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Thread: Is a "Full Wave" dipole a good idea or not?

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  1. #1
    Join Date
    May 2003
    Posts
    355

    Default Is a "Full Wave" dipole a good idea or not?

    I read all about half-wave dipoles, but what about a FULL-wave dipole?

    Are there any pro's or con's to putting one of those up if space permits? Any advantage over a half-wave? If I get the green light on this, is the same freg-div-into-468 formula what I would use for dimensions?

  2. #2

    Default

    If you're serious:

    1. No of course the formula would be 936/f for a "full wavelength" dipole.

    2. No, the reason people don't use these is because they're very difficult to match: Impossible to use coax altogether; you can use open wire balance line to match a full-wave dipole but it will still be a 5 or 10:1 VSWR...it just won't matter much because the line loss will still be reasonably low. Still, the risk of arching stuff is much higher with a voltage-fed antenna (like a full-wavelength dipole), and there's about nothing to gain overall.

    A better idea is the "double extended Zepp" which is fairly commonly used and about 0.64 wavelengths per leg, or 1.28 wavelengths overall. Its formula is 1198/f and it's a better choice than a full-wavelength doublet.

    WB2WIK/6

  3. #3

    Default

    A full wave dipole will show some gain in the broadside if it is installed high enough.

    A full wave dipole by definition is resonant. It cannot be fed directly with coax though. Lots of people substitute the term resonant for any antenna load that has a low SWR when fed with a hunk of 50 ohm coax. Good quality balanced line feed with a balanced output antenna "tuner" is a common way to have success.

    Google: pair of half waves in phase.

  4. #4
    Join Date
    Mar 2007
    Location
    Florida Swamp
    Posts
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    Default

    Check the last Bill Oar book on antennas. AES has it its $19.95. I built the off center fed full wave for 17M and like it. Has a off center figure 8 pattern that runs the same direction as the antenna is strung. 4 lobes. My antenna runs north and south. T he lobes run 380 to 340, 20 to 80, 11o to 170 and 200 to 260. Major nulls at 90, 180, 270 and 360. Hope this helps if you need more about it pm me.
    Brad
    WZ0O
    Good Judgement comes from experience.
    Experience comes from Bad Judgement.

  5. #5
    Join Date
    May 2003
    Posts
    355

    Default

    Quote Originally Posted by WB2WIK View Post
    If you're serious:

    WB2WIK/6

    I was serious in asking the question alright, but now after reading the responses, I see why no one does it.

    Gotta ask, ya know...it's the only way to ever find out....

    Besides wasting the time it would take to put one up and then find out the hard way it won't tune.

    Thanks

  6. #6
    Join Date
    Apr 2006
    Location
    Bristol, MD
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    2,222

    Default

    I actually use something like that - about 550 Feet long, fed in the center with 450 Ohm ladder lind and fed thru a Johnson Matchbox(tm). On 40 meters and up, it's great. Ok on 160 and 80. I've also fed it with TV foam twinlead and an MFJ 941 tuner.

    In theory, it shows a lot of lobes and tends to be directional off the ends., but you can develop some gain Mine runs NE>SW and pretty much covers the world, but is weak towards South America. The lobes haven't been much of a problem. On 80, a 1/2 wave dipole beats it for the most part, but neither is very high - 30-50 feet.

    Wire antennas are easy and cheap to make, and if you don't like it, recycle the wire into something else and try again.

    But as WIK has pointed out, you'll see some pretty strange impedences (I've never tried to measure them), but if you can match them, go for it.

    A half wave dipole provides a good match for 70 Ohm Coax, which is one reason it's popular. And it has a pretty easy to understand radiation pattern (like an infinity or a donunut). But there's nothing magic about it. A very old "all band" antenna is a half wave dipole for the lowest band, fed with open wire, so an 80- meter dipole run on 10 will five the same idea. Try it, you MIGHT like it. Else try something else.

    de Bob
    Ham radio is something you DO and LEARN. NOT something you BUY!

  7. #7
    Join Date
    Mar 2007
    Location
    Florida Swamp
    Posts
    219

    Default

    The OCF will tune to about 1.6 to 1 and mine works much better than the 1/2 wave it replaced. Went together in about an hr. and is fed with RG8. I did use a current balun at the antenna to cut down on feed line trouble.
    73
    Brad
    Good Judgement comes from experience.
    Experience comes from Bad Judgement.

  8. #8
    Join Date
    Feb 2002
    Location
    Right here
    Posts
    12,919

    Default

    A half wave dipole is doable but requires a quarter wave matching section (tuning stub) to match to coax. Think about a J pole antenna, same thing. The biggest problem you run into at HF is that the matching section is usually only good over a narrow frequency range and adjustment of the stub can be a royal pain. Even then, you would still want to use open wire line or ladder line as a feeder.

    As WIK pointed out there are easier ways to get gain, and a good match with a wire antenna than a full wave dipole. I just didn't want you to think it couldn't be done. It can, but not as easily as some other antenna configurations.
    I'm sorry you don't have the experience or understanding to realize that others possess a skill set that you seem to dismiss as fantastical.

  9. #9

    Default

    Quote Originally Posted by KA4DPO View Post
    A half wave dipole is doable but requires a quarter wave matching section (tuning stub) to match to coax.
    ::That was a typo! I'm sure you meant "a full wave dipole...."

    Anyway, "everything works," it's just a matter of how well. If you have a terrific antenna but can't transfer power into it because it's hugely mismatched, then it's not so great.

    A narrowband matching network can always be built to transfer power from a source to a load even if the natural mismatch is terrible....but then you have a narrowband matching network, which works at one frequency and not anywhere else. This is why most hams don't bother -- however it's a pretty acceptable solution for non-hams in the commercial/broadcast/aerospace/etc industries where a single-frequency match is just fine.

    WB2WIK/6

  10. #10

    Default

    A fullwave loop, on the other hand, is one of those antennas that many inexperienced hams dismiss as boring, but don't realize the potential they hold, even as multibanders...
    And now for something completely different...

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