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Old 10-11-2016, 03:04   #1
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Dual backstay antenna.

On our boat we have dual backstays. At the stern thy have a distance of about 2 meters and at the top they meet each other.
Now, what would be the correct approach to use this as an antenna.
Should you isolate both at the top and short them together at the bottom.
Then feeding it from the ATU in one of the corners.
Or, should you just use on of the backstays isolating it 1.5 m from the top ?
The deck is GRP. Is it ok to skip the isolator at the bottom and feed the antenna below deck ?

At the moment the antenna is made up by 2 isolators at the top.
Just below the isolators the 2 backstays are shorted with a short wire.
At the bottom there are no isolators. Below deck the stays are shorted with an ignition cable.
This is then feed at one of the corners close to to the ATU.

If you use one of the backstays. Will the other now non isolated backstay sitting close to the active interfere ?

We are about to replace the standing rigging shortly and are able to chose configuration.

The existing one tunes fine on all bands but it would be interesting to hear views on this matter.
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Old 10-11-2016, 07:51   #2
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Re: Dual backstay antenna.

From your description, it sounds as if the two backstays are shorted together at the top AND the bottom. Then the whole system is fed at the bottom corner. The ground side of the tuner then goes to ground?

If that's true, I can't see that feeding the two backstays essentially in parallel will accomplish any more than feeding one.

If the two backstays were shorted at the top and fed with the ATU hooked between the two bottom ends, I could imagine it being some form of loop antenna. But I can't see that that would perform much better than a single vertical wire.

I've also seen a couple of split backstays systems where the ATU was hooked between the two lower stays, feeding the small loop formed by the two lowers. This small loop couples into the upper which needs a top insulator.

You might check a good antenna book for loop antennas and parallel-fed vertical antennas and see what you actually have.
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Old 10-11-2016, 12:45   #3
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Re: Dual backstay antenna.

Quote:
Originally Posted by svinshallah View Post
From your description, it sounds as if the two backstays are shorted together at the top AND the bottom. Then the whole system is fed at the bottom corner. The ground side of the tuner then goes to ground?
Correct. The tuner is grounded with a large dynaplate.

Quote:
Originally Posted by svinshallah View Post
If that's true, I can't see that feeding the two backstays essentially in parallel will accomplish any more than feeding one.


If the two backstays were shorted at the top and fed with the ATU hooked between the two bottom ends, I could imagine it being some form of loop antenna. But I can't see that that would perform much better than a single vertical wire.
So you mean that the other one in parallel will not disturb the active one ?
It will be running at a distance of 2 meters at the bottom closing in to about 30 cm at the top.
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Old 10-11-2016, 13:06   #4
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Re: Dual backstay antenna.

If the ground terminal of the ATU connects to a Dynaplate, then whatever is attached to the antenna terminal is the antenna. As long as it's long enough, i.e. more than about 23', it will probably tune correctly and work. If the two backstays are connected together at BOTH the top and the bottom, they will function as a single driven element. I can't see any good reason that the existing complex system will work any better than using a single wire.
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Old 10-11-2016, 13:25   #5
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Re: Dual backstay antenna.

A bit more detail. If the two backstays were "far enough apart" they might act as two separate two separate driven elements. But, with them as close as they are, that's unlikely. I suspect, without actually trying to model it, that they will act as a single wire, with perhaps a slightly larger effective cross section. With a tuner, that won't make a difference.

In any case, I think that just feeding one side with a insulator at the top and bottom would work equally well. If the chainplate at the bottom is not connected to ground, you might not need the bottom insulator. Of course if the deck and chainplate get wet enough it might affect things, or somebody grabs hold of it while you're transmitting would be uncomfortable.
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Old 10-11-2016, 14:27   #6
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Re: Dual backstay antenna.

Quote:
Originally Posted by svinshallah View Post
Of course if the deck and chainplate get wet enough it might affect things, or somebody grabs hold of it while you're transmitting would be uncomfortable.
Thanks svinshallah for your feedback.
The backstays are insulated to 2.5 meters above deck so touching them will not
give you a shock. I always tell my wife if she wants curly hair to hold the backstays while I am transmitting but she has never tried it.

I think I will stick to the existing configuration.
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Old 10-11-2016, 14:48   #7
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Re: Dual backstay antenna.

Feed the tuner antenna to on of the backstays (best if you can feed it from below the deck at the chainplate. Isolate the other backstay completely top and bottom. In this case your antenna will start at the tuner run up though one of the backstays then continue all the way to the masthead insulator. I have a similar set-up and I get no degradation from the isolated backstay leg. Remove all the interconnecting wires. Make sure the mast top is grounded against lightning.
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Old 10-11-2016, 15:48   #8
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Re: Dual backstay antenna.

Thanks Pizzazz!
Your input is much appreciated.

Quote:
Originally Posted by Pizzazz View Post
Feed the tuner antenna to on of the backstays (best if you can feed it from below the deck at the chainplate.
Yes, this is what I do at the moment, or very close to. I do feed it at the chainplate.

Quote:
Originally Posted by Pizzazz View Post
Isolate the other backstay completely top and bottom. In this case your antenna will start at the tuner run up though one of the backstays then continue all the way to the masthead insulator. I have a similar set-up and I get no degradation from the isolated backstay leg.
How do you know there is no degradation ?

Quote:
Originally Posted by Pizzazz View Post
Remove all the interconnecting wires. Make sure the mast top is grounded against lightning.
The mast and all other standing rigging and guardrails are connected to the keel. The keel is external from the hull and thus exposed to sea water.
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Old 10-11-2016, 16:44   #9
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Re: Dual backstay antenna.

Quote:
Originally Posted by hasse_A View Post
On our boat we have dual backstays. At the stern thy have a distance of about 2 meters and at the top they meet each other.
Now, what would be the correct approach to use this as an antenna.
Should you isolate both at the top and short them together at the bottom.
Then feeding it from the ATU in one of the corners.
Or, should you just use on of the backstays isolating it 1.5 m from the top ?
The deck is GRP. Is it ok to skip the isolator at the bottom and feed the antenna below deck ?

At the moment the antenna is made up by 2 isolators at the top.
Just below the isolators the 2 backstays are shorted with a short wire.
At the bottom there are no isolators. Below deck the stays are shorted with an ignition cable.
This is then feed at one of the corners close to to the ATU.

If you use one of the backstays. Will the other now non isolated backstay sitting close to the active interfere ?

We are about to replace the standing rigging shortly and are able to chose configuration.

The existing one tunes fine on all bands but it would be interesting to hear views on this matter.
The configuration you have makes a nice broadband antenna that probably tunes very well. If it is working I would not change it.

You don't absolutely need insulators at the bottom and since it is working well today without any that is how I would keep it.

If you remove one of the upper insulators and the shorting wire that will likely create new problems. In that case the un-insulated wire will act as a receive antenna and couple some RF energy into the mast. This will likely reduce the antenna efficiency. I would not recommend this approach although it will probably work in most cases.
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Old 10-11-2016, 18:21   #10
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Re: Dual backstay antenna.

Quote:
Originally Posted by hasse_A View Post
How do you know there is no degradation?
In my setup, I started with the golden standard (simple wire on a halyard as an antenna (easy to adjust length and keep it away from the rigging) and household aluminium foil as a ground (just let it out in the water for a few feet, connect directly to tune ground with no wires, just the folio on a nut), then adjusted the wire to minimize the radio VSWR. Next I used Winmor to connect to various servers in different bands and maximize the ACK rate. Basically, I saw little difference once I got the antenna wire more than 1 ft away from the mast top and the port backstay. Then I cut my backstay accordingly and installed a copper strip to a through hull. It sounds complicated but it is not and it really helps you understand how the antenna impacts SSB operation.

I believe what you have on your boat is called a Delta loop antenna, basically a triangle oriented vertically. It do not think it will be as efficient as a long wire backstay antenna. You definitely need a good tuner as the length of the loop sides is smaller on a boat than what it should be (for example, at 14 MHz, the side of the triangle should be 23 ft, longer at lower frequencies which is difficult on a sailboat. Still, the tuner will tune it and you may check to see if it works. It really depends how much you want to play with it. I would say go for long wire, 30 ft or longer and be done with it.

Regards,
Pizzazz
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Old 10-11-2016, 22:48   #11
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Re: Dual backstay antenna.

Quote:
Originally Posted by transmitterdan View Post
The configuration you have makes a nice broadband antenna that probably tunes very well. If it is working I would not change it.

You don't absolutely need insulators at the bottom and since it is working well today without any that is how I would keep it.

If you remove one of the upper insulators and the shorting wire that will likely create new problems. In that case the un-insulated wire will act as a receive antenna and couple some RF energy into the mast. This will likely reduce the antenna efficiency. I would not recommend this approach although it will probably work in most cases.
For what it's worth, I completely agree with this. Keep the backstays connected top and bottom, and keep feeding the antenna from the bottom as you are doing. Your dual-backstay configuration is a very simplified version of a "vertical conical monopole" which is a broadband antenna with good efficiency. Your tuner should have a fairly easy time matching this antenna.
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Old 11-11-2016, 08:36   #12
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Re: Dual backstay antenna.

Quote:
Originally Posted by Pizzazz View Post
I would say go for long wire, 30 ft or longer and be done with it.
Pizzazz
The backstays are slightly over 40 feet. Something like 42.
Any concern I am feeding at one corner. Does it matter if I instead
connected the tuner halfway between the lower endpoints ?

/Hans
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Old 11-11-2016, 10:08   #13
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Re: Dual backstay antenna.

Hans,

No, it does not matter you are feeding at one end. In fact that is actually better. That way any 1/2 wave resonance will be mitigated. I would not change anything as long as it is working. I wish I had your setup.
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