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Old 11-07-2010, 11:24   #1
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Metal Boat - Antenna Lead-in Wire Through Deck

We have a steel Amazon 49 built by SP Metalcraft in 1988. There is an Icom M700Pro and an AT-120 tuner already installed. The lead in wire from the antenna tuner to the backstay has been cut, is now too short, and needs to be replaced.

While looking for a source of GT-150 wire, I came across a site in Germany called www.yachtfunk.com. In their site they make a statement that whenever the lead in wire goes through a metal deck or bulkhead, you need a minimum 30 mm (about an inch and a quarter) hole with an insulated feedthrough to keep the conductor centered. This is required to prevent "capacitive loss" as the wire goes through the deck. In my boat, there is a very small hole drilled in the deck, not much larger than the wire. It is sealed with some type of marine sealant, but the hole is certainly not very large.

Anyone have thoughts on this? On a steel boat, how did you get your wire from inside the cabin to the backstay? Appreciate any sharing.

Steve Brown
Orontes II
Amazon 49, currently lying La Paz, Mexico
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Old 11-07-2010, 11:34   #2
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CableClams - Blue Sea Systems

The largest size looks like it provides the gap you need.

When you drill this out, the diameter drilled will come out smaller than the drill bit you used because the rubber stretches somewhat around the bit. Don't overdrill the size you need though. You want nice tight stretched fit to make it watertight.

Use silicon grease as a lubricant when you push the coax through. Don't use a petroleum based lubricant like Vaseline on the rubber. Petroleum based lubricants will swell and weaken the rubber over time.
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Old 11-07-2010, 12:16   #3
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Thanks, David. The Blue Sea Systems Cable Clams unit looks like it will work.

How about the statement from yachtfunk that a 30 mm clearance hole is required when penetrating a steel deck? I have not seen this suggestion anywhere else, and would like to validate it before I take a hole saw to my boat.

When the lead in wire comes in close proximity to the deck during a penetration, are the losses significant? This is different from inductive coupling during a long parallel run. This is a perpendicular penetration of the deck.

Thanks!

Steve
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Old 11-07-2010, 13:10   #4
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Unfortunately, I do not know enough about the losses to give you an educated answer. Hopefully another member who is more knowledgeable in this area will be able to answer your question.

Perhaps the manufacturer of this equipment knows more than what was stated in the literature?
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Old 11-07-2010, 17:59   #5
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I wouldn't worry about it. Iv'e seen many installations on aluminum boats where the hole is just large enough for the GTO-15. Iv'e done on-air testing while having the GTO-15 placed directly against a grounded backstay (below the insulator) and then pulled over a foot away and while there me be some coupling of the signal to ground, there was no discernible difference of the signal level on the receiving end. Here's a picture of how the pro's are doing it on the new YP's at the Naval Academy. As you can see, it is strapped directly against an aluminum stantion that is welded directly to the superstructure which is grounded and this is on a 200w HF unit.

Eric

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Old 11-07-2010, 20:00   #6
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Agree with Eric....don't worry about it. Just run the GTO-15 thru a standard deck insulator designed for RG-8X coax, or any other appropriate waterproof fitting.

Bill
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Old 11-07-2010, 20:13   #7
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Thanks, gentlemen. This begs the questiojn on the effectiveness or necessity of standoffs. In my boat, the antenna tuner is located in a cockpit locker. There is about 4 feet of horizontal run to the lazerette, then another 4 feet in the lazerette before it turns vertical and comes through the deck. This leaves about eight feet of cable that right now is routed in a cable bundle very close to the hull. Should I isolate this run belowdecks and use standoffs to keep it a few inches away from the deck? Or is this even worth it?
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Old 11-07-2010, 21:48   #8
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If any of what you were told was true (that nearby steel could somehow effect the transmission line wire or coax), somewhere around 10,000 hams who operate mobile in their cars/trucks/SUVs could easily dispel that concern. Theory doesn't always follow actual experience.
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Old 11-07-2010, 22:29   #9
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Runs pretty close to metal leaving the transceiver, tuner and inside the coax. I would guess a tiny hole or a short run next to a rail makes no detectable difference.
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