Intermittent AIS Transmissions

[torrmundi]

Hi,

On recent passages, buddy boats and others have commented that they have “not seen my AIS for 5 hours” or “your AIS is missing sometimes”. These boats were sometimes as close as 3 miles. I have a Simrad NAIS-500 installed July 2018. I reprogrammed it with version 1.2.8.0.8 in October 2020. The VHF antenna is on my mizzenmast, at 59 ft air draft. The antenna measures VSWR of 1.6-1.7 from 156 to 162 MHz. Not great, but that will lose only about 1 watt in 20 (1.2 dB loss of forward power).

When I connect NAIS Configurator (app that interfaces via USB to NAIS-500) it shows no errors, and the number of transmissions and receptions increases over time.

On the receive side, I see others (including those buddy boats that could not see me) at up to 40nm distance. In OpenCPN, the AIS list may show 60 or 80 vessels beacons and ATONs in busy areas. So reception appears quite good.

Any ideas? I have not yet climbed the mast to inspect the VHF antenna and connection to the cable.

How do shops determine if AIS transceivers are transmitting correctly? Do they have equipment capable of measuring the brief spurts of TDMA output power?

[GILow]

I’m going for an intermittent poor connection somewhere on the antenna side. If it is something loose it may not show when testing vswr. I fear a trip up the mast is in your future.

[dougweibel]

Have you been able to observe if you are receiving targets during these periods when others cannot see you? If you can pin down that you have a transmission problem but not a reception problem that should point to something different about the Tx and Rx paths.

Do you use a single antenna for VHF and AIS. Maybe your antenna splitter is causing a loss of transmit power.

[ka4wja]

torrmundi,

A couple important points to remind you of, that should help....but first a couple quick "guesses".


1) As GILow wrote, "intermittent" signals / communications, are a tip-off to intermittent connections!


2) It is difficult to troubleshoot from afar....and even more so with limited info / data {we don't know what antenna you have, nor how old it is....nor the type/size and especially the age of your antenna cabling....nor how long of a cable run you have (but, I'm assuming ~ 75')....nor if you have any splices / cable junctions / connectors in this cable.....nor when was the last time you inspected the antenna and cabling, etc. etc...}

So, we can only answer in generalities...

Please understand I'm not being a jerk here....as it is very important to know the type and size of your antenna cable, and its age, in order to know if the good VSWR you see (anything lower than 2:1 is fine!) is real, or artificially low due to cable losses....(details below)



3) Even if you didn't mention Intermittent right up front (in the title, etc.), as I have written many, many times....the fact is that most marine electronic issues (not caused by lightning strikes) can be traced to the connections / wiring / weather-proofing (lack of) / programming / commissioning....

And in regards to radio comm issues, this is mostly about the antenna itself, the antenna-to-cable connector attachment, the connector-to-cable connection/attachment (actually one of the most common failure points), any cable/connector splices (the second most common failure point), and the cabling itself!!

So, your mizzen-mounted VHF (AIS) antenna, and your antenna cabling, especially the cable connectors at the cable ends and/or any splices, are the very first things to consider as the cause.



4) I'm making the assumption here that you already asked your "buddy boats" to verify that they were actually receiving many other AIS target signals at the same time that they were not receiving your AIS signal! (if you didn't, please ask them now....so they can more clearly remember....and assist you in trouble-shooting your issues)

Also, confirm that they were / are receiving the same (or almost the same) AIS targets that you were / are?

If they were not receiving many other AIS signals during that time (within the same minute / minutes), and/or they are not receiving the same AIS targets that you are, and you're all within a few miles of each other, then you cannot make the assumption that your AIS transmitted signal is at fault, as you have mentioned that you're receiving many other AIS targets just fine...and, it is possible that their AIS reception is faulty / intermittent (which can be an external cause, such as their on-board RFI....as well as internal, such as their antennas, cable, connections, etc..)...of course, with multiple other boats mentioning the lack of seeing your AIS signal, this is a lower possibility, but something to be very aware of.


Also, please verify / confirm that while you were on this passage, specifically during the times that your buddy boats were not seeing your AIS signal, that you were receiving many AIS targets, from those ranges you mention...
Meaning, did you see lots of distance targets all the time?



5) As for your VSWR reading.....it does show that you probably do have an antenna / antenna cable problem....but, due to only partial info, we cannot be certain...

Please understand that the 7% reflected power from a VSWR of approx 1.7:1, is NOT lost power, as it is re-reflected back up to the antenna, and, minus what is lost in the cable, is radiated ....the actual amount of this power is attenuated (reduced) by the cable losses, in both directions (forward and reflected), and of course, this continues for each reflection....so, as you see the cable loss (when matched) is what controls the system loss, and whatever reflected power you have is also reduced by this loss...

So...
So, if you had say 75' of new high-quality Belden RG-8x, that would be approx. 3.2db of loss going up to the antenna....(and a lot more loss...if lower quality cable is used, and/or the cable is old)....that would be approx 47% of the power getting to the antenna.....then let's say you have a VSWR at the antenna of approx. 3.5:1 (indicating a bad antenna and/or bad connection at antenna), reflecting approx. 30% of that 47% (which is approx. 14% of the original transmitter power) back....then you have that same 3.2db of loss back down to the transmitter (and VSWR meter), leaving you just a bit less than half of that signal....so your VSWR meter shows about 1.7:1 (about 6.7% reflected power), when in fact the VSWR at the antenna is 3.5:1!!!

And, that is with 75' of new, good-quality cable....the mismatch / poor VSWR at the antenna would be even worse with older and/or poor-quality cable....and if the cable is bad, filled with water, etc., the mismatch could even be a direct-short or open, or not even have the antenna connected at all, and you could still measure an acceptable VSWR at the transmitter!
So...

So, seeing an acceptable swr at the transmitter isn't too telling in most circumstances....

And, in your situation....I suspect that you might in fact have a serious antenna / antenna connection issue, at the top of the mizzen mast....(but, as I wrote above, I don't know anything about your antenna and your cable, nor do we know if your "buddy boats" were actually seeing many other targets during the same time that they were not receiving your AIS signal....so, I can only speak in generalities)




More later?

I do hope this has helped? (but, I suspect that you will need to provide more info / more details, in order to get more than generalities / guesses)

Fair winds.

John

[Snore]

Before you go crazy checking your gear. Is this one buddy boat or many? Try to find another boat and see if they also lose you.

[torrmundi]

Originally Posted by dougweibel

Have you been able to observe if you are receiving targets during these periods when others cannot see you? If you can pin down that you have a transmission problem but not a reception problem that should point to something different about the Tx and Rx paths.

Do you use a single antenna for VHF and AIS. Maybe your antenna splitter is causing a loss of transmit power.

Yes, I had continuous reception of all targets, as far as I can tell. Specifically, I could see the buddy boats as they told me they could not see me. I do not have any splitter; I have separate antennas for VHF radio and AIS.

[torrmundi]

Originally Posted by Snore

Before you go crazy checking your gear. Is this one buddy boat or many? Try to find another boat and see if they also lose you.

It was reported by two or three buddies.

[wholybee]

Originally Posted by ka4wja

torrmundi,
snip

So...
So, if you had say 75' of new high-quality Belden RG-8x, that would be approx. 3.2db of loss going up to the antenna....(and a lot more loss...if lower quality cable is used, and/or the cable is old)....that would be approx 47% of the power getting to the antenna.....then let's say you have a VSWR at the antenna of approx. 3.5:1 (indicating a bad antenna and/or bad connection at antenna), reflecting approx. 30% of that 47% (which is approx. 14% of the original transmitter power) back....then you have that same 3.2db of loss back down to the transmitter (and VSWR meter), leaving you just a bit less than half of that signal....so your VSWR meter shows about 1.7:1 (about 6.7% reflected power), when in fact the VSWR at the antenna is 3.5:1!!!

And, that is with 75' of new, good-quality cable....the mismatch / poor VSWR at the antenna would be even worse with older and/or poor-quality cable....and if the cable is bad, filled with water, etc., the mismatch could even be a direct-short or open, or not even have the antenna connected at all, and you could still measure an acceptable VSWR at the transmitter!
So...

So, seeing an acceptable swr at the transmitter isn't too telling in most circumstances....

I was about to post almost exactly the same thing, but I'll just highlight it. From what the OP posted, it sounds like a possible antenna/connection issue, and personally I would upgrade from 8x to something better on a mast that tall.

[torrmundi]

Please see my comments and answers embedded into the quoted section.

Originally Posted by ka4wja

torrmundi,
...
1) As GILow wrote, "intermittent" signals / communications, are a tip-off to intermittent connections!
...
So, your mizzen-mounted VHF (AIS) antenna, and your antenna cabling, especially the cable connectors at the cable ends and/or any splices, are the very first things to consider as the cause.
>>Agreed!


4) I'm making the assumption here that you already asked your "buddy boats" to verify that they were actually receiving many other AIS target signals at the same time
>>I did, and it looks like I may have received more than they did, but we were far apart at the time.


Meaning, did you see lots of distance targets all the time?
>>Only within 40nm of ports; other times we have far fewer targets, as expected in a December coastal passage.


5) As for your VSWR reading.....it does show that you probably do have an antenna / antenna cable problem....
...
so your VSWR meter shows about 1.7:1 (about 6.7% reflected power), when in fact the VSWR at the antenna is 3.5:1!!!
...
and if the cable is bad, filled with water, etc., the mismatch could even be a direct-short or open, or not even have the antenna connected at all, and you could still measure an acceptable VSWR at the transmitter!
...
I don't know anything about your antenna and your cable...
>>Nor do I; I bought the boat in 2018 and have not investigated the antennas yet. But I will now.



...
I do hope this has helped?
>>Absolutely, you have helped refocus my efforts with appropriate discussion and examples.

[torrmundi]

I have a run of 123ft of Ancor RG8x. So about 5.7 dB of attenuation when in perfect condition. I could replace it with MPD's 125ft LMR-400 cable for about $150 and save 4 dB of attenuation (not too shabby), and about half the RG213 attenuation.

[Snore]

Originally Posted by torrmundi

It was reported by two or three buddies.

Go KISS—- connect an antenna directly to the AIS antenna connection. Yes it will be below decks, but you should still get 1/2-1 mile of range.

So connect and ask a buddy boat to monitor. May cost you a few beverages, but that would 100% determine antenna or device.

[sv_pelagia]

Have you checked to see if you are showing up on Marinetraffic (or similar website)?

[ka4wja]

torrmundi,
1) Stop!
Before you do anything else....just read this brief message.
(that means do not spend any $$$ of LMR-400 cable, etc.)


2) FYI, it appears that your AIS antenna is not connected at all, or it (or the masttop connection) is damaged, etc.

Of course, we still don't know if you have any splices / junctions in this cable? And, that is a VERY important piece of info to know....so again, I still cannot be too specific....
But, I'm going to assume that if there were a mast-step splice, you would've mentioned it?
So, let's go forward....but, please give us as much info as possible, as it will save you time, money, and frustrations....and save all of us here some time.

Now that we have a bit more info (even though it would be good to know that age of the cable as well, let's just go with what we have, okay...)

Originally Posted by torrmundi

I have a run of 123ft of Ancor RG8x. So about 5.7 dB of attenuation when in perfect condition. I could replace it with MPD's 125ft LMR-400 cable for about $150 and save 4 dB of attenuation (not too shabby), and about half the RG213 attenuation.

Just read this first...(I'll be using some easy round numbers here, so those that wish to criticize, please chill)

100% power coming out of AIS transmitter (or any marine VHF radio), minus 5.5db loss = 28% at the other end (masthead)....if infinite VSWR at the masthead (100% reflected power), you have 28% of the original power reflected, minus 5.5db loss going back down to transmitter end....now you see 7.9% reflected power, which is a VSWR of 1.75:1 (perfectly acceptable to the transmitter)

So....


3) So, while you could mess around and try an antenna inside the cabin, but why not just fix the problem?
And, that means replace the cable, connectors, and antenna...

Now, if this was "new" cable, and all connections were professionally done, we would absolutely need to know about any splices, etc. and then also consider whether or not your AIS transmitter is faulty and/or whether your "buddy boats" have some AIS receive issues (such as RFI from a masthead LED light, etc.)
But, we are assuming this is old cable (> 5 years old) and unknown connections....so..
So, unless we find otherwise, you should plan on replacing it...

Now, I know others will send you around many troubleshooting steps, and I can do the same....but, probably not necessary.
Although, if you do have a spare VHF antenna you can connect it to your AIS and try that....
Perhaps just swapping your VHF and AIS antenna cables at the radio transmitter end, and see what results you get...
This would be a good first step, but we also introduce another "unknown" (the quality of your primary VHF antenna system)....

Again, torrmundi, I'm sorry to be just giving you some general answers, but we still don't have all the info needed to be specific.



4) And, finally in this brief message....
LMR-400 cable is a very bad choice! It has a solid, copper-plated aluminum center-conductor! Not a good choice for a boat!
(even the "UF" version can be an issue, as you MUST be certain that no water / moisture gets inside, as the dissimlar-metals in the shield will have issues, and well as the added loss as the moisture wicks thru the foamed dielectric)



Definitely want RG-213 here, with properly/professionally installed connectors!
{although, if you cannot "fit" RG-213, understand that a good quality RG-8x, is still okay....have a look at the margins we have in our Marine VHF system, and you'll see....fyi, the old standard called for a max of 3db of feedline loss, the new (ISAF, WorldSailing) standard is 2.7db not much difference, and "ironically" the loss in 100' of RG-213 at 156mhz....it's just great that a tech standard is written by non-tech folks who get their numbers of a brochure, huh?}



There is a LOT more to all of this....but that's it for now...I gotta go!

John




P.S.....just curious how tall your mizzen mast is? 'Cuz a 125' run of coax on a 47' boat seems like a lot?

[ka4wja]

torrmundi,
Your numbers are off a but....and again, you should not use LMR-400 on a boat, ever!!!

Originally Posted by torrmundi

I have a run of 123ft of Ancor RG8x. So about 5.7 dB of attenuation when in perfect condition. I could replace it with MPD's 125ft LMR-400 cable for about $150 and save 4 dB of attenuation (not too shabby), and about half the RG213 attenuation.

From Times spec sheet....125' of LMR-400 at 150mhz would be 1.9db...versus 125' of RG-213 which would be 3.3db....that's a 1.4db difference (+/- 0.05db)....

And, fyi, 125' of LMR-400UF, at 150mhz would be 2.25db loss...versus the 3.3db of 125 of RG-213....and that's a difference of only 1.05db....

Of course, the 125' of RG-8x, using Belden 9258 (which is much better than the marine store / Ancor crap), has a loss of 5.2db....or, rounding up to 5.5db for a better guess of "average" RG-8x...

Figure approx. 0.05db more loss at 156mhz, versus 150mhz, for 125' of any of these cables...

I'm taking my LMR-400 loss figures (at 150mhz), directly from the Times Microwave spec sheet, and with various sources of various "varieties" of RG-213, showing loss of 100' at 150mhz of 2.3db (Belden 8267 spec sheet and RF Parts catalog), up to 2.5db or 2.6db for 100' (qsl on-line calculator and Universal Radio website)....I'm sorry I don't have my reference texts with me here, but for many decades I always just used "two and a half db, per 100', at 150mhz" as my back-of-the-napkin calculations....
So...

So, I'm not sure how you calculated the greater differences / where you came up with your calculations?
But...

Regardless, as you see.....going from 125' of NEW RG-8x (~ 5.5db loss) to 125' of NEW RG-213 ( ~ 3.3db loss), would see a 2.2db reduction in loss....and going to 125' of LMR-400UF (2.25db loss) (not recommended), would see an additional 1db reduction, or about 3.25db of reduction...(not 4db)
But...

But, going from 125' of old RG-8x, with unknown connections, to just about any NEW coax with properly assembled connections, will almost certainly show you MUCH greater positive effect, possibly as much as 6db to 10db improvement...
{please note that this is the primary reason why so many sailors rave about whatever new VHF antenna cable they've just installed....not because of that specific cable's actual loss specs, but rather due to the fact that it is new, clean, etc., and most importantly has new properly-installed connectors!}






Also, you've not mentioned what your AIS transmission success is to Marine Traffic, etc....or on other boats?? (you know we still cannot be certain that there are issues on your "buddy boats"...)





There is still a lot more.....but quite busy, at the moment....gotta' go

John

[torrmundi]

See my answers within the quoted parts.
Thanks,
John

Originally Posted by ka4wja

torrmundi,
Of course, we still don't know if you have any splices / junctions in this cable? And, that is a VERY important piece of info to know..
>>I am not able to see all the cable run - but I would expect any splices to be available and there are none visible.


It would be good to know that age of the cable as well, let's just go with what we have
>>Probably ca. 2014.


3) So, while you could mess around and try an antenna inside the cabin, but why not just fix the problem?
And, that means replace the cable, connectors, and antenna...

unless we find otherwise, you should plan on replacing it...
>>agreed.


Perhaps just swapping your VHF and AIS antenna cables at the radio transmitter end, and see what results you get...
>>yes, this is possible. It has VSWR of about 1.1 with what is about the right diameter for RG213, and the same mast height and a possibly shorter run length inside the hull. Pretty good performance in VHF use.



just curious how tall your mizzen mast is? 'Cuz a 125' run of coax on a 47' boat seems like a lot?
>>the mast itself is 51ft. The rest of the run is up and down and around the circumference of the cabin to reach the radio, about 35.5ft. So about 87ft total. Obviously I made an error with my calculator summing up the bits and pieces. Pretty big error! Somehow I think I managed to add the mast & 2*bits. Thanks for forcing a redo.