AIS Transponder: Yes or No

[jeffrobbins]

In an ideal world, I think having separate VHF and AIS antennas is the way to go. But the mounting issues sometimes preclude either the antenna or cable run for some boats.

I don't recommend putting two VHF antennas (one AIS and one VHF) on the top of the same mast. These two antennas are operating on roughly the same frequencies and would be only a few inches apart. That's looking for trouble. Rather, if you want them both on the masthead, then I'd say go with a proper AIS splitter and a single antenna (in my opinion, optimized for AIS but with enough bandwidth for the VHF radio).

If you've got a ketch, mounting one on the main mast and one on the mizzen is probably the best. I don't know about mounting near a wind generator. There might be a lot of RF noise coming off the wind-gen.

Mounting on a spreader on a relatively small boat has it's problems too since the antenna will either be very close to the mast or near the rigging.

Lots of people mount their AIS antennas on an arch, radar pole (keep out of the beam path), bimini, etc. This generally works well in most cases.

[CarinaPDX]

xymotic-

How what I said is relevant is this: there is no difference between Class A and Class B position packets. Only the static packets are different, and all Class A transceivers are required to have been updated to recognize them. Thus they ALWAYS see Class B position transmissions, and should always see the static data. THERE IS NO WAY TO NOT SEE CLASS B POSITIONS!!!! There is no difference and there is no filter. Of course the occasional packet gets missed, due to interference or collisions. Other posters continue to spread the myth that commercial vessels "turn off" reception of Class B positions; Class A systems simply do not have the ability to do this. See this: The Class B A.I.S. Filtering Myth Revisited | Cruising World

[jeffrobbins]

Originally Posted by CarinaPDX

xymotic-

How what I said is relevant is this: there is no difference between Class A and Class B position packets. Only the static packets are different, and all Class A transceivers are required to have been updated to recognize them. Thus they ALWAYS see Class B position transmissions, and should always see the static data. THERE IS NO WAY TO NOT SEE CLASS B POSITIONS!!!! There is no difference and there is no filter. Of course the occasional packet gets missed, due to interference or collisions. Other posters continue to spread the myth that commercial vessels "turn off" reception of Class B positions; Class A systems simply do not have the ability to do this. See this: The Class B A.I.S. Filtering Myth Revisited | Cruising World

As far as I know, all Class A's were supposed to have been updated so they can decode the two Class B static data messages. All Class A's can receive Class B position reports. But the position report messages aren't the same. Class B has its own position report message which is different from the Class A position report messages.

However, it doesn't matter because all class A's can receive and process Class B position reports. Worst case, a non-updated Class A won't know your vessel name, type and size. This is the same thing you'll experience because static data is sent only every 6 minutes so for a time you won't know their name either. Of course, you'll both know the position, course, speed, etc.

[S/V Alchemy]

Originally Posted by jeffrobbins

In an ideal world, I think having separate VHF and AIS antennas is the way to go. But the mounting issues sometimes preclude either the antenna or cable run for some boats.

I don't recommend putting two VHF antennas (one AIS and one VHF) on the top of the same mast. These two antennas are operating on roughly the same frequencies and would be only a few inches apart. That's looking for trouble. Rather, if you want them both on the masthead, then I'd say go with a proper AIS splitter and a single antenna (in my opinion, optimized for AIS but with enough bandwidth for the VHF radio).

If you've got a ketch, mounting one on the main mast and one on the mizzen is probably the best. I don't know about mounting near a wind generator. There might be a lot of RF noise coming off the wind-gen.

Mounting on a spreader on a relatively small boat has it's problems too since the antenna will either be very close to the mast or near the rigging.

Lots of people mount their AIS antennas on an arch, radar pole (keep out of the beam path), bimini, etc. This generally works well in most cases.

Those are all good points. I have a bimini arch with panels and that's a good spot for it. But I still prefer dedicated antennas for the reasons I stated already. I use a splitter to get FM reception on the VHF antenna feed...and yes, I never play the radio when I transmit! but I'm familiar with the idea; just not sure if I like it.

[jeffrobbins]

Originally Posted by S/V Alchemy

Those are all good points. I have a bimini arch with panels and that's a good spot for it. But I still prefer dedicated antennas for the reasons I stated already. I use a splitter to get FM reception on the VHF antenna feed...and yes, I never play the radio when I transmit! but I'm familiar with the idea; just not sure if I like it.

If you've got an arch and don't mind stringing another cable, then go for it. You likely won't be disappointed and you'll also have a totally separate backup antenna you could use if you were to lose the rig or your masthead one should ever fail or be knocked off (bird?, bridge?)

Regards the splitter.... a properly designed splitter shouldn't require you to switch off the FM radio when you transmit on the VHF. If it does than it sounds like an accident waiting to happen.

[donradcliffe]

The ProAIS program that came with my WMP transponder has a screen that gives diagnostic information, including the number of successful packets transmitted and received. It also gives reflected power, which allows you to ensure that your antenna is well matched. I played around with some spare VHF antennas, and found that one of them gave pretty poor performance, but two worked fine -these were base-loaded masthead types.

I have both masthead and pulpit mounted antennas, and find that I usually pick up ships by 12 miles with the pulpit antenna and by 20 miles with the masthead. I usually pick up Class B rigs by 6 miles with the pulpit antenna, but there have been some Class B boats that weren't picked up until they were within 2 miles. With the masthead antenna, I have picked up some ships well over 100 miles out--BTW when I went through the Gulf of Aden we could hear other VHF stations over 200 miles away, so its probably the opposite of an AIS dead zone unless the ships are shutting down their transponders.

Talked to an American Flag vessel on the way back from Hawaii--he called me by name, and said that the newer one of his two Furuno AIS units had picked up my static data, but the older one just showed my dynamic data. I also was called by an aircraft carrier off Norfolk, who called me by location, course and speed and said he only had my dynamic data, so there still must be a lot of un-updated Class A units around, but they still see enough of my Class B data for collision avoidance purposes.

The first AIS unit I saw was on a Turkish ship, and they proudly pointed it out when I toured the bridge. They said it was a great improvement on radar, as it could see around corners and it gave the name of the other ship for VHF calls.

[svBeBe]

Originally Posted by CarlF

Judy, you're right that I don't constantly monitor my AIS transmit, but I do frequently check with passing ships that I'm showing up. There's never been a report of "dropping out" or not seeing me.

I also don't see "drop outs" on my receiver when looking at other boats that are presumably transmitting on class B units. Especially on the ICW, I watch them come and go past me over quite a distance.

Is there a chance that your AIS receiver or installation is at fault rather than the other boat's transmitter? Receiving class B signals is tougher than class A. A marginal setup might behave exactly as you describe (e.g. good reception of Class A and spotty reception of class B).

Carl

Sure, the problem could lie with our equipment or the installation; but I don't think so because it is only some Class B signals that drop. Other Class B signals never drop on our receiver. We have the Smart Radio receiver purchased from Milltech in early 2008 and it is installed with a dedicated antennae on top of the main mast. It is also strange that almost all Class B signals we receive show only their MMSI number, not the boat name. But some of the Class B signals do show yacht name. The yacht names on some Class B signals started showing up less than 2 years ago. So I think there are different quality Class B transponders in use.

Judy

[DJBrookster]

Originally Posted by Bash

My guess would be that there are alternative reasons for your lack of success getting ships to respond to hails on the VHF. It's highly unlikely that ship owners would make such a substantial investment in a large vessel and not equip in with an adequate radar.

The fact of the matter is that ships carry radars far more powerful than the 2kw or 4kw units most recreational boats use. And most ships have multiple units in use simultaneously. They get far better data than most of us imagine.

That's all well and good, but when the vessel 'Condor fast ferry' is still doing 37kts in fog and hits a fishing vessel, one member of crew died and captain did not initially know he'd hit them. It doesn't matter how many radars and AIS are inboard if nobody is looking or responding to them. This is a real case only last year. Both are brilliant but no substitute for "proper lookout at all times".

[S/V Alchemy]

Originally Posted by jeffrobbins

If you've got an arch and don't mind stringing another cable, then go for it. You likely won't be disappointed and you'll also have a totally separate backup antenna you could use if you were to lose the rig or your masthead one should ever fail or be knocked off (bird?, bridge?)

Regards the splitter.... a properly designed splitter shouldn't require you to switch off the FM radio when you transmit on the VHF. If it does than it sounds like an accident waiting to happen.

It's no accident; it's a 1970s Great Lakes freshwater boat with a fair bit of original wiring (yes, it's a time machine that sails). I simply know not to use the VHF with the radio on.

And no, I don't mind stringing another cable. I have a windvane and an autopilot and the hardware to make a rudder out of a lashed spinnaker pole and a head door, so obviously I'm comfortable with the idea of redundancy. Having separate but potentially interchangeable AIS/VHF antennas is just that idea in a fresh iteration.

[S/V Alchemy]

Originally Posted by donradcliffe

The ProAIS program that came with my WMP transponder has a screen that gives diagnostic information, including the number of successful packets transmitted and received. It also gives reflected power, which allows you to ensure that your antenna is well matched. I played around with some spare VHF antennas, and found that one of them gave pretty poor performance, but two worked fine -these were base-loaded masthead types.

I have both masthead and pulpit mounted antennas, and find that I usually pick up ships by 12 miles with the pulpit antenna and by 20 miles with the masthead. I usually pick up Class B rigs by 6 miles with the pulpit antenna, but there have been some Class B boats that weren't picked up until they were within 2 miles. With the masthead antenna, I have picked up some ships well over 100 miles out--BTW when I went through the Gulf of Aden we could hear other VHF stations over 200 miles away, so its probably the opposite of an AIS dead zone unless the ships are shutting down their transponders.

Talked to an American Flag vessel on the way back from Hawaii--he called me by name, and said that the newer one of his two Furuno AIS units had picked up my static data, but the older one just showed my dynamic data. I also was called by an aircraft carrier off Norfolk, who called me by location, course and speed and said he only had my dynamic data, so there still must be a lot of un-updated Class A units around, but they still see enough of my Class B data for collision avoidance purposes.

The first AIS unit I saw was on a Turkish ship, and they proudly pointed it out when I toured the bridge. They said it was a great improvement on radar, as it could see around corners and it gave the name of the other ship for VHF calls.

Thanks for providing the "real world" results, Don. Very helpful information and actually a little better than I would have guessed.

[jeffrobbins]

Originally Posted by svBeBe

Sure, the problem could lie with our equipment or the installation; but I don't think so because it is only some Class B signals that drop. Other Class B signals never drop on our receiver. We have the Smart Radio receiver purchased from Milltech in early 2008 and it is installed with a dedicated antennae on top of the main mast. It is also strange that almost all Class B signals we receive show only their MMSI number, not the boat name. But some of the Class B signals do show yacht name. The yacht names on some Class B signals started showing up less than 2 years ago. So I think there are different quality Class B transponders in use.

Judy

Something to keep in mind when using a single channel receiver like this one... Class B sends position reports every 30 seconds when the vessel is moving >2K, and every 3 minutes when <2K. It sends it's static data (name, etc.) every 6 minutes. It alternates channels on each transmission of this info.

Since your receiver can only receive every-other-message this results in position updates once a minute or once every 6 minutes, depending on vessel speed. It will receive static info only every 12 minutes. Could this explain what you are seeing?

[Auspicious]

I spent an hour on the phone with Furuno today. I give them credit for very good customer service. I called in to tech support and the person that answered the phone was a bit at sea with regards to my question and bumped me up to 'Larry'--presumably the same Larry referred to earlier in the thread--who in turn bumped me to Shaun (Shawn? not sure).

As a courtesy to Furuno I'm writing up my understanding from Shaun (again my apologies if I am spelling his name wrong) and sending it to him to ensure I don't put words in the mouth of Furuno. I am also sending a link to this thread. Furuno can decide if they would like me to post my understanding or respond directly.

Originally Posted by svBeBe

it is only some Class B signals that drop. Other Class B signals never drop on our receiver.

Hi Judy & Bill,

Remember that Class A transmitters are higher power than Class B, and use antennas higher above sea level. Class A transmissions happen more often. Class B static information is even less often transmitted. I'm not surprised to see Class B positions suddenly get boat names after half an hour.

[Zanshin]

I have a transponder and have seen cargo vessels at 40nm distance and TCA of an hour change course so as to avoid getting too close. Certainly much more efficacious than a radar reflector or lights in such a situation, as they have my callsign, MMSI, course, and can see that I am a sailing vessel.

[David M]

Originally Posted by Auspicious

I spent an hour on the phone with Furuno today. I give them credit for very good customer service. I called in to tech support and the person that answered the phone was a bit at sea with regards to my question and bumped me up to 'Larry'--presumably the same Larry referred to earlier in the thread--who in turn bumped me to Shaun (Shawn? not sure).

As a courtesy to Furuno I'm writing up my understanding from Shaun (again my apologies if I am spelling his name wrong) and sending it to him to ensure I don't put words in the mouth of Furuno. I am also sending a link to this thread. Furuno can decide if they would like me to post my understanding or respond directly.



Hi Judy & Bill,

Remember that Class A transmitters are higher power than Class B, and use antennas higher above sea level. Class A transmissions happen more often. Class B static information is even less often transmitted. I'm not surprised to see Class B positions suddenly get boat names after half an hour.

Did Larry or Shawn give a reason for a FA150 (Class A AIS transceiver) requiring a special AIS antenna? Power does not seem to be the reason because VHF antennas are connected to transmitters that are limited to 25 watts....unless Class A is greater than 25 watts? Could that be the reason?

[jeffrobbins]

Originally Posted by Auspicious

Remember that Class A transmitters are higher power than Class B, and use antennas higher above sea level. Class A transmissions happen more often. Class B static information is even less often transmitted. I'm not surprised to see Class B positions suddenly get boat names after half an hour.

Yes, the higher power and antennas makes a difference and Class A position reports occurs more frequently (for moving vessels only), but Class B static info is actually sent at the exact same interval as Class A.