AIS greatly reduced range

[double u]

suddenly our AIS (Matsutec) is showing only targets at greatly reduced range, nothing further out than say 9nm, often only 4-5nm before an approaching target is shown/a passed one is lost, when before 24nm was no rarity. I suspect the VHF antenna-installation, cable or connectors (the Matsutec is only 1/2 year old & otherwise working perfectly)
the question now is: what sort of test can be done to see how much power comes out of the VHF masthead antenna (the AIS shares the antenna with the VHF via a coupler).
any other suggestions? (of course I could "just" change the mast cable & the aerial at the masthead as "regular maintenance"...but if I can be sure of the fault before going that way...)
btw: it's not the coupler/splitter, directly connecting the antenna cable made no difference

[rom]

Originally Posted by double u

suddenly our AIS (Matsutec) is showing only targets at greatly reduced range, nothing further out than say 9nm, often only 4-5nm before an approaching target is shown/a passed one is lost, when before 24nm was no rarity. I suspect the VHF antenna-installation, cable or connectors (the Matsutec is only 1/2 year old & otherwise working perfectly)
the question now is: what sort of test can be done to see how much power comes out of the VHF masthead antenna (the AIS shares the antenna with the VHF via a coupler).
any other suggestions? (of course I could "just" change the mast cable & the aerial at the masthead as "regular maintenance"...but if I can be sure of the fault before going that way...)
btw: it's not the coupler/splitter, directly connecting the antenna cable made no difference

I had the same problem last year. Marinetraffic & vesselfinder confirmed I was not being seen anymore. Closeby MRCC could see me but I suppose that's because they have very high gain antennas. Vesper Marine (I have XB8000 and splitter) was not very helpful, the equipment was showing a rather good signal/noise ratio. I ended up changing my VHF antenna & cable. Problem solved.

[double u]

...yes rom, that's the "maximum effort" solution...
should there be connectivity between center & screen on the coax while it is still connected to the antenna? how many ohms? (of course I know that on the cable alone, antenna unplugged there should be no connectivity...

[DeepFrz]

Did anything significant happen just before you noticed the drop in range? Strong storm? Maintenance around the antenna/cable? Install new electronic equipment?

Temporarily remove the splitter from the equation and see if that makes any difference. See if you can find someone with a coax tester that you can borrow.

[Dsanduril]

Any chance you have a new LED light fixture up the mast? When our new, "guaranteed no RFI" LED nav light went up our range went down similarly. At least at night with the LED powered on (and we frequently forget to turn it off during the day). This is common enough with enough different AIS/LED combinations that the USCG issued a safety advisory last year (August/September?).

[double u]

no change in eqipment, no elctrical storm - the change occurred during passage from Curacao to Portobello (or at least that was when we noticed ships "popping up" on the AIS at close raange & disappearing from the display at 3 miles or so).
no apparent change with splitter left out & antenna connected directly

[smac999]

Buy one of these , tempory stick it in the deck and plug it in. and see if the range comes back. You should be carrying one anyone's incase you ever break the antenna or mast.

If it does. Time to replace ant and or cable.

5911 Classic Emergency VHF Antenna | Shakespeare Marine Antennas

[S/V Alchemy]

Originally Posted by smac999

Buy one of these , tempory stick it in the deck and plug it in. and see if the range comes back. You should be carrying one anyone's incase you ever break the antenna or mast.

If it does. Time to replace ant and or cable.

5911 Classic Emergency VHF Antenna | Shakespeare Marine Antennas

This is good advice irrespective of your results. A rail-mountable antenna could save your bacon one day.

[brownr377]

You've got attenuation somewhere in the system, check the cable and all the connections. Look for chafe and damage on the coax anywhere you can. If the shield got damaged somewhere down the line that easily could cause this.

Does the Matsutec have a VSWR measurement function? If it's higher than 2:1 you have problems with the cabling and or antenna.

Could also be connections worked loose inside the unit, I have no insight into the build quality of Matustec but I'm suspicious of it.

[donradcliffe]

The 'connectivity' between the center lead and the screen on a vhf antenna depends on the antenna. Some show open, while some use a coil and show good continuity. If you know the model number, look on the manufacturer's website for technical specifications.

I agree you should make sure the range isn't a function of having electrical equipment on or off, especially solar controllers and led masthead nav lights. As rom mentioned, see if other AIS receivers are picking up your transmitted signal normally--if you show up well on Marinetraffic, the problem could be local noise sources on your boat instead of an antenna/wiring problem.

[stormalong]

Find someone with an antenna analyzer. A cruiser ham might have one. It will tell you if the antenna or coax have a problem.

[a64pilot]

I think there are a lot of poor xmitters out there, just two days ago I was following a nice, expensive Trawler, but losing his AIS at less than two miles, it was intermittent.

[Chuck Hawley]

Originally Posted by double u

what sort of test can be done to see how much power comes out of the VHF masthead antenna (the AIS shares the antenna with the VHF via a coupler).

When we (Adm Chris Oxenbould, Stan Honey, and I) wrote about sailing high speed sailboats in congested waters after the collision in the Volvo Ocean Race, we identified that several of the race boats had similar problems which were directly related to water intrusion at the masthead antenna connection. Please read the report for how to diagnose this and how to prevent it in the future.

The report also makes some recommendations on how to make racing sailboats more obvious to vessels in their vicinity, as well as improvements to radars used in anti-collision roles.

Cheers,

Chuck Hawley

[toddster8]

All good advice above. One more possibility, particularly with Marine Traffic, is that a local volunteer ground station (or repeater) may have gone off the air. There is a way to drill down on the site to check the status of specific ground stations, but I don't recall the specifics at the moment. It has happened here before.

[Jim Cate]

How is your VHF working these days? If there is a problem with the antenna or coax, it should show up on the VHF range as well. Also, have you checked with other folks to see if your AIS signal is being received at normal ranges? Faults in cable and antenna would s how up there as well.

Jim