AIS Antenna VSWR exceeds Limit

[newbie-]

Originally Posted by wsconner

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....

Em-trak Marine Electronics A100 AIS Class A Transceiver

I looked in to the splitters but I wasn't sure about that direction. The purpose built AIS antennas tend to be about 4' in length and my VHF is 8'. Felt like this wasn't a good overlap.

I have the room for 2 antennas so it wasn't a big deal in my case.

[continuouswave]

Originally Posted by wsconner

...when I use ProAIS2 to get some diagnostics, the VSWR [ratio] is 12:1...

With typical line loss involved at VHF, a measured VSWR at the transmitter end of a transmission line of 12:1 indicates there is probably NO ANTENNA connected at the other end of the line.

[continuouswave]

Originally Posted by s/v Jedi

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)....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 that you need a special wide band antenna.

I believe that use of Channels 75 (156.775-MHz) and 76 (156.825-MHz), which were formerly GUARD BANDS for Channel 16 (156.800-MHz) for long-distance AIS transmission was approved at the 2012 World Radio Conference. See

https://continuouswave.com/forum/vie...hp?f=13&t=1561

In addition to using a separate radio frequency for transmission, these satellite AIS devices also transmit a slightly different message, AIS Message 27.

The above link gives considerable information about AIS satellite reception and problems created by the presence of too many signals.

Re a transmitting antenna, your observation about antenna VSWR bandwidth is very cogent.

The considerable separation of the two new 156-MHz AIS satellite channels from the 162-MHz AIS channels will likely require use of a wide-VSWR-bandwidth antenna with these special AIS satellite transmitters.

What is not clear to me: does every CLASS-B SOTDMA AIS device being sold now make use of the satellite channels?

[wsconner]

So I did some troubleshooting, and ultimately after connecting a new antenna (shakespeare 5215 AIS model) and new cable, and only using the B924 transponder (E80 MFD off), I got the same error conditions. At first the unit gets a green light and the VSRW is low (2:1), then it jumps to 10+:1 and the unit throws a high VSWR error (monitoring using ProAis connected to it). So I'm beginning to thinks its something wrong with the unit itself...?

[donradcliffe]

You could put an external SWR meter in the cable to see what is going on.

[s/v Jedi]

Originally Posted by continuouswave

I believe that use of Channels 75 (156.775-MHz) and 76 (156.825-MHz), which were formerly GUARD BANDS for Channel 16 (156.800-MHz) for long-distance AIS transmission was approved at the 2012 World Radio Conference. See

https://continuouswave.com/forum/vie...hp?f=13&t=1561

In addition to using a separate radio frequency for transmission, these satellite AIS devices also transmit a slightly different message, AIS Message 27.

The above link gives considerable information about AIS satellite reception and problems created by the presence of too many signals.

Re a transmitting antenna, your observation about antenna VSWR bandwidth is very cogent.

The considerable separation of the two new 156-MHz AIS satellite channels from the 162-MHz AIS channels will likely require use of a wide-VSWR-bandwidth antenna with these special AIS satellite transmitters.

What is not clear to me: does every CLASS-B SOTDMA AIS device being sold now make use of the satellite channels?

Yes, every SOTDMA transceiver, class A and class B+ transmits Msg27 on AIS channels 3 and 4. The two antennas I linked to earlier both support the full 6 MHz bandwidth.

[continuouswave]

Originally Posted by donradcliffe

You could put an external SWR meter in the cable to see what is going on.

The AIS transmissions are very brief, only about 25-milliseconds in duration. The interval between these transmission when the boat is stationary could be several minutes long.

Making VSWR measurements using a conventional in-line directional wattmeter may be quite difficult with such short transmissions sent at such slow intervals.

A VHF Marine Band radio cannot transmit on the 162-MHz AIS channels.

You could use a VHF Marine Band radio to transmit at 156.800-Mhz to measure the VSWR of the antenna at that frequency.

[Brian.D]

I posted in Dec 2020 about the NanoVNA. It can be used to measure SWR on a transmission line. It does so by injecting a very low level signal on the line to the antenna and measures the return loss. The NanoVNA is inexpensive and easy to use (if one follows the directions).

I encourage all boaters to have such a small and inexpensive device on their boat. It will pay for itself after a few uses, like in this case of SWR error on AIS antenna/cable.

Here is a link to the posting I made.

[s/v Jedi]

Originally Posted by Brian.D

I posted in Dec 2020 about the NanoVNA. It can be used to measure SWR on a transmission line. It does so by injecting a very low level signal on the line to the antenna and measures the return loss. The NanoVNA is inexpensive and easy to use (if one follows the directions).

I encourage all boaters to have such a small and inexpensive device on their boat. It will pay for itself after a few uses, like in this case of SWR error on AIS antenna/cable.

Here is a link to the posting I made.

Indeed, the NanoVNA is the perfect tool for this.

[stormalong]

Befriend a ham radio operator and get him/her to your boat with an antenna analyzer. Invite him/her sailing. Many hams have antenna analyzers and have good troubleshooting skills.

From the original post I would lean towards a bad installation, probably improperly installed connectors. They are not easy to install until you have done several of them. Sure you can replace everything, at a price, or you could figure out what is wrong and fix it.

[wsconner]

Just a quick update to my OP #9 after talking with a few vendors about my issue. I think I need to solve or make sure I have good cable and antenna connections. Even though I redid all the fittings (soldered) some of the VSWR readings I got with the antenna on top of mast (cable down mast connected to another cable to nav panel), I was still getting variable results in the 8:1 to 12:1 range. To further troubleshoot, I took a new antenna and new cable and connected it directly to the AIS transponder, and was getting a consistent 3:1 VSWR. However, the AIS data was still not transmitting to the E80 (as it did successfully a few weeks ago). One hypothesis posed by a vendor suggested the E80 could not handle an abundance of data on the NMEA 38000 port. In my set up, I'm using the B924 as a multiplexer (per it's manual and specs) to send both instrument data and AIS data from the 38000 port to the NMEA port on the E80. When I monitor the transponder on ProAis, I am seeing AIS traffic, but only 10+ targets. I can't imagine that is "too much data" for the E80. The other thing I'm wondering is if I can send the AIS data from the transponder via the NMEA2000 port to my Seatalk2 input (using appropriate converter cables) on the E80. So maybe I'm dealing with two separate issues.. wondering what others think. THX!

[Brian.D]

I know nothing about all the data connections within a boat, but I wonder if one is trying to crap 5 lbs of crap in a 2 lb bag. May try disconnecting everything except the AIS and see if the data is now being received/transmitted. If yes, then start reconnecting until it stops. Might be time consuming but at least you might find the source. Just a thought.

JMHO

[wholybee]

Originally Posted by stormalong

Befriend a ham radio operator and get him/her to your boat with an antenna analyzer. Invite him/her sailing. Many hams have antenna analyzers and have good troubleshooting skills.

From the original post I would lean towards a bad installation, probably improperly installed connectors. They are not easy to install until you have done several of them. Sure you can replace everything, at a price, or you could figure out what is wrong and fix it.

I agree, it is probably a bad connector(s). Most connectors sold at marine stores are not high quality. I only buy Amphenol, but if you go to a HAM radio store they will be sure you get a good connector.

Second, installation problems are very common. If the shield doesn't make a good connection to the connector, or a single strand of shield shorts to the center, or the dielectric is melted from poor soldering, all can cause high VSWR. Water intrusion into the cable can cause problems that simply replacing the connector won't fix.

The NANO is (maybe) a great tool for this, that will sweep the system and give a display of VSWR vs. Frequency, and also can tell you the distance from the unit to the mismatch, so you will know what connector is the issue. (base of mast vs antenna)

But a word of caution. I don't believe something like the NANO is an easy device to learn without some training or experience making such measurements. Asking about what happens to VSWR if there is no antenna shows lack of understanding of the very basics of what VSWR is. So, I suggest finding a HAM, and they can help, and teach you some basic RF knowledge. They might have a NANO, and if they don't might be happy to play with one. It will probably be fun and interesting for both of you.

Also, I have experience with more expensive and professional analyzers. I was disappointed with the NANO. I expect that all the issues I had were related to varying quality of different sellers on Amazon. I suggest not buying from Amazon or Ebay, and getting on the official site and buying from an approved seller directly. I can't recommend a specific seller, but I see no way to know if what you are getting from Amazon is decent or crap.

By far the best suggestion though, is to contact a local HAM club and ask them to take a look.

[s/v Jedi]

Okay, I’m gonna be the blunt one again:

An AIS transponder transmits to other AIS transponders and receivers, not to a chart plotter. The receiving of targets on the plotter has nothing to do with VSWR or the receiving of targets by the transponder.

It’s becoming clearer that you have a datalink problem as well as a RF link problem.

You talked about a new coax cable and antenna before with horrible results for VSWR but it is unclear how you determined VSWR and you also seem to ignore the info posted about NanoVNA (=antenna analyzer). I now read that with “new cable” you mean one where you soldered the connectors DIY.

I recommend you start with a ready made cable for testing from Amazon (like 20’ RG58 with UHF connectors, only costs a couple of dollars) and a temporary rail mount for the antenna, or hoist it up a bit with a spreader flag line.

Now connect that to the transponder and check VSWR. If above 2:1 then get a NanoVNA or find someone (a HAM or pro) to help. For the masthead antenna you may need that anyway.

Also, besides a NanoVNA you still need a good VSWR meter with coax patch cord.

Here is a link to a “good” meter: https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

And here to a good NanoVNA: https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

I own both of these and can confirm they will do what you need done. On how to use these it’s best to search YouTube.

[wsconner]

Thanks-- very helpful for me as I formulate my own hypothesis. Agree I likely have two issues occurring. The unit was working properly for two weeks, and was transmitting AIS target data, plus instrument data from the B924 AIS to the E80 chart plotter. So it's likely I developed an antenna connection issue (existing cables are 20+ years old), and maybe that is the root of all my issues. I did troubleshoot by isolating the antenna connection (new cable, antenna and fittings) connected directly to the AIS and the VSWR reading on AIS ProAIS software was only 3:1. I thought this would have solved it, but the E80 was still getting the same error. Anyways, appreciate the time folks take to comment.. it does help my problem solving approach to hear the different ideas. thx.