AIS-VHF Antenna Splitter

[dlochner]

No, they boats will see each other at a little under 10 miles. Same as with landfall, you can see mountain tops long before you can see the beach.

AIS and VHF are line of sight. Once the other antenna pokes above the horizon your AIS will see it. There are a few other considerations, like atmospheric conditions that can lengthen or shorten the distance, but those are somewhat rare.

Transmission power does have an impact, I think AIS transmits at about 2 watts, however, transmitting a data burst is more efficient than transmitting analog voice, which is why VHFs are 25 watts. It is also why DSC emergency signals are much better than voice Maydays.

 

[dLj]

No, they boats will see each other at a little under 10 miles. Same as with landfall, you can see mountain tops long before you can see the beach.

AIS and VHF are line of sight. Once the other antenna pokes above the horizon your AIS will see it. There are a few other considerations, like atmospheric conditions that can lengthen or shorten the distance, but those are somewhat rare.

Transmission power does have an impact, I think AIS transmits at about 2 watts, however, transmitting a data burst is more efficient than transmitting analog voice, which is why VHFs are 25 watts. It is also why DSC emergency signals are much better than voice Maydays.

I understand AIS and VHF are line of sight.

But if my AIS can see the ship at 25 nm, they must also be able to see me at 25 nm. How can I see them and they not see me with a line of sight device?

dj

 

[dlochner]

I understand AIS and VHF are line of sight.

But if my AIS can see the ship at 25 nm, they must also be able to see me at 25 nm. How can I see them and they not see me with a line of sight device?

dj

They should be able to see you. Higher up you will be able to see them further away and often behind some land forms which is handy in some places, like the entrances to harbors or behind islands. The entrance to the St Lawrence river is one example. The shipping channel runs N and then makes a right turn into the river. There is a headland that blocks the view of the approach. A high antenna on both vessels allows them to see each other long before the turn onto the river, which is only a couple of miles away.

 

[dLj]

They should be able to see you. Higher up you will be able to see them further away and often behind some land forms which is handy in some places, like the entrances to harbors or behind islands. The entrance to the St Lawrence river is one example. The shipping channel runs N and then makes a right turn into the river. There is a headland that blocks the view of the approach. A high antenna on both vessels allows them to see each other long before the turn onto the river, which is only a couple of miles away.

@dlochner I'm going to have to retract several of my earlier comments. I went looking through my manual to see what the power rating of my AIS was as you had mentioned a value and I wanted to see if my unit met that or if for some reason it may have been more powerful. I then came to the understanding that the antenna I have mounted on my stern rail is not my AIS transmit/receive antenna, but only the GPS antenna that the unit uses... I'd always wondered why that antenna never looked right for a VHF unit...

The VHF antenna that the AIS uses is indeed on my mast head!

So indeed my AIS unit is running close to 65 feet above sea level! HA! These conversations are great for figuring things out!

dj

 

[kappykaplan]

These conversations are great for figuring things out!

I have two GPS antennas on my pushpit. One is for the chart plotter and the other is for AIS.

 

[Ward H]

The VHF antenna that the AIS uses is indeed on my mast head!

Hi Dave,
Now that begs the question, where is your VHF Radio antenna? Do you have only one and use a splitter?

 

[dLj]

Hi Dave,
Now that begs the question, where is your VHF Radio antenna? Do you have only one and use a splitter?

It is also on the mast head and no, I do not run a splitter. This means if I'm using my VHF, it likely interferes with my AIS. My manual says to mount the two at least 5 feet apart. They clearly are not 5 feet apart. They are mounted one on port and one on starboard of the mast head. Looking at them, I'd say they are maximum maybe a foot apart. I have not noticed any issue with these mounted like this. But then, I mostly just monitor my VHF radio. I do use it sometimes coming into port, but really haven't paid any attention to if it interferes with my AIS, although at that junction, I don't think it would matter much...

I have depended heavily on my AIS unit, and it has performed magnificently! Frankly, I don't know how I would have done this trip without it; running up the coasts of Nova Scotia/Newfoundland and then especially crossing the shipping lanes coming into Europe! Man, there were some big ships that clearly would have just run my a$$ over....

dj

 

[dlochner]

I think the concern about antenna placement is not due to potential interference, it is signal strength from the VHF broadcast over whelming the AIS receiver and damaging it. The AIS broadcast is very low wattage and very brief, just a second or so, thus it probably is not an issue for the VHF.

 

[Ward H]

Frankly, I don't know how I would have done this trip without it;

@jssailem and @LeslieTroyer introduced me to benefits of AIS back in 2019 while sailing on the Georgia Straight. We were entering Annette Inlet in the Gulf Islands during a snow squall. We were watching a fast moving ferry on AIS and were not sure if they would cross in front of us or turn towards us as we were coming in the Inlet. They were apparently watching us as well as they soon called us to let us know their direction of travel and Les let them know ours. We only had a brief glimpse of them thru the snow as we passed well off of their stern.

 

[dlochner]

Second only to GPS, AIS is a valuable asset if there is any significant boat traffic. On the ICW it let powerboats hail you by boat name rather than "white sailboat by Maker 103" when they wanted a slow pass. On the ocean at night, it is much easier to see other boats with AIS and as @dLj notes know their course and speed.

It also serves as entertainment, looking boats as they show up on the screen.

 

[dLj]

I think the concern about antenna placement is not due to potential interference, it is signal strength from the VHF broadcast over whelming the AIS receiver and damaging it. The AIS broadcast is very low wattage and very brief, just a second or so, thus it probably is not an issue for the VHF.

I can't comment about this - reading the manual, it only states how it should be mounted, not what happens if you don't...

I can say, both the VHF radio and the AIS transceiver have both been mounted as they currently are configured for the almost 3 years I've owned the boat, and for the numerous years prior when the PO had the AIS transceiver installed. So from the documentation, that would mean almost 10 years with this configuration and both work well...

dj

 

[dLj]

@jssailem and @LeslieTroyer introduced me to benefits of AIS back in 2019 while sailing on the Georgia Straight. We were entering Annette Inlet in the Gulf Islands during a snow squall. We were watching a fast moving ferry on AIS and were not sure if they would cross in front of us or turn towards us as we were coming in the Inlet. They were apparently watching us as well as they soon called us to let us know their direction of travel and Les let them know ours. We only had a brief glimpse of them thru the snow as we passed well off of their stern.

I had been a skeptic of the need for AIS but now I'm a staunch convert! If you are boating with traffic (and where can you be today without?), it's indispensable. Doing over night passages, it is indispensable - I run with an alarm that is set such that if the closest point of contact is 2.5 nm or less, then the alarm goes off. At 3:30 am (pick a difficult time) after a very tough passage, it's really nice to have this backup!

Running up the Delaware Bay, I had a tug hail me by name asking for me to move to the far side of the channel as they were coming down with a very wide barge. There are so many advantages to running an AIS transceiver it's hard to list them all....

dj

 

[stephen_i_w]

My separate AIS antenna is mounted on my Bimini Frame. That puts it a few feet higher than my stern rail. It is one of those 8 foot, white, power boat type antennas. Whereas, my vhf antenna is atop my mast.

I can definitely see AIS freighters 10nm away on my chart plotter. As for my AIS transmit range, usually, my boat's AIS shows up on the Marine Traffic web site ( MarineTraffic: Global Ship Tracking Intelligence | AIS Marine Traffic ). So it must be reaching whatever receivers Marine Traffic uses. This seems sufficient to me.

 

[dLj]

It also serves as entertainment, looking boats as they show up on the screen.

Ya can't under-rate this one!!!!! Hahahaha - it was so much fun looking at where all merchant ships were from (an often going to) - the entertainment value is very high!

dj

 

[dlochner]

My separate AIS antenna is mounted on my Bimini Frame. That puts it a few feet higher than my stern rail. It is one of those 8 foot, white, power boat type antennas. Whereas, my vhf antenna is atop my mast.

I can definitely see AIS freighters 10nm away on my chart plotter. As for my AIS transmit range, usually, my boat's AIS shows up on the Marine Traffic web site ( MarineTraffic: Global Ship Tracking Intelligence | AIS Marine Traffic ). So it must be reaching whatever receivers Marine Traffic uses. This seems sufficient to me.

For the Great Lakes, the stern rail is probably adequate in almost all situations. Commercial traffic tends to stay in the shipping lanes which are well defined, at they are on Lake Ontario. On the high seas, the greater the range, the better. AIS will see boats long before your eyes will allowing for plenty of time to alter course.

Here on the south shore of Lake Ontario it took a couple of years before I saw any traffic on my AIS. Guess I was an early adopter on this lake.

Marine traffic relies on mostly volunteer stations along the shore for the free accounts and adds satellite tracking for paid accounts. If you have an account and follow your boat you'll get an email when you arrive at a port. Handy if you didn't notice that you arrived.

 

[jssailem]

Marine Traffic is a 3rd party supplier of AIS data. They use the many coastal repeater sites and if you pay the price they use the satellite feed to give you the location data of the many commercial and pleasure craft with AIS transponders.

MarineTraffic was one of the sources used to track @dLj on his trip up the coast. Once he got more than about 35nm from the coastal repeater, he went off the map. Occasionally during his crossing he would come close enough for a passing ship’s AIS to register his location. But this was infrequent. MarineTraffic is a great tool. Additionally, VessleFinder provides a similar service.

The issue of Line of Sight is physical issue. The earth is curved. standing on the flat surface of the earth you can see about 2.9 miles. At that point the curvature of the earth makes the object disappear. Let’s raise your antenna 10 feet in the air. Your signal takes a laser beam shot out from the antenna. It is so fast that even gravity can not affect it. But at approximately 5 mile the beam just keeps going in a straight line as long as the energy permits. Right off the planet into space. If you have a friend standing on the earth at the same elevation as you above sea level, the beam of energy shoots over his head and as far as he is concerned you are not existent.

Here is a free calculator to use to decide how far the height of your antenna can send a signal. Remember if the receiver is higher than you site than they can have a better chance of receivneing the signal and returning a message to you.

Line of Sight Calculator | Southwest Antennas - High Performance RF and Microwave Antennas & Custom Antenna Manufacturing

The Line of Sight calculator will determine both the "line of sight distance" and "radio horizon / service range" of an antenna placed at a specific height above the ground, represented by "antenna height (h)" in this equation. The calculation assumes there are no obstructions (trees, buildings...

www.southwestantennas.com

 

[Hello Below]

I understand AIS and VHF are line of sight.

Here is a handy calculator to determine the LoS depending on each antenna height.
Radio Line of Sight Distance Calculator

Of course atmospheric conditions (rain, fog) antenna type (AE gain) and more importantly transmission power (class A or class B) will influence the actual maximum range. Here is an excellent primer for AIS in general.
Everything there is to know about AIS

 

[Hello Below]

If you have an account and follow your boat you'll get an email when you arrive at a port.

And a limited number of friends vessels (5 for free account) if you have them in your “Fleet”.
What is a MarineTraffic Fleet

 

[Hello Below]

I don't know if this is Raymarine MFDs (Lighthouse) limitation only or not; but, as I was transiting to Anacortes, WA several weeks ago I noticed a Washington State Ferry barreling down on me from about 3NM at greater than 16Kts. Although I was able to track it on radar and MK1 eye ball, the AIS target did not show up on my Raymarine e95 MFD. I had recently installed a Vesper antenna splitter so I considered it as my first suspect. Once alongside the marina, I did some faultfinding of the system (AE/cable, splitter, Em-Trak AIS and MFD). Employing PROAIS2, I found that the splitter was reporting over 250 targets out as far as 72NM yet the MFD was only displaying targets out to 1.26NM. With further investigation, I found that Raymarine Lighthouse software limits the number of tracked targets to 100:
"Up to 100 AIS targets can be tracked simultaneously. If more than 100 targets exist, within your range, the 100 targets nearest to your vessel will be displayed."
Here is a thread that refers to Raymarine Lighthouse version 3.6 but is also applicable to their newest version 4.
Raymarine Lighthouse 100-target AIS limit

Of the 100 AIS targets that my MFD listed, 80 were recording 0.0kts so were either at anchor (8) or tied up alongside (72). The moral of the story is turn off or silence your AIS transmitter when you are not actually using it.

FWIW in areas that are not as congested with AIS signals my Raymarine MFD mirrors what PROAIS2 reports up to 100 of the closest targets.

 

[dlochner]

I don't know if this is Raymarine MFDs (Lighthouse) limitation only or not; but, as I was transiting to Anacortes, WA several weeks ago I noticed a Washington State Ferry barreling down on me from about 3NM at greater than 16Kts. Although I was able to track it on radar and MK1 eye ball, the AIS target did not show up on my Raymarine e95 MFD. I had recently installed a Vesper antenna splitter so I considered it as my first suspect. Once alongside the marina, I did some faultfinding of the system (AE/cable, splitter, Em-Trak AIS and MFD). Employing PROAIS2, I found that the splitter was reporting over 250 targets out as far as 72NM yet the MFD was only displaying targets out to 1.26NM. With further investigation, I found that Raymarine Lighthouse software limits the number of tracked targets to 100:
"Up to 100 AIS targets can be tracked simultaneously. If more than 100 targets exist, within your range, the 100 targets nearest to your vessel will be displayed."
Here is a thread that refers to Raymarine Lighthouse version 3.6 but is also applicable to their newest version 4.
Raymarine Lighthouse 100-target AIS limit

Of the 100 AIS targets that my MFD listed, 80 were recording 0.0kts so were either at anchor (8) or tied up alongside (72). The moral of the story is turn off or silence your AIS transmitter when you are not actually using it.

FWIW in areas that are not as congested with AIS signals my Raymarine MFD mirrors what PROAIS2 reports up to 100 of the closest targets.

Dig deeper into the software for the AIS and CP. There is often a way to filter the targets to eliminate those that are not moving or not a danger. When transiting NY Harbor, I turned off the AIS as there were so many targets I couldn't see the chart!

If the description of the target is a number or seemingly random 2 or 3 letters it may be an EATON, Electronic Aide to Navigation. These targets are actually buoys and the the AIS signal is not broadcast from the buoy, rather it is broadcast over land based stations which cover many more miles.