is on a larger boat, let's say 60' above sea level, then combined range will be about 13 miles. If it's on a ship at 100' then 16 miles is about the range. I think those ranges are OK for AIS given the likely speed of the vessels involved, others may disagree.

You can increase range by mounting the whip antenna on a pole - at 20' above sea level this would present the signal at 7 miles, combined range of 14 miles to a similarly fitted boat and 19 miles to the ship.

Two yachts with masthead antennas 40' above sea level will communicate at 16 miles. Masthead antenna to a big ship, about 20 miles.

These are theoretical figures, taking into account the slight gain through the curvature of the earth, but can be exceeded in certain atmospheric conditions - unfortunately you can't count on those conditions.

 

[nigelmercier]

... I think those ranges are OK for AIS given the likely speed of the vessels ...

Quite.

 

[dewent]

I worried about this too when I first fitted AIS. SWMBO was concerned when watching me sail from Falmouth to South Wales that I had disappeared in the Bristol Channel! However after some research on AIS traffic there are some huge blackspots on data reception. At the end of the day this is a ship to ship system and works perfectly for that even if fitted to a stern rail. Marine traffic is just for fun I think.

 

[funk-an-bord.de]

I agree to the formula of the radio horizon as:
Distance in nautical miles = 2,5 x square root out of antena high in meter.
Before changing the antena you might check in marinetraffic.com
your route.
When you know the position of the AIS receiving antena,
then you can analise the results.
And calculate the radio horizont of the receiving antena and your own.
So go outside of the range of the receiving AIS antena and see where you are reported.
When at sea you will have less metall parts around you which work in the marina as reflectors or absorbers.
Wilhelm

 

[lw395]

I would just say that an aerial on the pushpit is very vulnerable to being leaned on, ropes, etc etc.
Also the top of it really, really should always be above the height of someone's eye, as poking yourself in the eye is a far more common injury from antennas than radiation.
Put it on a 2m post at least.

 

[Koeketiene]

I worried about this too when I first fitted AIS. SWMBO was concerned when watching me sail from Falmouth to South Wales that I had disappeared in the Bristol Channel! However after some research on AIS traffic there are some huge blackspots on data reception. At the end of the day this is a ship to ship system and works perfectly for that even if fitted to a stern rail. Marine traffic is just for fun I think.

AIS transmissions are also being monitored by satellite.
Data from those in generally not available on 'free to view' websites such as Marine Traffic but is available on subscription (fee-paying) websites and some websites reserved for government use and the security services.

Last month, a friend sailed his AIS equipped yacht across the Atlantic and I was able to give his family who had stayed behind twice daily updates of his course/speed/position.
You can run, but you can't hide.

FWIW: I've only very recently fitted and AIS transponder, but it is my intention to only transmit when the situation requires it: crossing a TSS at night/dense fog/...
Big Brother is out there, there's no reason why I should make it easy for him.

 

[Daydream believer]

So I am about to purchase an AIS transponder plus a chart plotter ( good old LIBS chose it there)

if height of an aerial is a problem what would happen if i undid my backstay ( No the mast would not fall down) swung it in to the mast
stood on the cabin & taped the aerial to the backstay as high as possible ( stand on a box etc) then re rigged the backstay
I would get plenty of height, have little to snag on- lines sail etc
BUT-- here is the big question---would the backstay wire upset the aerial reception?

I also intend to fit an EChomax active radar reflector- could I do the same with that?

 

[Salty John]

BUT-- here is the big question---would the backstay wire upset the aerial reception?

The antenna needs to be vertical and in that position wouldn't be affected by the backstay. However, I think it might waft around considerably and that would cost perfomance.

 

[st599]

To me asking the height of an AIS antenna is like asking which size of anchor to use.

In a perfect world you want it as high as possible, a second Ariel at the top of the mast would be better. Or there is hot topic of the ariel splitter.

The thing to watch for is not picking up big ships at 20 miles in open water, its how far you can see smaller vessels yachts, fishing boats etc If entering or leaving a walled harbour (Dover?) you AIS range will be limited till clear. As others have said sounds like this effect is stopping your signal getting to the receiver for Marine Traffic.

Parents had a motor sailor with pushpit mounted VHF ariel, they soon moved it to the top of the mast because every time they called for lock in they had to turn the boat round.

Its all a compromise whats acceptable to one may not be to another.

A second aerial is not better. They'll be within each others interference zones and will no longer be omnidirectional.

A splitter is also bad news. To many points of failure in a safety critical
system.

 

[neil1967]

So I am about to purchase an AIS transponder plus a chart plotter ( good old LIBS chose it there)

?

Slight thread drift. May I ask which transponder and plotter you have chosen? I am going through the same process, but still not 100% convinced about the need for the transponder?

Neil