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Old 18-04-2006, 19:25   #1
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VHF interference from instruments

While my fixed mount VHF was out of commission last week, I was forced to use my handheld from the helm. I discovered that the wind instruments are creating a good deal of interference with the receiver...to the degree that I couldn't "squelch it." Turn off the instruments and all is well.

Any idea on diagnosing and correcting? The wind instruments are on the same circuit as the rest of the instruments so I can't really isolate. I believe it is the masthead wind unit, though, because the sound changed as the windspeed changed.

Curtis
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Old 21-04-2006, 01:01   #2
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Interference

An interesting scenario, as I had thought that the VHF in transmission mode might interfere with instruments. With this in mind I elected to mount radio aerials on a frame over the pushpit built to mount solar panels etc.

When discussing with the supplier of instruments I selected, their advice was that there should be no problem if a good quality sheilded cable was used. I decided to mount on the tower over the stern anyway.

I know that in a land based installation there is a "separation of services" protocol that required power, data and telephone cabling to be run in separate ducts or conduits. This is to eliminate or minimise the possiblity of power transmission (240v) interference corrupting the data transfer or telephony.

Now I just don't know.

Fair winds

Steve
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Old 21-04-2006, 01:23   #3
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Curtis, please give more detail. What is the make and model. Once I know that, I or someone may have further questions on wiring details.
For RF interference like that, it is more than likely coming from the internals of the instruments and is being created by an oscillator that could be driving a uP or even the LCD display. I am suprised they are producing that much interference. I doubt it is coming from the mast head unit itslef, but I've been wrong before.
You may find it is a combination of all the instruments, not just the wind. You would have to isolate each individualy to know that for sure. Check and see if the installation is as to the recommended procedure, especially in the area's of cable sheilds if they have them.
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Old 21-04-2006, 04:42   #4
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It's possible you have always had a general problem but the weaker handheld notices it more. With the fixed VHF the antenna is on top of the stick the stronger signal can easily be squelched to a clear sound. VHF on the very top of the mast is the best. Thick insullation and shielding all the way to the unit.
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Old 21-04-2006, 08:30   #5
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Curtis, are you using radio frequency instruments, such as Tack Tic?

I can't scan with my handheld unless I have a band pass filter between it and the antenna. It picks up radio signals from all over the area. As soon as I put a bandpass filter on or operate on a single channel everything is good. However the filter is large and not practical when using the handheld on the boat.

You may have to look for a handheld with a stronger front end (larger unit), possibly a commercial grade marine handheld or operate without scanning. But try before you buy. I don't know of any bandpass filters that would be practical for a handheld.

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Old 21-04-2006, 08:54   #6
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I have no problems with TackTick. It use a different frequency.
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Old 21-04-2006, 12:13   #7
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IF conversion frequency

Curtis your VHF handheld un-squelch problem is probably caused by your instruments generating a wideband spectrum of noise that directly invades the handheld internals via the IF of the radio and not at the VHF frequencies like you might imagine. Actually the IF can be interferred with either through the case or the antenna.

I've "solved" similar problems by altering the squelch circuit to vary the rf or IF gain simultaneous with the squelch yet for you that is not an option. The best solution would involve filtering the masthead wires serving the wind unit AT the masthead, not easy to do.

Less effective, yet easier to do, is to first wrap several turns of the wires exiting the mast from the wind transducer around a ferrite core rated to 70MHz frequency response. You might also merely try to use clamp-on ferrites designed to minimize switch-mode power supply noise from power leads like you see used on most computer component cables to note any improvement in squelch settings that work.
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Old 24-04-2006, 06:00   #8
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Thanks for all the responses....

To catch up on some questions, the instruments are all Raymarine ST-60 series that are interconnected with their proprietary cables. No LCD displays as I use a laptop for a plotter. I've eliminated the laptop from the cause by turning it off. Turning each individual instrument off will be a bigger challenge as they are "daisy chained" together and access is tight.

I installed the instruments myself and am fairly sure that they were done according to the spec. The instruments did include a number of ferrites with no guidance as to where to place them....I'll probably start experimenting with moving them around the system....

Thanks,

Curtis
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Old 24-04-2006, 13:26   #9
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Can't argue with the instrument quality. OK, so how about the H/Held. Is it a cheapy or a good one??
Use the ferrites as Rick explained. I would start with the easiest access and see what happens. So that would mean the cables closest to the instruments. I would also suggest that this is where the noise is being created, not at the mast head itself, so keep the ferites as close to the instruments as possible is important. It is also possible that the wind instrument is not the cause of the noise. It could be that it is simply being used as a radiator of that interference. If you could possibly get to those instruments ( and lets face it, you need to get to the cable of the wind instrument anyway), you need to isolate each instrument individually to see which one or more is causing the issue. I suspect that somewhere in that "daisey chain" you have a sheild or earth fault.
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Old 26-04-2006, 17:14   #10
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Curtis, I've used my HT and the main VHF (with masthead 1/4 wave antenna installed maybe a yard from the ST-40 windset) without hearing any problems on either one. I'm only rashly guessing if the ST40 and ST60 wind heads both are the same design, but I think your best bet is a phone call toll-free to Raytheon/Raymarine, whichever they are calling themselves this year. They're a tech company, so if you explain that you need tech support for an RFI problem, they may have some specific ideas about this.

I don't know what else you've got connected into the mix (maybe a seattalk/nmea converter too?) but if the main VHF, with the masthead antenna, does NOT pick up the interference, and the handheld VHF does, the problem may be in the handheld. I'd suggest getting an antenna adapter to connect the HT to the masthead antenna (typically an SMA-to-PL259 connector, $5-10 from a ham radio supplier). That will give your HT better range and power in case you need it in an emergency, so it is not wasted.
Now, if you find that using the main antenna "solves" the interference...then you can presume you are getting RFI from something down below, rather than from the wind set.
You can also snoop around for the noise source by taking the antenna off your HT, turning down the squelch, and running it along the instruments and cabling, trying to pick up the signal with your "weaker" HT, missing an antenna. That may localize it.
But it also is possible that something aboard is generating "intermod", a signal that is mixing internally with something in your HT and not coming in via the antenna. In that case, you've got to find the culprit and find a way to filter it--but replacing the HT with a different model may be much simpler, if more expensive.
Borrowing another HT, and seeing if the interference affects "any" HT or just yours, would also be a good way to see if intermod is the problem.
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