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Old 06-06-2021, 22:36   #1
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AIS Antenna VSWR exceeds Limit

I noticed this alert come up after installing an AIS and after a brief google search on the spot... I decided I wanted dinner instead of figuring it out.

Now I'm home researching the alert and it looks like the VSWR exceeds limit is likely due to a short. I've just purchased a new AIS tuned antenna and cable. That should resolve the issue.

But I don't plan to return to the boat for 6 more days. Am I going to damage the AIS by leaving it on with the VSWR alert for a week?
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Old 06-06-2021, 22:57   #2
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Re: AIS Antenna VSWR exceeds Limit

It's probably not the best thing for the AIS, but shouldn't cause (fingers crossed) any damage. I'd wager the same mechanism that sent you the alert is also attenuating the transmit power while the alert exists to protect the circuitry. That being said, protections sometimes fail and it would be less risky to shut the thing off if you have a way to do so.
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Old 11-06-2021, 05:00   #3
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Re: AIS Antenna VSWR exceeds Limit

Most modern solid-state-output-circuit transmitters are smart enough to reduce their power output when they encounter an antenna load that is not well-matched. Whether the specific device on your boat has that feature is unknown to me.

A general upper limit for VSWR before indicating an alarm condition is perhaps 2:1 or higher, or perhaps even 3:1 or higher.

If the antenna for the AIS transmitter were not specifically tuned to 162-MHz and was instead just a general VHF Marine Band antenna that would be tuned to 157-MHz, the general 157-MHz-tuned antenna would likely have a rather high VSWR at 162-MHz, and this may be the source of the problem.

On the other hand, if the VSWR alarm alert is something new and the AIS transmitter did not previously alert about VSWR, then a new problem in the antenna, the transmission line, or any connections involved may be the cause of the VSWR alarm alert. An open circuit or a short circuit at any point in the circuit would cause a radical change in the VSWR on the transmission line as seen at the AIS transmitter trying to input power into the transmission line.
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Old 11-06-2021, 08:49   #4
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Re: AIS Antenna VSWR exceeds Limit

It's an old repurposed VHF antenna original to the boat. My AIS isn't being picked up on marinetraffic.com unlike my neighbors so I'm guessing the VSWR alert has disabled transmission. Which is fine.

I've ordered a Shakespeare 396-1 5' Classic AIS Antenna and it should get here Monday.
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

I did see another AIS antenna at a local store: 5396-AIS Galaxy Little Giant™
AIS/VHF Antenna
It's twice the price. But comparing the specs.. I can't really see any difference. So I'm just going to wait for the cheaper one to arrive on Monday - unless anyone can point out why the 5396 is a better option.
https://shakespeare-ce.com/marine/pr...isvhf-antenna/
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Old 11-06-2021, 09:19   #5
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Re: AIS Antenna VSWR exceeds Limit

Since that is a repurposed VHF antenna, using it on AIS means the antenna is too long. And, I doubt that you will reuse that antenna on VHF, so just cut off about 1" to start. That should put close to 162MHz. Observe your SWR indicator to see if the warning is eliminated. If not, start cutting off 1/4" then check again. Keep going until you see the SWR drop to a usable level, what ever that is for your AIS device.

Have fun, experiment, learn.
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Old 11-06-2021, 17:19   #6
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Re: AIS Antenna VSWR exceeds Limit

Would you guys expect a VSWR exceeds limit alert without any vhf antenna connected at all?

I disconnected the old antenna until the new one arrives and... the alert still comes up after dismissing it.
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Old 11-06-2021, 18:09   #7
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Re: AIS Antenna VSWR exceeds Limit

If there is no antenna, there is no load, a really bad SWR will exist. On the order of 10+ even possibly 100+. So yes, if you disconnect the antenna, turn off the AIS. Better yet, do not operate the AIS without an antenna attached.
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Old 11-06-2021, 20:57   #8
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Re: AIS Antenna VSWR exceeds Limit

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Originally Posted by Brian.D View Post
If there is no antenna, there is no load, a really bad SWR will exist. On the order of 10+ even possibly 100+. So yes, if you disconnect the antenna, turn off the AIS. Better yet, do not operate the AIS without an antenna attached.
Thanks!

I think there's a way to disable AIS transmission. But I don't think it's possible to turn the unit off otherwise. Unless the power cable is disconnected.

The manual says the Class A AIS should be connected to emergency power. I believe that's the same as auto bilge pumps. So there's no "off" switch. It stays on indefinitely.

But I'll disconnect the unit from the battery as per your suggestion tomorrow.
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Old 18-06-2021, 04:36   #9
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Re: AIS Antenna VSWR exceeds Limit

This is a timely thread as I have a similar issue I'd like to run by the experts. Installed a EmTrack B924 AIS transponder this winter and integrated it with my Raymarine E80 MFD. Also added a new Metz 206 Manta-6 VHF antenna. All was working fine for about a week, then I started to get a "antenna VSRW fault" on the E80 (see pic). After some troubleshooting, it seems like the AIS unit is getting an error (red light), but then stabilizes (green light). But when I use ProAIS2 to get some diagnostics, the VSWR ration is 12:1 (way too high?). And the E80 never gets a valid connection to the AIS...

Emtrack support seems to be at a loss.. not sure what changed-maybe my vhf cable connections are lousy and weakening? many thanks.
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Old 18-06-2021, 05:00   #10
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Re: AIS Antenna VSWR exceeds Limit

So here’s a consideration if those who have a SOTDMA AIS transponder (the new class B+ transponders as well as newer class A transponders). With many choosing the new SOTDMA class B transponders, this has become common but the issue is largely ignored:

SOTDMA transponders support satellite AIS with two extra transmit-only channels. They transmit on those channels every couple minutes. The issue is that these channels are not near 162 MHz but centered around 156.8 (ch 16) MHz. This means thatyou need a special wide band antenna.

I just bought new antennas and found two good replacements: for the stainless steel whip style antenna, you can buy the Vesper antenna: https://www.milltechmarine.com/Vespe...nna_p_284.html
which I will mount on my mizzen mast to replace a Shakespeare 5215 whip.

The other one is to replace my Shakespeare Galaxy Little Giant on the main mast. The replacement here is the Shakespeare Phase-III 5400-WB: https://www.milltechmarine.com/shake...f-antenna.html
It requires the extra female-female ferrule for mounting. The advantage over the Little Giant is that this one has a female UHF connector for the coax cable instead of a coax coming out of the antenna. It’s more expensive but top of the line.

These antennas give good VSWR for both VHF and AIS.
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Old 18-06-2021, 05:11   #11
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Re: AIS Antenna VSWR exceeds Limit

Thanks. So you think my issue is caused by not having the right antenna? Just not sure why it would have worked for a period of time, then started giving errors?
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Old 18-06-2021, 09:20   #12
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Re: AIS Antenna VSWR exceeds Limit

WSCONNER, that could very well be the case. Even a home made antenna I use at my house has a narrow band width. It is cut for 146MHz but generates almost a 2:1 SWR a few MHz away. So with most marine antennas, they are tuned to the 156MHz area and AIS is 6MHz away.

The other possibility is a short in the cable. But the 'out of tune' theory is more plausible.
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Old 18-06-2021, 13:05   #13
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Re: AIS Antenna VSWR exceeds Limit

It looks like marine band radios transmit on a range from 152.150 to 155.500MHz. The center frequency for that should be 158.825 which is where I'd tune my antenna to. A straight half wave antenna for 146MHz should be 3'2-15/32" or 0.977M long and the antenna for 158.825MHz should be 2'11-3/8" or 0.898M long. I might actually consider using a full wave antenna for 158.825MHz which would be 5'10-3/4" or 1.796M.
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Old 18-06-2021, 13:59   #14
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Re: AIS Antenna VSWR exceeds Limit

Replacing the vhf cable and installing a dedicated AIS antenna resolved my VSWR alert.
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Old 18-06-2021, 16:51   #15
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Re: AIS Antenna VSWR exceeds Limit

Quote:
Originally Posted by newbie- View Post
Replacing the vhf cable and installing a dedicated AIS antenna resolved my VSWR alert.
Thanks.. may I ask what type AIS you had installed? Mine has a built in splitter, so the goal was to re-use the single antenna for VHF and AIS....
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