Advantage of Class B+ AIS on a sailboat?

[Paul L]

Originally Posted by Dockhead

That's really good to know; thanks.


But Message 27 is for satellites, right? And only active when well offshore? Do we really care if SWR is suboptimal for these messages? Surely we care a lot more about what we broadcast on the normal frequency, no?

Yep, don't let perfect be the enemy of good. The class B+ units are far more forgiving of the install with their 5 watt power than the class B 2 watt units.
My class B+ going through a switch and a standard masthead VHF antenna gets easily picked up by Sat AIS.

[BillKny]

Originally Posted by Telemark43

My current mast is 61 feet high, and I routinely see AIS offhoreyat thirty miles. Sailboats with mast rigged antennas really get great AIS vision

This ^^^^

We have a standard Class B system, the antenna on top of the 50 foot mizzen mast. We see large ships 30 to 50 NM away. They see us ~10 miles away. This is the performance you should expect from a good install.

We see MANY sailboats with totally crappy installls who do not appear until we are within 1 or 2 NM. It is very unlikely a change in the transmitter specifications will have anything other than a minor improvement without upgrading the cable and antenna.

[Pitchondesign]

A bit off topic but, can a VHF antenna be shortened to retune it to AIS frequencies? If so, how much should be it be shortened?

[donradcliffe]

Originally Posted by Discovery 15797

BTW...I did upgrade to a B+ with dedicated antenna, and also have a AIS receiver built into my Standard Horizon VHF radio as backup.

How do you get your standard horizon radio AIS to stop telling you that your separate transponder is a dangerous target??

[s/v Jedi]

Originally Posted by Dockhead

That's really good to know; thanks.


But Message 27 is for satellites, right? And only active when well offshore? Do we really care if SWR is suboptimal for these messages? Surely we care a lot more about what we broadcast on the normal frequency, no?

Msg 27 is always active but a shore station can send a Msg 23 IIRC to tell receiving units to halt sending msg27 temporarily. There were some reports about interference on ch.16 during emergency traffic and this is how they dealt with it.

Yes, you care about VSWR, which should be better than 2:1 at all times because if not, you get effects like that interference on ch.16

Also, these wide band antennas are good for everything so very well suited for situations where a VHF antenna is lost etc.
The reason Vesper made that antenna is because of their automatic switch. Many people are using one so they all need a wide band antenna.

[s/v Jedi]

Originally Posted by Pitchondesign

A bit off topic but, can a VHF antenna be shortened to retune it to AIS frequencies? If so, how much should be it be shortened?

What you want to have is an antenna analyzer; you can buy a NanoVNA for $60 and be able to perfectly tune your antenna.

Tune it to 159MHz center frequency, which basically makes it behave like the Vesper antenna.

[s/v Jedi]

Originally Posted by Paul L

Yep, don't let perfect be the enemy of good. The class B+ units are far more forgiving of the install with their 5 watt power than the class B 2 watt units.
My class B+ going through a switch and a standard masthead VHF antenna gets easily picked up by Sat AIS.

No, the VSWR for all transceivers aboard should be better than 2:1, which is as bad as it’s allowed to get. Perfect would be 1:1 and at 2:1 you get half the signal reflected back into the transceiver which is pretty bad.

The problem isn’t just a shorter range, it is interference. Years ago, every new VHF installation was tested and it wouldn’t pass when VSWR was 2:1 or worse. No ship station license until fixed.

[Bycrick]

Measuring just VSWR is almost useless. Comparing just VSWR between systems is useless. For VSWR to be meaningful as an indicator of how well your antenna is radiating, you need to account for the loss in the cable. If you don’t, the VSWR number is meaningless.

The VSWR is simply the RATIO of the power that goes up to the antenna compared to what bounces back if the antenna isn’t perfectly matched. If the coax has 3 db loss, even if the antenna is gone (as in a lightning strike) the VSWR won’t be much worse than 4:1 and perhaps even better.

The radios measure it to see if too much power is coming back. But without knowing the coax loss, you have no idea about how well it’s working.

[akopac]

For those who want to see the before and after of changing a standard Shakespeare squatty body 5247 antenna to a vesper AIS/VHF VA159 antenna. I agree totally on the coax, connector, and installation quality too.
Shakespeare was 3.6:1
Vesper is 1.6:1
Attachment 1

[Paul L]

Originally Posted by s/v Jedi

No, the VSWR for all transceivers aboard should be better than 2:1, which is as bad as it’s allowed to get. Perfect would be 1:1 and at 2:1 you get half the signal reflected back into the transceiver which is pretty bad.

The problem isn’t just a shorter range, it is interference. Years ago, every new VHF installation was tested and it wouldn’t pass when VSWR was 2:1 or worse. No ship station license until fixed.

Well having checked the SWR on my system and checked that my class b+ gets satellite received while the class B units anchored around me are not, I'm confident that my install is fine. So your absolute No is just being pedantic, as much of CF has turned into,
I'll go back to weather passage planning and not worry about the edges of the Ais install.

[s/v Jedi]

Originally Posted by Bycrick

Measuring just VSWR is almost useless. Comparing just VSWR between systems is useless. For VSWR to be meaningful as an indicator of how well your antenna is radiating, you need to account for the loss in the cable. If you don’t, the VSWR number is meaningless.

The VSWR is simply the RATIO of the power that goes up to the antenna compared to what bounces back if the antenna isn’t perfectly matched. If the coax has 3 db loss, even if the antenna is gone (as in a lightning strike) the VSWR won’t be much worse than 4:1 and perhaps even better.

The radios measure it to see if too much power is coming back. But without knowing the coax loss, you have no idea about how well it’s working.

Exactly, for a system like this to work well, all parts of it need to be done right. There is a large safety margin and unfortunately some people like to use that up for normal operation, leaving not enough margin to cope with situations like bad weather, compromised installations at the other boat/ship etc.

Originally Posted by akopac

For those who want to see the before and after of changing a standard Shakespeare squatty body 5247 antenna to a vesper AIS/VHF VA159 antenna. I agree totally on the coax, connector, and installation quality too.
Shakespeare was 3.6:1
Vesper is 1.6:1
Attachment 1Attachment 246329

Exactly, the difference is huge.

Originally Posted by Paul L

Well having checked the SWR on my system and checked that my class b+ gets satellite received while the class B units anchored around me are not, I'm confident that my install is fine. So your absolute No is just being pedantic, as much of CF has turned into,
I'll go back to weather passage planning and not worry about the edges of the Ais install.

So what was your SWR on channel 16? If it was 2:1 or worse, your installation isn’t okay and is causing interference on other systems aboard or at other boats. The cases where channel 16 traffic was problematic because of AIS interference are documented.

[Paul L]

Originally Posted by s/v Jedi

....
So what was your SWR on channel 16? If it was 2:1 or worse, your installation isn’t okay and is causing interference on other systems aboard or at other boats. The cases where channel 16 traffic was problematic because of AIS interference are documented.

About 1.3, 1.4, as I remember. No doubt there are documented cases of problem installs for any radio. The vast majority of class b+ units are functioning just fine using standard antennas, many shared via switch.

[s/v Jedi]

Originally Posted by Paul L

About 1.3, 1.4, as I remember. No doubt there are documented cases of problem installs for any radio. The vast majority of class b+ units are functioning just fine using standard antennas, many shared via switch.

Yes, when using a standard antenna, SWR on ch 16, as well as satellite transmissions on channels 75 and 76 is fine. The antenna is tuned to that exact frequency.

But this moves the question to what your SWR for channels AIS1 and AIS2 is. Like Acopak posted above, they found 3.6:1 which is unacceptable. If other stations still receive you at distance with just 1W transmitted power (you’re left with only 20%) is because of good antenna placement and the higher power of a SOTDMA transceiver to start with. But it negates the higher power advantage of a SOTDMA unit right there, plus will still cause interference but likely on channels that are little or never used.
Upgrading to the Vesper antenna costs $99 and results in an enormous improvement in AIS transponder performance, longer lifespan of the transponder and increased safety in general considering not just the better performance but also the suppression of interference.

Please don’t mistake me for a Vesper fan… I actually dislike their new offerings; I pick their antenna because it is the cheapest stainless whip that covers the used frequencies.

[Paul L]

Originally Posted by s/v Jedi

Yes, when using a standard antenna, SWR on ch 16, as well as satellite transmissions on channels 75 and 76 is fine. The antenna is tuned to that exact frequency.

But this moves the question to what your SWR for channels AIS1 and AIS2 is. Like Acopak posted above, they found 3.6:1 which is unacceptable. If other stations still receive you at distance with just 1W transmitted power (you’re left with only 20%) is because of good antenna placement and the higher power of a SOTDMA transceiver to start with. But it negates the higher power advantage of a SOTDMA unit right there, plus will still cause interference but likely on channels that are little or never used.
Upgrading to the Vesper antenna costs $99 and results in an enormous improvement in AIS transponder performance, longer lifespan of the transponder and increased safety in general considering not just the better performance but also the suppression of interference.

Please don’t mistake me for a Vesper fan… I actually dislike their new offerings; I pick their antenna because it is the cheapest stainless whip that covers the used frequencies.

The unit works fine. I scanned the swr with a nano vna and did not see any jumps up to 2, like the 3.6 you mention, so I'm happy with my install. Feel free to do whatever you want for your install while informing CF readers of what you think is optimum. Just skip the absolutes that imply everybodies install is crap and only your equipment choice is acceptable.

[s/v Jedi]

Originally Posted by Paul L

The unit works fine. I scanned the swr with a nano vna and did not see any jumps up to 2, like the 3.6 you mention, so I'm happy with my install. Feel free to do whatever you want for your install while informing CF readers of what you think is optimum. Just skip the absolutes that imply everybodies install is crap and only your equipment choice is acceptable.

Well, then you have an antenna that covers both bands already and you’re good to go. My posts are about installations with SOTDMA transceivers that use either a standard VHF or standard AIS (stainless steel whip) antenna.

I have a Shakespeare Galaxy Little Giant regular VHF antenna and my SWR for the AIS channels is at 1.9:1 which isn’t optimal but good enough at the time. But that also isn’t the standard antenna most boats have.

With a NanoVNA you can shorten a standard VHF whip antenna to a center frequency of 159MHz and have the equivalent of a Vesper whip. But for people who don’t own a NanoVNA, buying the Vesper antenna isn’t much more than a NanoVNA.