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Old 21-06-2011, 18:50   #46
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Re: AIS reception issue

An antenna analyzer will give you much more information than an SWR meter alone, but they are more expensive.
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Old 22-06-2011, 01:52   #47
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Re: AIS reception issue

What more does an analyzer do? I've ordered an MFJ-822 which measures forward and reflected power plus SWR.
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Old 22-06-2011, 02:06   #48
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Re: AIS reception issue

An analyzer can measure the complex impedance of the antenna and cable (reactance and resistance), and some analyzers can generate test signals that let you measure this impedance over a broad range of frequencies.

I don't think you really need one for what you're doing -- a couple of SWR (and power level) measurements can tell you most everything you need to know, at least to solve the problem you're working on. Antenna analyzers and other more sophisticated instruments are great for designing antennas, matching networks, etc, but that's not what you're doing. Also, and no disrespect intended, you probably need some training or RF background to get the most out of the more complex test gear.
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Old 22-06-2011, 02:30   #49
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Re: AIS reception issue

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Also, and no disrespect intended, you probably need some training or RF background to get the most out of the more complex test gear.
No doubt about that, but this is slowly bringing back all my AC circuits classes from 30 years ago. Trouble is, I'll once again forget it all in a few weeks....
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Old 22-06-2011, 04:42   #50
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Re: AIS reception issue

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I spliced the cable using one of the Shakespeare splicing devices,
Those damn Shakespeare CenterPin connectors! I can't tell you how many of these I have had to replace. I have a whole box of them. The problem is usually loss of contact with the cable shield. You can check continuity after installing them and you think everything is ok but you have no way of knowing how many and how well any of the six shield "fingers" are making contact with the shield. It could be that just one tiny piece of one of them is making contact. After a little vibration, movement and/or oxidation, and the contact is lost. There are also issues with the center conductor. Usually, the pin does not go into the center of the stranded center conductor, but goes off to the side squishing the center conductor between the dielectric and shield. With foam cable, it can come very close to shorting out. Easy to install, but a failure waiting to happen in my professional opinion.

Eric
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Old 22-06-2011, 05:59   #51
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Re: AIS reception issue

Does the radio have any NMEA 0183 (or NMEA 2000) connections? Most do for DSC. My radio says it will send information to my chartplotter to show the location of DSC signals (I've never hooked the talker side up because I was afraid it might interfere with the AIS display).

If a VHF NMEA 0183 talker is messing up your network, you'd see this the moment you turned on the radio.

Carl
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Old 22-06-2011, 06:13   #52
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Some antenna analyzers are capable of showing where along a cable run faults more subtle than shorts and opens exist, though the SWR meter should do fine to give you a go/nogo for your system.
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Old 22-06-2011, 06:16   #53
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Re: AIS reception issue

I'm expecting that coupler to be the problem as well. What's the preferred method to extend a cable? A barrel connector and two PL-259s?
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Old 22-06-2011, 06:25   #54
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Re: AIS reception issue

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Does the radio have any NMEA 0183 (or NMEA 2000) connections? Most do for DSC. My radio says it will send information to my chartplotter to show the location of DSC signals (I've never hooked the talker side up because I was afraid it might interfere with the AIS display).

If a VHF NMEA 0183 talker is messing up your network, you'd see this the moment you turned on the radio.

Carl
A few posts back I described the communications layout, but to answer your question the VHFs are the only devices on 0183. Both are listeners, and only the icon is a talker since you can only have one. The DSC messages are different from the AIS messages so the plotter should have no issue handling both. If your concern is having two fallers on the same 0183 but, you are correct that it's a no no to do that. To me that's one of the most compelling reasons to use N2K, which I've done everywhere else. For whatever reason VHFs are very slow to support N2K.
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Old 22-06-2011, 07:16   #55
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Re: AIS reception issue

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A few posts back I described the communications layout, but to answer your question the VHFs are the only devices on 0183. Both are listeners, and only the icon is a talker since you can only have one. The DSC messages are different from the AIS messages so the plotter should have no issue handling both. If your concern is having two fallers on the same 0183 but, you are correct that it's a no no to do that. To me that's one of the most compelling reasons to use N2K, which I've done everywhere else. For whatever reason VHFs are very slow to support N2K.
Sorry for the typos - That's my iphone "improving" my poor typing.

I meant "if your concern is having two talkers on the same 0183 bus, you are..."
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Old 22-06-2011, 07:19   #56
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Re: AIS reception issue

Sorry, I missed the page about the network. But if the antenna doesn't fix it, I'd go another round with the network. One of those radios could be intermittently throwing something on the network that confuses the chartplotter. All of that NMEA and DSC stuff seems to be an after-thought for the radio guys (except for the red buttons)

I find a complex NMEA 2000 network when mixed with NMEA 0183 to be a twitchy creature. I have a Simrad autopilot (same model as yours) and I've never been able to keep it happy using NMEA 2000 to the Garmin chartplotter and Airmar weather station). It will work for a while but after 10-20 minutes the network would crash. I replaced connectors, analyzed impedance, etc. Finally I just used 0183 between the two systems and it's completely stable.

You might also want to consider one of the active splitters for the antenna. I know people have their concerns about this approach but it means you have a "known good" cable and only one thing transmitting at a time. I use one (Digital Yacht SPL250) and have seen no negative impact on VHF performance and great AIS reception (60+ miles offshore from a 60ft high mast). It cost about $225 and installed in about 10 minutes. It came with all the interconnect cables and adapters made up.

Carl
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Old 22-06-2011, 09:16   #57
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Re: AIS reception issue

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Some antenna analyzers are capable of showing where along a cable run faults more subtle than shorts and opens exist, though the SWR meter should do fine to give you a go/nogo for your system.
A "Time Domain Reflectometer" (TDR) is extremely useful for troubleshooting cable and connector issues. It will show you the impedance at every point from beginning to end of a cable run. Now that I'm retired, I sometimes make a crude TDR using a pulse generator and high-speed oscilloscope. It's not as accurate as a lab TDR but still works great for finding bad connectors, etc.

Still, the good old SWR meter is cheap, simple, easy, and effective, especially for what twistedtree is trying to do.
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Old 22-06-2011, 10:36   #58
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Re: AIS reception issue

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A "Time Domain Reflectometer" (TDR) is extremely useful for troubleshooting cable and connector issues.
Oh yes, I remember those. We rented them once or twice to do design verification of a product in a previous life. They were VERY expensive, but did exactly what you describe and were key to validating the design from chip, through board traces, connectors, cables, etc. But I never touched the thing - just signed the PO.

I think go, no-go is a good description of my needed, and an SWR reading should give me that in a cost effective manner. After all, the only things I can "correct" are the connectors and replacement of defective items.
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Old 22-06-2011, 11:17   #59
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Re: AIS reception issue

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I think go, no-go is a good description of my needed, and an SWR reading should give me that in a cost effective manner. After all, the only things I can "correct" are the connectors and replacement of defective items.
Not trying to be the smart a$$ here but actually... it isn't. I think somebody already mentioned that a dummy-load gives a perfect 1:1 SWR reading combined with a total failure to communicate. I have seen many cases where a 1:1.5 - 1:2 SWR was shown while there was no electrical connection between radio and antenna (just moisture in coax).

I really still recommend the emergency antenna. Somebody replied that it isn't a real good antenna... I agree... but I meant using it for testing. This antenna must be outperformed by the real antenna, so you have a good comparison. It's also the cheapest test.

ciao!
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Old 22-06-2011, 11:44   #60
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Re: AIS reception issue

Nick is right about a "good" SWR not being proof of a good cable. In a marine or outdoor environment, coax has a limited useful life due to moisture absorption. You might give some thought to replacing your existing spliced together coax with a nice, fresh undivided run of new coax to the antenna if you keep having trouble finding the problem. Particularly if the coax is old and/or connectors are exposed to weather. Good RG-8X is only about 35 cents a foot from someone like Ham favorites therfc.com or thewireman.com -or $2 a foot from West Marine.
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