AIS Fault Message

[Emerald Sea]

We have a Raymarine AIS650 fitted and Im getting the following error message "External AIS Unit Fault, Antenna VSWR Fault" with an alarm going off every 10mins or so. Anyone out there experienced this one?

We are presently in Namibia without a Raymarine tech about and the Raymarine tech forum has not solved it.

Some background: with this fault the AIS reception remains strong and I receive targets up to 20Nm away. The 650 is connected to an AIS 100 Splitter which is connected to the Raymarine VHF - also working fine. A few months back I upgraded all Raymarine system software.

What did I do before just this fault? I changed out the antenna, located on our mast top as it blew off, and replaced it with a similar 6db one (open circuit). Also changed out the cable from RG58 to RG233. Ive checked continuity on all connections and these are fine. And as we get radio and targets it would seem this is not the concern.

Has anyone come across this fault?

Steve

[continuouswave]

Originally Posted by Emerald Sea

We have a Raymarine AIS650 fitted and Im getting the following error message "External AIS Unit Fault, Antenna VSWR Fault" with an alarm going off every 10mins or so.....

What did I do before just this fault? I changed out the antenna, located on our mast top as it blew off, and replaced it with a similar 6db one (open circuit). Also changed out the cable from RG58 to RG233...

If the antenna is located atop a mast, I assume the transmission line length is rather long. If the previous transmission line was RG-58/U cable, it was quite lossy. There was probably a loss of more than -3dB in the transmission line.

If you replaced the transmission line with RG-233/U able, you significantly decreased the loss by a factor of about ten, and transmission line loss is now probably less than -0.5dB.

This change in the loss in the transmission line between transmitter and antenna has caused the VSWR as seen at the transmitter to be closer to the actual VSWR at the antenna.

In case you are not familiar with radio and transmission line theory, line loss can mask a high VSWR at the antenna. A simple way to consider this is as follows:

--assume an antenna has a VSWR 3.0 or 25-percent reflected power, that is, for every 100-Watt of incident power at the antenna there will b3 25 watts of reflected power.

--assume there is a -3dB transmission line loss

--assume the transmitter puts out 10-Watts (to keep the math simple)

--there is a -3dB loss in the transmission so only 5-Watts reaches the antenna;

--the antenna reflected power is 0.25 x 5 = 1.25-Watts back toward the transmitter; this is a VSWR of 3.0

--the reflected power is attentuated -3dB, so only 0.625-Watts reaches the transmitter

--the apparent VSWR at the transmitter is based on a reflected/forward power ratio of 0.625/10 = 0.0625, or a VSWR of 1.67

--the apparent VSWR is 1.67 but the real VSWR at the antenna is 3.0

In your case, the line loss of the RG-58 transmission line was probably more than -3dB, and this was masking the actual antenna VSWR. When you changed to a much lower loss transmission line, with a loss of only about one-tenth as much as before, the apparent VSWR at the transmitter increased significantly and now is more correlated to the actual antenna VSWR.

As a result, your AIS transmitter is now giving you an alarm that indicates the VSWR of the antenna is too high. This is probably a valid alarm because the VSWR of the actual antenna is probably too high, unless you happened to have purchased a special antenna that was specifically tuned for the AIS transmit frequencies.

Note that the AIS transmit frequencies are at the very high end of the VHF Marine Band, about 162-MHz, while the usual vessel transmit frequencies are at the very low end of the VHF Marine Band, about 156-MHz. Most vessel antennas are tuned for best VSWR at the 156-Mhz end of the band, not the 162-MHz end. This is why one sees specialty antennas sold for AIS transmitters.

This is just my armchair guess, but it is based on over 50-years of experience in radio communication, both as a radio amateur and in commercial radio and broadcast radio.

[Emerald Sea]

CW. Thanks for the good reply. Im not a conversant electtech but this makes sense. It would seem here the solution would be to revert back to a RG58 cable or consult Raymarine to either remove the fault message or have it reconfigured for the improved resistance.

[continuouswave]

My first move would be to actually measure the VSWR on the transmission line at the AIS frequencies

161.975-MHz
162.025-MHz

This is somewhat difficult to accomplish because your typical VHF Marine Band radio cannot transmit on those frequencies so it can't be used as source of RF for making the measurement.

If my guess is correct, then the antenna is not presenting "an improved resistance." The antenna probably has a higher VSWR than the AIS transmitter would prefer to see, which I would also guess to be probably a VSWR greater than 3.0 to 1.

It is also possible there is something really wrong with the new antenna and transmission line, and the VSWR is very bad, perhaps 10 to 1. You can still receive signals with an antenna like that, but you cannot transmit very well. If that were the case, you'd also see a high VSWR at the normal frequencies that a VHF radio could transmit, so checking the VSWR with the communication radio could also be useful.

[Emerald Sea]

CW, your time here is much appreciated. I haven't a voltage standing wave ratio meter available, nor my neighbours, unfortunately. Not that Id know what to do with one!

Note, that Ive had feedback from a cruising friend near us, coming up the Namibian coast the other day, that he was able to 'see' me some 8-10Nm away. Thats fairly good transmission I believe.

[witzgall]

How long is the entire run of cable? Take that number and plug it into any line loss calculator on the net (ham radio sites have these). You will find that anything over about 50-55 feet is not viable, because the 2w of output power is lost though the cable, so that there is no transmit power at the end.

Reception is not the same as transmit power, which is what I think is causing your error.

Chris

[Emerald Sea]

Chris. The total cable length is 21m. By replacing the RG58U coax with a RG213U I decreased the dB loss by some 55% to the antenna ie its more efficient, there are now more watts running up and down this cable.

Im curious that it may no be a transmitting fault error at all but rather a receiving error
where the AIS650 has not been configured to receive a stronger signal (?).

The complete Raymarine system has been designed and installed by Raymarine (Lagoon) and Ive only change this coax and antenna out where the antenna is the exact same spec.

[continuouswave]

Originally Posted by Emerald Sea

Chris. The total cable length is 21m. By replacing the RG58U coax with a RG213U...

There is a big difference between RG-233/U (as you first wrote) and RG-213/U.

Which did you actually use?

If RG-213/U, then transmission line loss was reduced compared to RG-58/U, but not nearly so much as would have occurred with RG-233/U.

[Emerald Sea]

CW. Oops. I just reopened the access panel to be sure and its actually a RG223/U. Typos! It was recommended to me by Radio Holland in Cape Town, who also sold me the antenna.

[Paul Elliott]

Steve, the change from RG58 to RG223 improves the cable loss by 0.7dB at 100 MHz. At the AIS frequency of around 162MHz the improvement will be a bit more, but still less than 1dB. I don't think that is enough change to cause the error you are seeing.

Here is a chart showing SWR measurement through a lossy line:


If we assume your antenna has an SWR of 2:1 then with the RG58 (assume 4dB loss) your transponder's SWR monitor would measure about 1.3:1. Switching to RG223 (assume 3dB loss) the measured SWR value rises to 1.4:1. This is a very small change, and while it might possibly cross an alarm threshold, I would not expect the transponder alarm to be so easily triggered.

What is your antenna? A short "sailboat" style whip is pretty broad, and if tuned to VHF Channel 16 (156.8 MHz) it will still probably present a reasonable SR on the AIS channels. If you have a longer "gain" whip, these can be quite out-of-tune up at the AIS end of the band. In that case the SWR may be worse than 2:1.

Have you checked your connectors on the new cable? I had a radio / AIS transponder that "worked", but the AIS and VHF range was suspiciously short. I discovered a dead short at the radio-end of the antenna cable -- it was a shield whisker that was hitting the center conductor at the plug. The SWR was horrible, but it still worked after a fashion.

[Emerald Sea]

Paul. The brand of the antenna eludes me and Id have to go up the mast to check. Unfortunately my receipt doesn't include the name. Its a 2.4m fiberglass 6dB VHF antenna, similar to the Shakespeare Mariner type. I did check all the connections and did a continuity check at the base connection which showed 0.2 which, when I called Radio Holland about this, said it was acceptable.

If you had a it was a shield whisker that was hitting the center conductor at the plug this would have surely shown up on a continuity check, no?

Not sure if this part of the puzzle, but the previous RG58 coax had a short, 1m long, RG58 pup piece between the main coax and the AIS 100 splitter. I removed this and plugged the new RG233 direct into the splitter (PL259). I didnt understand this need for this pup length so just left it out. I havent tried to replace it, simply as the coax fittings (cable to cable, not a PL259) weren't in my stock.

PS, we leave Namibia for St Helena later today so it will be 10-12 days before I receive CF emails. This support is great! Beers on me ... Should we meet up.

Thanks

[continuouswave]

OK--now we have changed the cable specification for the newly installed cable for the third time.

We began with RG-233, a cable with very much less loss than RG-58.
Next we changed it to RG-213, a cable with less loss than RG-58.
Now we arrive at RG-223, a cable with the SAME loss as RG-58.

There are the loss factors for the three transmissions lines, in terms of loss in dB per 100-feet at 100-MHz:

RG-58 = -6.5 (the original line)

RG-233 = -0.3 (the first line, and clearly a very low-loss line)

RG-213 = -2.3 (the second line, and having somewhat lower loss)

RG-223 = -6.5 (the third line, and having the same loss as RG-58)

Source for loss figures: http://stdwire.thomasnet.com/Asset/RG-Attenuation.pdf

Clearly there can be no expectation that my explanation of the cause of the problem will hold up when it was based on changing the transmission line to one with very significantly less loss, when the actual transmission line loss did not change at all.

Let's go back to my first suggestion on what to do: measure the actual VSWR on the transmission line.

[Emerald Sea]

Apologies for the errors. Age is catching up to me!

[witzgall]

I have been told by an ais tech that anything over 50ft of a run cannot be split. Use a separate antenna without a splitter, and put it on your Bimini or the like, and it should work well.

[Paul Elliott]

Originally Posted by witzgall

I have been told by an ais tech that anything over 50ft of a run cannot be split. Use a separate antenna without a splitter, and put it on your Bimini or the like, and it should work well.

What could the reason possibly be for this??? I can think of nothing. If anything, a longer line might make it easier on the splitter. I highly doubt that this is the case.

Now if you have a long run of lossy cable, and your splitter itself introduces significant loss, then that is taking a poor situation and making it worse. Perhaps this is what the ais tech was thinking of?