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Shielding Question

Old 07-13-2004, 04:31 PM
  #1  
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Default Shielding Question

I have been having this problem with my vhf and sounder since installation. In fact, I posted about it last year.
In laymans terms, when the sounder and VHF are on together the VHF is picking up the transducer signal and causing intermitent static on the vhf as it sounds out echoes.With sounder off signal is crytsal clear on VHF.
Garmin said I need to put aluminum duct tape on the antenna connection to the vhf unit and connection to the sounder itself.
Have you heard of "aluminum" duct tape and where could I buy some?

Thanks very much-

Rick
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Old 07-13-2004, 05:29 PM
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Default RE: Shielding Question

Home depot has it I think.
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Old 07-13-2004, 07:10 PM
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Default RE: Shielding Question

Your transducer cable and maybe your bottom machine is acting like a transmitter antenna. Try putting snap on ferrite chokes on the transducer cable (as shown here http://www.rfparts.com/choke.htmland part number fercke b or c based on the diameter of your transducer cable). 2 to 4 placed on the cable at the machine and perhaps at the transducer should suppress the EFl. RF Parts is very reputable, but expensive. Search around the net and you should find them for about a buck each. Or ask a ham radio friend. He can help you out.

Also try routing the antenna coax and transducer coax (and the radio/bottom machine for that matter) as far apart as possible. This will help a lot.

Finally, the aluminum duct tape is a last resort and probably won't help much.
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Old 07-15-2004, 01:24 PM
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Default RE: Shielding Question

skipperbrown's suggestion regarding separating the coax cables as far apart as possible is a good start. However, there may be a problem with the common feed of the +12 volts and ground to both units.
Just as a test, try feeding the VHF radio from a pair of wires that go directly back to the battery. You can try a pair of #14 wires. If that eliminates the problem, then forget all of that shielding stuff and find out whether the +12 or ground is the cause. Leave the +12 from the battery connected to the VHF and connect the VHF ground back to the normal ground return. If the noise comes back, then you will have to beef up the normal ground return or run a separate lead from the VHF ground back to the battery. If the noise does not come back then the problem is in the +12 supply and you may have to put in a +12 volt decoupling device. Radio Shack sells one that is pretty good.
If the problem does not go away even with the separate power supply wires to the VHF, then give the shielding a try. I would suggest that you use plain old aluminum foil instead of that foil duct tape. Wrap the coaxs' indivdually with the foil and secure with quarter inch spiral wrap over the foil. Also, the foil shields should be grounded to power supply ground. Skin regular #18 wire about 2 inches and wrap the wire around the foil under the spiral wrap. Return the other ends of the 18's to ground.
Good luck.
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Old 07-15-2004, 04:32 PM
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Default RE: Shielding Question

Thanks very much for the advice everyone. Will report back.

Rick
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