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Old 17-09-2012, 19:50   #1
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Dipole Antenna

Hello,I have a 50ft ketch and I'am looking into streaching a wire antenna between my 2 masts..I have seen this on fishing boats for C.B.,SSB,VHF and am radio.Does anyone have any info on this subject....
Thanks in advance,
d.evans
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Old 17-09-2012, 20:11   #2
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Re: dipole antenna

I could write a book on this but, don't be scared, I promise not to!

First, if you're going to use a dipole antenna, it would either be a single-band tuned dipole (for one band only) or a multi-band dipole. The latter would require open wire or balanced feedline and, while an effective antenna, is not particularly well suited to the marine environment. The feedline tends to blow and bounce around a lot in rough weather. However, at anchor or in port, this type antenna can work well. The G5RV and G5RV Junior antennas are a good example.

Rigging an efficient SSB antenna on a ketch is always a challenge. I have a friend with a 56' LOD ketch who's solved the problem by using the topmast shroud on his mizzen antenna and a jumper wire up to the triatic stay. He's using a 4:1 un-un at the base, going to an automatic tuner belowdecks. It works pretty well.

Many ketches use a single backstay on the main. Some use whip antennas on the stern.

There are many potential solutions, depending on your specific rigging details. Few are really great antennas because of the incredible amount of rigging required for the ketch rig. Most are workable, though.

FWIW,

Bill
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Old 17-09-2012, 20:12   #3
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Re: dipole antenna

Bill's Marine Pix
Go to the link and open the Marine Antenna album. Bill Trayfors has posted a nice "How To" for vertical dipoles that work well for him.

Oop, I posted this while Bill was posting.
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Old 17-09-2012, 20:43   #4
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Re: Dipole Antenna

The term dipole implies it is cut to a resonant length which would limit you to only a small frequency range. You can construct a multi-band dipole using different resonant lengths of wire fed at a common point as well.

This doesn't require open wire feed line. You can run coax to the common feed point typically a balun. There are various frequency calculation charts and formulas all over the internet for reference. The added benefit is that no tuner should be required.
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Old 17-09-2012, 21:05   #5
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Re: Dipole Antenna

Quote:
Originally Posted by S/V Illusion View Post
The term dipole implies it is cut to a resonant length which would limit you to only a small frequency range. You can construct a multi-band dipole using different resonant lengths of wire fed at a common point as well.

This doesn't require open wire feed line. You can run coax to the common feed point typically a balun. There are various frequency calculation charts and formulas all over the internet for reference. The added benefit is that no tuner should be required.
That's true, but:

1. a multi-band dipole fashioned from different resonant lengths of wire fed at a common point is, IMHO, totally impractical for a marine application;

2. multi-band dipoles can be constructed in several ways, including the one I mentioned: using a balanced feedline instead of coax, and a tuner. This works very well but, again, isn't really practical for a seagoing boat.

3. another way to make multiband dipoles is to use traps. However, these, too, are impractical in the marine environment.

Bottom line:

1. single-band VERTICAL dipoles are GREAT DX antennas, and work very well in the marine environment, especially for 10 mHz and above. They do not require a tuner, and can take all the power you can feed them.

2. multi-band dipoles -- however constructed -- are not likely to be long-lasting in a real marine environment.

3. the best versatile, all-band antenna for a sailboat is either a random-length wire (like an insulated backstay) fed with an autotuner, or a vertical whip also fed with an autotuner.

On my 42' sloop I have all three: an insulated backstay with an auto-tuner; vertical dipoles for 20m and 15m; and a stern-mounted mobile whip antenna with resonators for various bands. It's interesting to compare their effectiveness on various bands and over various signal paths...something I've done for several decades.

Bill
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