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Old 04-01-2012, 19:42   #46
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Re: Attaching Antenna Cable to Backstay

I think you folks are forgetting that chainplates, standing rigging, lifelines, stanchions, etc., are all connected via your boat's bonding system to ground. Unless you cut off the bonding wire the SSB transmitter would be pumping its signal directly to a ground.

The RF burn warning and even shock is real which is why the antenna feed is attached above the maximum human standing height on deck. Remember the "standoffs" need to be about 4" roughly from the grounded portion of the backstay so just putting a plastic cover over the wire will not totally eliminate the hazard.

Using a shroud makes more sense as it is very near to vertical but then the shrouds are sitting over your spreaders which connect to grounded mast (via the bonding system). Trying to pass your shroud through some kind of insulator where they touch the spreaders is a problem I have never been able to solve. Also the tension on shrouds are a magnitude of order greater than the tension on your stays which would put some serious loads on any insulators.

So you end up with the three most common SSB antenna solutions - insulated backstay; 23ft Whip; and free copper wire (usually run inside some double braid line/rope and hauled aloft by a halyard or whatever.

Whatever method chosen it needs to be able to withstand the stresses of a boat in a storm out in the ocean without causing the mast/rig to fail.
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Old 04-01-2012, 19:55   #47
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Re: Attaching Antenna Cable to Backstay

Suppose the spreader was insulated from the mast with a rubber pad, say 3mm thick? Maybe the spreader bolts could be isolated from the spreader with rubber bushings and washers. If that would work you could attach the anetnna to the shroud chain plates inside the cabin. Just wondering.

P & S, I have one shroud that goes over the spreader to the masthead. But I have two lowers that go only to part way up to the base of the spreader. Does the antenna need to go full height?
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Old 04-01-2012, 20:02   #48
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Re: Attaching Antenna Cable to Backstay

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Originally Posted by Sam Plan B View Post
Suppose the spreader was insulated from the mast with a rubber pad, say 3mm thick? Maybe the spreader bolts could be isolated from the spreader with rubber bushings and washers. If that would work you could attach the anetnna to the shroud chain plates inside the cabin. Just wondering.

P & S, I have one shroud that goes over the spreader to the masthead. But I have two lowers that go only to part way up to the base of the spreader. Does the antenna need to go full height?
Sam,

I'm in DC, too, as is my boat (at Capital Yacht Club).

Give me a buzz tomorrow and I'll be happy to try to help you.
My email is bill at wdsg dot com

Bill
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Old 04-01-2012, 20:06   #49
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Re: Attaching Antenna Cable to Backstay

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Originally Posted by osirissail View Post
I think you folks are forgetting that chainplates, standing rigging, lifelines, stanchions, etc., are all connected via your boat's bonding system to ground. Unless you cut off the bonding wire the SSB transmitter would be pumping its signal directly to a ground.
When we discuss feeding the backstay at the (insulated) chainplates, of course any grounding wire to the chainplates needs to be disconnected. The top insulator is still needed as well.

Quote:
Originally Posted by osirissail View Post
The RF burn warning and even shock is real which is why the antenna feed is attached above the maximum human standing height on deck. Remember the "standoffs" need to be about 4" roughly from the grounded portion of the backstay so just putting a plastic cover over the wire will not totally eliminate the hazard.[...]
A plastic cover over the accessible section of the backstay will reduce the shock hazard just as well as the insulation on the usually-exposed GTO-15 feedline does. The standoffs aren't there to eliminate spark-over, they're used to reduce the capacitive coupling from the feedline to the grounded stay. This capacitance can make the antenna difficult to tune, and can reduce the antenna efficiency.

The RF voltage we're talking about is usually well under 1000V. It doesn't take a lot of insulation to prevent a shock-hazard.
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Old 04-01-2012, 20:33   #50
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Re: Attaching Antenna Cable to Backstay

Drew -

I see you are planning to use an IC-7000 for your shipboard radio. This is a great radio, and I use mine on the ham bands while at sea too. There are some things I have done to ensure top performance:

1. The radio runs hot, even in receive only modes. I'm sure ICOM 's design factors this in, but it still makes me nervous, especially if operating in a tropical clime. I made a fan mod that keeps the cooling fan running constantly at a slow speed, but still kicks into high when needed. See it here: Fan Mod

2. It doesn't like lower voltages much. I use a TGElectronics boost regulator to feed it a constant 13.8 volts even if battery voltage has sagged. Boost Regulators

3. If you plan to use it with an SCS Pactor modem, you'll find that you have to set modem to radio drive levels very low because of the very high input sensitivity of the IC-7000. This increases the possibility of RF getting into the modulator and causing distortion again. I wedged an 8:1 voltage divider composed of a couple of 1/8 watt resistors into the connector at the radio end. This let me set the modem drive levels higher while reducing the effect of stray RF currents.

Naturally you can get away without doing any of these things if you are careful, but they do offer me a higher degree of confidence.

Chip
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Old 04-01-2012, 20:43   #51
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Re: Attaching Antenna Cable to Backstay

I got the stays and shrouds tension relationship backwards. Here is link to a chart based on the LOOS Tension gauges: http://l-36.com/pt2.php

But anyway putting "rubber" anything in the path of moving and highly loaded steel wire and aluminum parts (spreaders) will most assuredly result in the demise of the rubber rather quickly. Everything in your standing rigging moves when underway. And then rubber or any plastic also gets degraded by sun's heat and UV rather rapidly.

Wooden spreaders might be the answer rather than metal, but those things are as scarce as hen's teeth these days and would require constant maintenance to protect the wood.
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Old 05-01-2012, 06:21   #52
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Drew:

Not sure where you are, but I have't seen a boatyard yet that didn't have a pile of old wire. That's how I got mine. It's not like it's holding up the mast, so old wire should be fine.

Good luck and have fun -- I've got a 7000 on board too...

take care...
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Old 05-01-2012, 08:41   #53
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Re: Attaching Antenna Cable to Backstay

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Hey Bill,
I actually went down this path, but by the time the wire, insulators, turnbuckles and fittings are purchased the difference in price between having my backstay cut and building a new backstay are negligible... I have a quote for under $400 for the backstay work, including two insulators! I'm just wondering if I should go with the two insulators or if I'll benefit from using just one?
I had a single backstay to a triangle plate just above the bimini then dual wires down to the transom. Although the historical record of the backstay insulators is pretty good, I didn't want to totally trust them.

So I moved the triangle plate up to the masthead and ran "dual" backstays from it to the transom. One of the backstays has the insulators in it and is used as the backstay antenna. If for some unknown reason an insulator breaks, I have a pure unbroken wire "other" backstay holding up the mast.
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Old 06-01-2012, 18:32   #54
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Quote:
Originally Posted by osirissail

So you end up with the three most common SSB antenna solutions - insulated backstay; 23ft Whip; and free copper wire (usually run inside some double braid line/rope and hauled aloft by a halyard or whatever.
There is another solution. I was quoted an estimated $1000 to cut and install two insulators in my backstay from a well knowing rigger here in New England. I went looking for another solution an came across the GAM solution. Far less than $1000.
http://gamelectronicsinc.com/ssb.htm

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Old 06-01-2012, 20:04   #55
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Re: Attaching Antenna Cable to Backstay

Interesting idea - a dual external set of wires parallel to the backstay -
http://gamelectronicsinc.com/ssb.htm
But I will have to defer to the RF electronic wizards on the forum but it appears to be that since the active RF wires are mere millimeters from the grounded backstay you will get into the same problem that the 4" stand-offs are used to alleviate. See:
Quote:
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. . . The standoffs aren't there to eliminate spark-over, they're used to reduce the capacitive coupling from the feedline to the grounded stay. This capacitance can make the antenna difficult to tune, and can reduce the antenna efficiency. . . .
Anybody actually use this "Gam/McKim" Split lead antenna?
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Old 06-01-2012, 20:07   #56
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Re: Attaching Antenna Cable to Backstay

Bill has the right idea. The point is to get RF power from your tuner through 30 (or so) linear feet of some kind of conductor and to get it as high as possible and insulate it from the ships ground system. Using a halyard to haul up a hunk of wire makes good sense. NI7X
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Old 06-01-2012, 20:56   #57
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Re: Attaching Antenna Cable to Backstay

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Originally Posted by osirissail View Post
Interesting idea - a dual external set of wires parallel to the backstay -
http://gamelectronicsinc.com/ssb.htm
But I will have to defer to the RF electronic wizards on the forum but it appears to be that since the active RF wires are mere millimeters from the grounded backstay you will get into the same problem that the 4" stand-offs are used to alleviate.
Anybody actually use this "Gam/McKim" Split lead antenna?
The 4" stand-off is more of a guideline than a rule. The GAM antenna does indeed have a lot of capacitance between the twin antenna wires and the grounded backstay. In most cases the tuner can compensate, the radio will see a reasonable load, and a signal will get out. At some (most?) frequencies the excess loss will be minimal and the GAM antenna will work fine. Many people use the GAM and report good results.

On the other hand, some people try the GAM and are not satisfied with the performance. I personally know of one boat that had the GAM and couldn't reliably communicate with it. They got rid of the GAM and put up a synthetic line with buried wire, and had much better results. It is possible that the problem was due to the combination of the GAM, their particular ground system, and the frequencies they were using.

In general you want to minimize stray capacitance between your antenna and ground. Antenna tuners can compensate for a lot, but losses can be excessive at the extremes. If the GAM works for you then that's great. Any boat antenna system is a compromise, and the GAM is certainly a convenient way to string an antenna.
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Old 07-01-2012, 07:08   #58
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Re: Attaching Antenna Cable to Backstay

That seems to follow what I have found on some boats - the bonding system wires are badly corroded and even broken. In which case the backstay would be "ungrounded."
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Old 07-01-2012, 10:33   #59
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Re: Attaching Antenna Cable to Backstay

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Using a halyard to haul up a hunk of wire makes good sense. NI7X
...assuming you're never going to want to use your radio and sail at the same time.
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Old 07-01-2012, 19:17   #60
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Re: Attaching Antenna Cable to Backstay

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...assuming you're never going to want to use your radio and sail at the same time.
That depends upon how many halyards your boat has - - I have two forward and two aft halyards on each mast. So I always have a spare available for something just like the "rope SSB" antenna.
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