Plotter Interference with VHF

rwidman · 06-30-2009, 03:18 PM

I own a trawler with a fly bridge. I recently installed two Garmin 5208 plotters, one on the flybridge and one in the lower helm, and a GSD22 sounder using the Garmin network cables. I also have a Standard Horizon marine VHF mounted in the lower helm with a remote mic at the upper helm.

When scanning the VHF with either or both 5208s operating, the VHF picks up enough interference from the 5208(s) to stop on channel 67 or 68. Channel 67 is no problem, I don’t need to scan it, but channel 68 is heavily used in my area and used by the marinas including my own marina at times. I can increase the squelch on the VHF so it doesn’t stop on 67 or 68 but that eliminates all but the strongest signals.

If unplug the Garmin network cable that connects the 5208s together (at either end) the interference stops, but of course, connecting them together is necessary for proper operation.

The network cable and VHF antenna cable run in the same chase to the upper helm but I have separated them as much as possible. There is only one chase.

I purchased and installed two ferrite clip on filters from Radio Shack and they seem to provide a slight reduction in noise, but not enough to solve the problem.

Suggestions?

BTW: I also sent this to Garmin.

t3rockhall · 06-30-2009, 03:43 PM

Captian Obvious, here.

Have you tried some bigger toroid chokes? Can you slip the Garmin cable into a shield, like the outer braid of 1/2" coax? Have you tried chassis bonding the 2 components?

rwidman · 06-30-2009, 04:20 PM

Quote:

Originally Posted by t3rockhall

Captian Obvious, here.

Have you tried some bigger toroid chokes? Can you slip the Garmin cable into a shield, like the outer braid of 1/2" coax? Have you tried chassis bonding the 2 components?

I had trouble finding the filters I managed to get at Radio Shack. The lady had to call someone on the phone. She thought I was saying "ferral choke". And they no longer have a print catalog where you can look it up yourself.

I suppose I could try a "real" electronics supplier.

The garmin cable has a really big plug on each end, I had trouble getting it through the chase as it is. The plugs are an inch or more in diameter. I would have thought that if the Garmin cable needed to be shielded it would have come that way. These systems have been around for a while.

I haven't tried bonding the plotter and VHF together but I did try connecting the VHF chassis to ground with a cliplead. I don't believe there's a spot on the plotter to connect a bond wire to but there are two and either one causes the interference.

t3rockhall · 06-30-2009, 04:23 PM

try radioworks.com I can look up others if you need. PM me.

rwidman · 06-30-2009, 05:11 PM

It just occured to me that somewhere I have an old handheld so if the battery can be resurected, I could use that for testing. Follow along the wires.

capybob · 06-30-2009, 09:33 PM

raymarine was selling ferrite chokes. I suspect you might have a problem with the vhf antenna though as the Garmin network cables are shielded. Is the Garmin stuff newer than the vhf antenna? How old is the vhf antenna? What condition is the antenna shield in? If it is corroding inside then that is most likely your problem. You could remove the PL259 plug on the radio end of the antenna lead and check the shield condition. If it's good reassemble. Check all the grounds from each of the Garmins and the vhf to be sure they are clean and solid. Check the voltage at each of the units power cables also. If all of that is good. Then try replacing the Gnet cable or as others have mentioned you could case either the vhf in another shield or the gnet. The vhf plug is cheap and easy to replace and would be smaller to re=fish through your chase.

rwidman · 07-01-2009, 05:39 AM

Quote:

Originally Posted by capybob

raymarine was selling ferrite chokes. I suspect you might have a problem with the vhf antenna though as the Garmin network cables are shielded. Is the Garmin stuff newer than the vhf antenna? How old is the vhf antenna? What condition is the antenna shield in? If it is corroding inside then that is most likely your problem. You could remove the PL259 plug on the radio end of the antenna lead and check the shield condition. If it's good reassemble. Check all the grounds from each of the Garmins and the vhf to be sure they are clean and solid. Check the voltage at each of the units power cables also. If all of that is good. Then try replacing the Gnet cable or as others have mentioned you could case either the vhf in another shield or the gnet. The vhf plug is cheap and easy to replace and would be smaller to re=fish through your chase.

I did replace the plug on the antenna because the original was a "no solder" type. I suspected the plug because I was able to reduce the noise by wiggling it, but I suspect the wiggling was causing a loss of the center pin contact because with the new plug the noise is back. The ground and positive connections are clean and solid because they are new and on a new fuse block.

I tried moving the VHF to the other battery bank with a clip lead but that didn't help. I also tried a capacitor from hot to ground and that didn't help (all before I determined where the noise was comming from). Grounding the drain wires from the antenna and the NMEA 0183 cables didn't help either.

The antenna cable shield was fine at the plug end where I soldered it. It's possible that it's damaged at the other end from lowering and raising the antenna. I got my handheld working so I can use it to see how far from the Garmin cable I can pick up the noise. In theory, there shouldn't be any.

The Garmin stuff and installation is brand new. The antenna is probably original to the boat and seems to work fine. That may be my next step but I hate to be one of those guys who keeps throwing parts at a problem until it goes away.