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Old 25-04-2007, 21:05   #16
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Bill-
" lots of copper strapping or plates INSIDE the hull (for a capacitive coupling to the seawater);"

Which makes me wonder, if there is a capacitive coupling to the ground, what's the range of the capacitance?? How much does that vary (or doesn't it matter) with hull construction, and become another variable in how well things work, i.e. somewhat unpredicatbly from boat to boat?
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Old 25-04-2007, 21:43   #17
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SSCA Discussion Board :: View topic - RF GROUNDS IN THE MARINE ENVIRONMENT
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Old 26-04-2007, 11:58   #18
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Originally Posted by btrayfors
Other possibilities: lots of copper strapping or plates INSIDE the hull (for a capacitive coupling to the seawater); tanks, etc. This will work, too, and is more effective over time than is an external ground plate.Bill
This is what I have, but I am unsure of the maximum distance the strapping can be from the other "capacitor plate" (seawater, tanks, etc). For example, I have copper screen laminated into the bridgedeck of my catamaran. It sits 24" off the water. Will it still capacitively couple with the water? The aluminum tanks sit on the bridgedeck and are separated from the copper screen by 0.5". Can I assume that I am getting the full benefit of the 100sqft of tank surface area through capacitive coupling? The tanks are 6'Lx3'Wx1'D. Does the rf see these as 50sqft of surface area (each) or as two blocks of 18sqft, or a single block of 36sqft (they are side by side and only separated by one inch)?

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Old 26-04-2007, 13:37   #19
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Hi, Mark...

Seems like a nice setup. In your case the objective is not to capacitively couple to the seawater, but to form a pseudo-ground which will serve as the "other half" of the antenna.

While there may be some degree of coupling between the screen and the tanks since they're so close, I think I'd strap them together, i.e., bridge that inch or two with wide copper or aluminum strapping.

I doubt there'd be much problem with electrolysis as a result of different metals, but you certainly don't want to compromise the water tanks. Therefore, I think I'd make the strapping from aluminum so that the copper-to-aluminum interface is away from the tank structure itself.

Together, the tanks and the screen should make a good RF pseudo-ground. If you have problems tuning on any band(s), you can always add 1/4-wave radials for that band.

Let us know how it works out for you.

Bill
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Old 26-04-2007, 14:40   #20
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Thanks for your reply Bill. My question was limited to getting an understanding of how the function of the bridgedeck copper screening might be enhanced by the tanks and the seawater. This screen is only a portion of the counterpoise. My complete counterpoise is much larger - it consists of a 14'x10' aluminum arch/bimini connected to copper foil radials that run through both hulls slightly below the waterline and also to the copper screening spanning the bridgedeck. I guess also connected to the counterpoise is 200' of lifeline, since the lifelines connect to the bimini structure. The coupler and antenna are mounted on top of the arch, so all of this is below the radiating components.

I'm not sure I understand your description of connecting the tanks. Did you mean for the two tanks to be connected together, or for the copper screen to be connected to the tanks?

In general, how much distance can be tolerated for capacitive coupling with rf? Will the copper straps running through the hulls couple with the seawater through 3/8" fiberglass? Does the "dielectric" part of the capacitor make a difference (air, fiberglass, wood)?

Also, in the case of tanks, what shape does the rf see them? Does it think a tank is a two-dimensional sheet of metal, or does it see a tank as a larger three-dimensional surface area?

Thanks for your help with these questions. The boat was set up this way when I bought it and I have been racing to learn HF radio. I am now at the point of wondering about these types of things.

Presently, I am up a river in a marina surrounded on all sides by large metal pole barns. Experimenting with my radio setup is out of the question. I barely receive time signals and can't even get FM radio reception.

Mark
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Old 26-04-2007, 14:52   #21
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Mark,

Wish I'd known that before.

Sounds like you've got more than enough RF ground to satisfy any eventuality. I wouldn't worry about the screen or the tanks.

Remember, you're not trying to recreate the entire earth under you. You're only trying to create the other half of the dipole!

You need to take the boat out of the marina and do some tests before you worry any more about it, IMHO.

Bill
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Old 26-04-2007, 23:16   #22
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Mark,
I agree about getting out of the marina for some testing.
The ground you have should be sufficient, but I doubt the screen in the bridgedeck will have any significant capacitive coupling to the water.

The formula for capacitance (coupling in your case) is proportional to the area of the plate (screen, tanks etc) also proportional to the operating frequency you are using, and it's also inversly proportional to the square of the distance from the ground plate to the water.


I am a retired electronics technician as well as an Advanced class HAM.

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Old 27-04-2007, 02:16   #23
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Earth ground connections are always capacitive. Otherwise, a 1/4" bolt head in saltwater and connected via a low inductance conductor would be a perfect ground. An earth ground acts to dissipate RF energy.

A large non-inductive metal mass used as a ground is known as a counterpoise. It may be a single mass like the skin of an aircraft, or multiple masses like an engine and tanks of a boat. A counterpoise also acts by dissipating RF energy. (Note: A counterpoise located within a few inches of seawater also acts to connect to earth ground.)

A radial is a horizontal length of wire located above ground and near the base of a vertical radiating antenna element. Radials are part of the antenna and act much like a dipole bent in half. That is way it is offen recommanded to cut the radials to a quarter wavelength. This can give an vertical antenna with radials up to 1.25 dB of gain over a vertical with a dissipative ground. (Note: Radials run close to an earth ground or counterpoise will also couple to those grounds.)

However, a dissipative ground may do more to improve communications by reducing noise then the antenna gain provided by radials.

Guess what? You don't have to choose! When connected together, radials act as radials should and dissipative grounds act as they should.

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Old 22-02-2008, 08:38   #24
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Time to Replace These

OK so back to the original problem...


I will remove the remnants of the original bolts and replace. I would prefer not to remove the plate itself from the hull. So what sealant do I use? I'm concerned about water intrusion of course - sealing around the bolt heads will do nothing except seal them to the plate. Do I fill the bolt holes first before I insert the new bolts? Must I remove the plates , put 5200 around the holes and then re-mount?

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Old 22-02-2008, 08:52   #25
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I'd think that if you drill the old bolts out, fill the holes with 5200 around the bolts (or 4200, or any bedding compound designed for permanent use below the waterline) and then replace the bolts, keeping them fully bedded with no voids, with washers inside the hull, that would do it. barely snug them down, allow the bedding to cure and get solid, and THEN snug the nuts down a bit more, to compress and expand the sealant.

5200 cures up practically as solid as the FRP around it, I don't think you'll have any leaks considering the bedding will be in compression from the bolts and washers anyway.
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Old 22-02-2008, 09:01   #26
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Originally Posted by hellosailor View Post
...5200 cures up practically as solid as the FRP around it, I don't think you'll have any leaks considering the bedding will be in compression from the bolts and washers anyway.
That makes sense for the inside of the hull. But on the outside I'll have no such compression washer. I'm concerned that water would be inside the hole with the bolt and stopped at the interior washer? Eventually water would migrate into the hull layers wouldn't it?
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Old 22-02-2008, 09:32   #27
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Hi Mark:

I'm definteley not an expert but here's my logic for what it is worth. I've been studying up on how to replace thru hull fittings. Gord May posted this wonderful link Replacing Thru-Hulls and Seacocks Photo Gallery by Maine Sailing at pbase.com if you scroll down the page there is a section that talks about using a countersink tool on both the outside and the inside of the boat. The site's author explains that the reason for this is that you fill the countersink holes with sealant and the sealant fills the void and acts like a washer. Makes sense to me. Although you are talking about two different items that are attached you seem to have the same attachement problem.
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Old 22-02-2008, 09:42   #28
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Mark-
The bolt head itself will form the "washer" on the exterior side, and if you have made sure there are no voids--there is no way the water will get into the hull through the bolt holes. For all intents and purposes, a solid fill with 5200 might as well be epoxy, it is equally impenetrable and stable in the hole alongside the bolt.
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Old 22-02-2008, 09:56   #29
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Mark,

I agree with hellosailor. You won't have any leak problem if you follow his instructions.

Steve B.
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Old 22-02-2008, 10:36   #30
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My 36' steel hull makes a good ground plane also.
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